Category Archives: Diseases

Science snippets

Three short and easy-to-digest snippets of science this week. After last weeks’ overly-long DIY extravaganza I thought I’d try and be a little more succinct this time. 

The following triptych is based on three separate papers, two published in the last month or two and one from last year. Individually these incremental advances in our understanding probably do not justify a post of their own. 

The first paper, on sodium butyrate, was included here following a question I was asked during an evening talk to a beekeeping association last week. I don’t think I answered the question particularly well, so thought I’d elaborate here for clarity. The science is cool, but the paper is rather odd.

The other two papers are on a related topic, the bacteria in the gut of bees. The first of these came out last week and had a very catchy title. Reading that paper resulted in my burrowing back a year or two into the literature. While doing this I found a related paper that has got me thinking again about feeding winter bees honey, syrup or fondant.

Sodium butyrate reverses DWV-induced memory loss

All honey bees, perhaps with the exception of those in Australia 1, are infected with deformed wing virus (DWV). Historical studies that report 30% or 60% or whatever virus prevalence were probably using an insensitive assay. Even bees in Varroa-free regions carry DWV. 

And, in the absence of Varroa, DWV is not a problem to the bee 2. It is present at low levels and is apparently not pathogenic.

However, when transmitted by Varroa, the virus levels are amplified about a million times are a range of symptoms are clearly present. These include pupal death or the emergence of workers with overt developmental defects, including the classical ‘does what it says on the tin’ deformed wings.

DWV symptoms

DWV symptoms

In laboratory studies up to 70% of the bees exposed to high levels of virus exhibit these catastrophic symptoms.

However, some bees emerge with high levels of virus, but ‘look’ normal.

But they don’t behave normally.

In particular they have defects in memory and learning. 

Forgetfulness and getting lost

Impairments in memory and learning are bad news for bees. 

Foragers need to be able to learn where sources of nectar and pollen are by interpreting the waggle dance. Perhaps more importantly, they need to remember where the hive is so that they can successfully return from a foraging trip. If they forget where they are going, they’re doomed. 

A social insect like a honey bee cannot become a solitary bee without inevitably becoming a dead bee.

There is a long history of using sodium butyrate (NaB) to either enhance memory, or to reverse memory loss. It belongs to a class of compounds called HDACi’s (histone deacetylase inhibitors) which have been used in medical studies including anti-neural degeneration and anti-Alzheimer’s disease. 

NaB has even been used in studies with honey bees. In these it has been shown to enhance expression of genes involved in the immune response, detoxification and learning/memory. 

In addition, NaB restores learning ability in neonicotinoid-treated bees … it was therefore a logical extension (particularly since this neonic study was conducted by the same Taiwanese researchers) to investigate a role for NaB in counteracting DWV-induced learning and memory loss, the topic of the first paper this week 3 .

CCD and ‘massive disappearance’ of bees

This paper is a bit of a curate’s egg. The science is detailed and appears to be done well. The experiments are logical, mostly well-controlled and involve a combination of detailed molecular studies with monitoring bee behaviour in the field. 

However, the attempted link between memory loss and bee loss, the association with CCD (colony collapse disorder) and the superlatives in the paper make for a rather strange reading experience.

I’m not going to give a detailed account of the science. The key points are as follows:

  • NaB increased honey bee survival after oral DWV challenge 4. Disappointingly they did no virus assays.
  • NaB reversed memory loss in the standard proboscis extension assay (PER)
  • Gene expression studies indicated that NaB (an HDACi) resulted in increased expression of numerous genes, including reversing the suppression of some genes caused by DWV infection. Some of these genes were involved in memory, but many other gene classes were also differentially expressed. NaB was also shown to restore some neurotransmitter activity in the brain.
  • Colonies fed NaB and then fed DWV experienced a much reduced loss of bees than those that just received DWV.

Sodium butyrate reverses bee loss due to DWV (A) in/out ratios and (B) lost bee ratios

I need to re-read some of the methods and data on the in/out ratios (the graph on the left above) as it appears that more bees returned to the hive than left the hive! These field experiments used an automated hive monitoring system , but did not apparently use any form of tagging on the bees. It is not clear how they could be certain that the ‘returning’ bees originated from the monitored hive.

Smells fishy?

The conclusions of the paper end with the sentence a diet incorporating histone deacetylase inhibitors could be used to maintain the overall wellbeing of the bees and integrity of the colony”.

Well … perhaps.

I’d argue that prevention is always better than cure.

It is preferable to minimise DWV levels in the hive – by killing Varroa – than it is to try and counteract the deleterious effects of DWV by adding additional chemicals. Studies from my lab and others show that effective Varroa control results in very low virus levels.

But if you are going to feed them HDACi’s, then it probably should not be sodium butyrate. 

The ‘butyrate’ bit of the name is derived from butyrum meaning ‘butter’ in Latin. Sodium butyrate is a fatty acid and is famously smelly. It reeks of spoiled milk, or sour butter, and is the compound that gives vomit that distinctive ‘smell-it-a-mile-off’ odour.

Hmmmm …. nice 🙁

So, although it doesn’t smell fishy … it certainly does smell. 

Possibly not something you’d want anywhere near hives producing honey 😉

Chicken eating bees

I wrote recently about how important catchy titles are to scientific papers. The title of this next paper was the only thing that made me read the study …

Why Did the Bee Eat the Chicken? Symbiont Gain, Loss, and Retention in the Vulture Bee Microbiome by Figuerosa et al., (2021) mBio 12:e02317-21

How could you not want to read a paper with a title like that?

Well, one reason might be that you don’t know the words symbiont or microbiome.

Let’s see if we can change that … 😉

Vulture bees

Honey bees, along with all other bees, are classified with the sawflies, wasps and ants as members of the Hymenoptera. Of these, bees are wasps that switched to a vegetarian lifestyle, eating pollen and nectar.

Vulture bees dining out on chicken

However, some stingless bees also dine on carrion (literally ‘the decaying flesh of dead animals’) and a few species – the aptly named vulture bees – only feed on carrion for protein and no longer collect pollen.

You are what you eat

The microbiome is a collective term for the all the bacteria 5 in a particular environment.

For example, the gut microbiome or the skin microbiome. 

In the gut, these microbes help the host exploit novel food resources. For example, honey bees have bacteria (Gilliamella apicola) that help them digest toxic sugars 6.

The host (bee) benefits from the presence of the bacteria, and the bacteria benefits from the protection and food provided by the host … which is exactly what the term symbiotic means.

You are what you eat is, of course, not meant literally 7.

However, it is certainly true that the symbiotic microbiome is significantly influenced by diet. And the symbiotic microbiome also influences what can be consumed.

The Figuerosa et al., study compared the gut microbiome of a variety of stingless bees from Costa Rica. Some of these bees were pollenivorous and others were facultatively or obligately necrophagous.


  • Pollenivorous – pollen eating
  • Facultatively – some of the time 
  • Obligately – all of the time / only
  • Necrophagous – feeding on corpses or carrion

The microbiome of vulture bees

By analysing the microbiome of these different types of bees the authors determined that reversion to a purely necrophagous lifestyle had resulted in the acquisition of a unique range of additional bacterial species.

Gut microbial communities in pollen-eating (absent), or facultatively or obligately necrophagous stingless bees.

However, the gut microbiome was not entirely unique. Many species were also found in facultative necrophagous bees, or in the pollenivorous species. 

It’s not yet clear what all these new species actually ‘do’ in the gut of these carrion eating ‘vulture’ bees. Further studies will be needed to determine this.

And, of course, this study begs the additional question.

Which came first, the chicken or the bacteria? 8

Did the microbiome change in response to a change in diet, or was the change in diet enabled by the change in the microbiome?

And, the topic of changes in the microbiome is the topic of the third paper … which, you’ll be relieved, is on honey bees.

The microbiome of summer and winter bees

I really used the ‘vulture bees’ paper to introduce the concept of the symbiotic microbiome.

In honey bees, the microbiome has been extensively studied over the last decade or so.

A striking feature is that it includes relatively few species of bacteria, and is dominated by less than 10 in total 9. These species are conserved regardless of geography, life stage (nurse bee, forager etc.) or season.

Almost every study of the honey bee microbiome has been a qualitative one. By that I mean the scientists determine the species present, but ignored the quantities of the different bacteria. 

In comparison, a quantitative study would have determined the amounts of some or all of the core species in the microbiome. 

And that is exactly what Kešnerová et al., (2020) did in their study entitled Gut microbiota structure differs between honeybees in winter and summer 10.

Multi-year, multi-hive studies

As you would expect from a paper in the ISME Journal 11 this is a thorough study, involving sampling of one hive on a monthly basis for 24 months, and fourteen hives in two different locations in the summer and winter. Each sampling involved multiple individual bees that were analysed. There are some additional experiments on colonisation of the gut that I’m going to largely ignore here.

The authors qualitatively and quantitatively studied only five of the core species and two non-core species from the gut microbiome. The names don’t really matter, but are shown in the figures below.

The gut microbial community differs between summer foragers and winter bees

There’s a huge amount of date in this figure.

However, simply by looking at the monthly changes (A), or the community composition (B), it is clear that summer foragers and winter bees have significantly different microbial populations within the gut.

In addition, the overall levels of many of the species tested (C) were significantly increased in the winter bees population.

Summer bees, nurse bees and winter bees

From a physiological and dietary point of view, there are some similarities between nurse bees and the long-lived winter bees. The authors therefore tested the bacterial population the gut of each contained in summer (foragers and nurse bees) and winter (winter bees).

Bacterial load and community composition in foragers, nurse and winter bees

Nurse bees and winter bees contained at least 10 times the population of bacteria as present in the gut of foragers (A, above). Of these, nurse bees were intermediate in the range of species between the foragers – which had a greater range – and the winter bees which contained fewer species.

Finally, detailed statistical analysis of the populations in the three bee types indicated that they were distinct (C), despite conservation of several of the core microbiome species. This latter analysis showed that, whilst each was distinct, all of the populations were similarly variable within a particular bee type i.e. none of the dots are more clustered/scattered in the third panel above.

Diet and the microbiome

Numerous studies have shown that diet influences the bacteria in the gut – of honey bees, vulture bees, flies, mice and men. It’s therefore very likely that the diet of nurse and winter bees at least partly accounts for the differences in the bacterial community present in their gut.

Foragers need an energy-rich diet and mainly feed on nectar and honey. In contrast, nurse bees and winter bees also consume pollen.

In studies I don’t have time to discuss, Kešnerová et al., (2020) also showed that feeding gnotobiotic bees 12 pollen and syrup resulted in significant increases in the amount and levels of bacterial colonisation i.e. they resembled nurse or winter bees. In contrast, bees fed syrup alone developed a gut microbiota that resembled that of foragers.

All of which made me think about feeding bees syrup/fondant for the winter vs. feeding/leaving them honey.

Honey is better for bees in the winter … really?

Beekeepers who leave their bees with a super or so of honey are often convinced of the benefits to the colony.

When pressed they unfortunately provide little evidence to support their expensive decision 13

I’m not aware of a single study that convincingly i.e. statistically, demonstrates that colonies are more successfully overwintered on a diet of pure honey, rather than a colony fed syrup or fondant. 

I’ve discussed this before – for example, see my response to this comment in the post entitled ‘Cut more losses’. 

Is the microbiome a marker of colony health?

However, this paper on the winter bee microbiome got me wondering whether – in the absence of evidence supporting better overwintering survival – bees fed syrup/fondant or honey have a different bacterial population.

It would be very interesting if they did.

Furthermore, as scientists further untangle the role of these bacteria, we would be able to tell whether syrup/fondant was better, worse or neutral in terms of the changes it induced in the bacteria that inhabit the winter bee gut.

Unfortunately, the Kešnerová et al., (2020) study has no details whatsoever of the hive management regime. The work was done in Lausanne, Switzerland, but it doesn’t say how or what they were fed for the winter.

Nor is there any mention of whether the diet was supplemented with sodium butyrate 14. This will also need to be studied as butyrate is a natural product of some gut microbes and there is evidence that – in humans at least – it is involved in communication between the gut and the brain.

And I think my gut is telling my brain that it would like some pizza …


Socially distanced bees

A real skill when writing scientific papers 1 is to give them a suitable title.

Choosing the title involves a combination of art and science.

It must look appealing … you want the viewer to become a reader.

Since it is always indexed by search engines you must make sure it includes suitable keywords or phrases.

It needs to be informative. At least sufficiently so that the ‘take home message’ is clear. Even if the viewer does not become a reader they should still remember the title and so know the gist of what the article concludes.

The art of good title writing goes beyond this though. To increase the appeal, if it includes humour, some sort of half-hidden pun or some clever word play, then all the better.

And there are some great examples out there:

  • You probably think this paper’s about you: narcissists’ perceptions of their personality and reputation by Erika Carlson et al. (2011) in Journal of personality and social psychology 101:185-201. doi:10.1037/a0023781
  • Fifty ways to love your lever: Myosin Motors by Steven Block (1996) in Cell 87:151-157

There’s another variant of the latter and a host of additional variously funny or insensitive titles in this post on Slate. This also includes mention of the contrived efforts some scientists make to include Bob Dylan song titles in their publications (see Freewheelin’ scientists: citing Bob Dylan in the biomedical literature in the BMJ) as part of a long-running bet with colleagues.

Making it topical

Failing humour – and you could argue that some of the examples above 2 or linked are failing humour – a good way to get a paper some attention is to use a title that overtly hints at topicality.

In this regard, two papers caught my eye 3 this week:

The first of these is topical because travel restrictions to limit infectious disease transmission is a near-daily news item. However, it goes further than that in also including the Blofeld-like quote. The paper also has an entertaining abstract which finishes with the words We only live once, and sub-sections entitled The man with the golden gut: food safety and infections and The fly who loved me: arthropod-borne diseases. 

However, I’m not going to discuss the analysis of Bond’s hand-washing, potential Toxoplasmosis or the disturbingly high mortality rate of his sexual partners.

You’ve seen the film(s), now read the book paper 😉

Instead I’ll briefly focus on the second paper which managed to sneak ‘social distancing’ into the title, thereby ensuring it was picked up by almost every newspaper in the UK.

Socially distanced bees

‘Briefly’ because it’s a long paper and because rather too many of the figures are uninspiring bar charts like this one:

Spatial shift in allogrooming behaviour

… which, if you read the legend shows that there is almost no significant (ns) difference in allogrooming behaviour (which I’ll come to shortly) between Varroa-infested and -uninfested bees.

However, some of the graphs do have bars of different heights (and that are statistically significantly different) and there’s an interesting contradiction between studies conducted on full colonies and individual cohorts of bees.

So, rather than work through the entire paper I’m going to just focus on a few points and then discuss a couple of things that I found interesting.

Hypothesis driven science

Social insects, like ants and bees, are particularly at risk from pathogens and parasites. Their large populations, high density and ample food reserves means they have had to evolve both individual and social immunity.

The former prevents or mitigates infection of the individual, the latter reduces the chances that the colony will get infested (or restricts the impact of any infestation or infection to help ensure the survival of the colony).

The authors hypothesised that the presence of Varroa might induce some of these social immune responses. For example, bees might increase grooming activity in areas of the hive where Varroa were most frequent, or they might decrease antennation or trophallaxis with infested nest-mates, all to reduce the chance of mite transmission.

They focused on two particular aspects of social immunity and colony organisation, and made two predictions (hypotheses) for each:

  1. Space usage.
    1. Spatial shift of waggle dances to the periphery of the brood nest in infested colonies when compared with uninfested colonies.
    2. Spatial shift of grooming activity to the core of the colony in infested colonies when compared with uninfested colonies.
  2. Social behaviour.
    1. Infested bees would be expected to show changes in social behaviour including an increase in allogrooming, and decreases in antennation and trophallaxis.
    2. Changes in the structure of the social network in the infested hive, with decreases in connectivity and centrality.

Using colonies with high and low (almost negligible – I’ll return to this later) mite levels they then conducted observational science – they watched waggle dances, allogrooming etc. – to see if their predictions were correct.

Compartmentalisation of the colony 

When we open a hive all we often see is a mass of bees covering every frame.

Lots of bees

Beekeepers are often too busy trying to find the queen, or judge whether there are eggs or sufficient stores present, to appreciate that the bees are organised into two main ‘compartments’ within the colony:

  • an outer one occupied by foragers (the older bees) located nearer the hive entrance.
  • an inner one containing the young nurse bees and the queen, all of which are mainly arranged on brood.

The authors reasoned that since foragers represent a potential entry route of Varroa into the hive, you might expect the waggle dancing foragers to move the ‘dance floor’ to the periphery of the colony.

Does this make sense to you? To me it only really makes sense if you assume that the forager picks up a mite from elsewhere, for example when robbing a mite-infested collapsing colony elsewhere and returning to the hive. The alternative is that that forager was already carrying a mite, though I suppose that’s still a mite being introduced (or, more correctly, reintroduced) to the colony

Whatever the reason – and this wasn’t really elaborated – the changes in space usage and social behaviour would be expected to increase the compartmentalisation of infested colonies, so reducing mite spread.

Remember, mites predominantly associate with nurse bees and need to spend several days ‘surfing’ around the colony on these bees before entering a cell to reproduce.

Experimental details

Two month before the experiments started observation hives and other colonies were treated with dribbled oxalic acid. The colonies destined to be “Varroa-free” were then treated once a week for two further weeks with trickled oxalic acid.

Six weeks later, at the start of the observations, Varroa levels were strikingly different. The infested colonies were about ~6.2% and the “Varroa-free” uninfested colonies ~0.1%.

6% means six mites for every 100 bees sampled.

The team recorded the location of waggle dances and allogrooming in observation hives. Independently, using individually marked populations of caged bees, they recorded allogrooming, antennation and trophallaxis.

And, just so we all know what these terms mean:

  • allogrooming – is where one bee removes foreign particles and parasites from another bee
  • antennation – is how bees identify nestmates in the hive, by touching with the antenna
  • trophallaxis – is where one bee feeds another bee liquid food

Spatial shifts in waggle dancing and allogrooming

The colony is approximately spherical, sliced through by the vertically-hanging frames. The authors distinguished between the central frames and the lateral frames, and the position on the frames being closer or further away from the hive entrance 4.

In uninfested colonies the waggle dance and allogrooming activity occurred on both central and lateral frames, and predominantly on the lower half of the frame.

In contrast, infested colonies showed a significant shift of waggle dancing activity to lateral frames, and to positions closer to the hive entrance on these lateral frames. The allogrooming activity also shifted, but in the opposite direction, becoming concentrated on a larger area of the central frame.

These spatial changes were statistically significant and they should have the effect of keeping the forager and nurse bee populations better separated, and of concentrating the grooming activity to the centre of the colony.

Spatial organisation of nurse bees (yellow) and foragers (red) in mite-infested and uninfested colonies

Did the latter occur because that’s where most of the mites are located … hanging around waiting for a suitably-aged late stage larva to snuggle up with?

Or, does allogrooming become concentrated in the core because the nurse bees – which are responsible for most allogrooming activity – have relocated from other areas within the colony?

Or both? … these are not mutually exclusive.

The diagram above is my half-assed rather poor attempt to demonstrate the changes in compartmentalisation within the colony. In the colony on the left there is much more mixing and overlap between the nurse and forager bees. On the right there is much less mixing, and therefore less opportunities for mite transmission.

Social behaviour

The studies on social behaviour were somewhat less definitive, or produced unexpected results. These studies were all done using caged bees from infested or uninfested colonies. Allogrooming, antennation and trophallaxis can all be divided into ‘giving’ and ‘receiving’ activity, all of which was recorded, as was whether the bee from the infested colony was activity carrying a mite.

The expectation was that these activities – all of which are likely to increase the opportunities for mite transmission – might all be reduced in bees from Varroa-infested colonies, with one or two caveats.

In fact, in the majority of cases there were no significant differences between the levels of allogrooming, antennation and trophallaxis.

The exceptions included Varroa-parasitised bees which were – perhaps understandably – more likely to be the recipients of grooming.

Infested colonies overall exhibited slightly increased antennation, with Varroa-carrying bees receiving significantly more attention from cage-mates and – in turn – performing less antennation.

Finally, although there was no overall difference between trophallaxis between bees from infested and uninfested colonies, bees actively parasitised by Varroa received more trophallaxis … an unexpected result considering the potential for mite spread.

The final hypothesis that was tested was whether the social network changed in infested colonies. This was based upon analysis of high resolution videos of caged bees, recording the interactions between and then calculating the connectivity and centrality of the network.

I’m deliberately being brief in my description of the methodology here, for two reasons; 1) it’s complicated and would take 500 words to describe more fully, and 2) there were no differences in the measured parameters of the social network in the infested bees when compared with the bees from the uninfested colonies.


Looking back at the predictions (see above) it seems clear that there were large scale changes in space usage within the colony … perhaps justifying the phrase ‘social distancing’ in the title.

However, when the authors looked at individual cohorts of bees they did not detect evidence of increased small scale separation – either within the social network they formed, or in terms of avoiding activities that would be expected to lead to mite transmission.

In fact, the caged bees showed increases in activities that were commensurate with ‘care giving’ … increased grooming and trophallaxis of Varroa-carrying individuals.

These appear to be contradictory observations.

How can the large scale spatial reorganisation occur without changes in the bee-to-bee interaction that occurs at a smaller scale?

The authors skirt around this a little, but don’t really tackle it head on.

Loose ends

I think a couple of things warrant further investigation.

The large scale spatial reorganisation was of activities (dancing and grooming) not of bees, though there was an unwritten assumption that the activities were observed to move because they were conducted by particular ages of bees (which did move).

That could be tested by high resolution video observations of a colony containing marked cohorts of nurse bees and foragers. The expectation would be that – like the red and yellow circles I’ve drawn above – you would expect to see a more distinct separation of the two groups.

With sufficient time, money and video recording you could also use this in place of the studies of small cohorts of caged bees. For example, using lots of bar coded bees. Perhaps these don’t perform in the same way outside the hive as inside it?

Oxalic acid treatment

The authors used oxalic acid to reduce mite levels in the “Varroa-free” hives.

Unusually – at least in my experience – they used three weekly treatments of trickled oxalic acid.

This seems to have been very effective in reducing mite levels – compare the 3 x treated (0.1% infestation) to the 1 x treated (>6% infestation) – five to eight weeks respectively after the treatment started.

I was surprised it was that effective in a colony that was activity rearing brood, where the majority of the mites would be hidden in capped cells.

However, there are numerous studies that show that trickled/dribbled oxalic acid damages open brood 5. Therefore, in the studies conducted in this social distancing paper there’s a possibility that an entire generation of brood were missing due to the three successive treatments with trickled oxalic acid.

How this would have affected the results is unclear.

Although bees display temporal polyethism they also exhibit developmental plasticity and can change roles if and when needed. This doesn’t appear to have been considered and is certainly not discussed in the paper.

How is social distancing achieved?

But, let’s take their clever and topical title at face value and accept that bees do socially distance in response to mite infestation 6.

What level of mite infestation is needed to initiate this activity?

What are the molecular (chemical) or behavioural signals that trigger this activity?

Can we, as beekeepers, exploit them to improve the efficacy of rational mite management?

All of which will involve wild speculation and precious few hard facts, so I’ll save it for another time 😉


Cut your losses

The stats for winter losses in the UK, Europe and USA can make for rather sobering reading.

In the UK, losses over the last 12 years have fluctuated between 9% and 34%. This self-selecting survey includes responses from about 10% of the British Beekeepers Association membership (primarily England and Wales, despite the name). The average number of hives maintained by a BBKA member is about 5, meaning – all other things being equal 1 – that most beekeepers should expect to lose about 1 hive every winter.

BBKA winter losses survey

About 30 countries, mainly Northern hemisphere, contribute to the COLOSS survey which is significantly larger scale. The most recent 2 data published (for the ’16/’17 winter) had data from ~15,000 respondents 3 managing over 400,000 hives. Of these, ~21% were lost for a variety of reasons. COLOSS data is presented as an unwieldy table, rather than graphically. Further details, including recently published results, are linked from their website.

In the USA the Bee Informed Partnership surveys losses – both winter and summer – and claims to have results that cover ~10% of all the colonies in the country (so probably between 250,000 and 275,000 hives). Winter losses in the USA are rarely reported at less than 20% and were as high as 35% in the ’18/’19 winter 4.

Bee Informed Partnership annual colony losses

Are these figures to be trusted?

Who knows?

Each survey is accompanied by a variety of statistics. However, since they all appear to be based upon voluntary reporting by a subset of beekeepers, there are opportunities for all sorts of data to be included (and even more to be missed entirely). 

The problem with surveys

Is the successful beekeeper who managed to get all her colonies through the winter more likely to respond?

A form of ‘bragging rights’.

What about the beekeeper that lost all his colonies?

Does he respond out of a sense of responsibility?

Or does he keep quiet because he doesn’t want to be reminded of those cold, quiet, mouldy boxes opened on the first warm day of spring?

One and two year beekeepers

What about the high level of annual ‘churn’ amongst beekeepers? They buy a nuc in May, filled with enthusiasm about the jars of golden honey they’ll have for family and friends in late summer.

To say nothing of all the “saving the bees” they’ll be doing.

But by late summer the colony is queenless and has an unpleasant temperament

Beekeeping should be enjoyable ...

Beekeeping should be enjoyable …

Psychopathic you might say … if you were feeling uncharitable.

Consequently the Varroa treatment goes on far too late,. Or is quietly forgotten. The winter bees have high viral loads and ‘die like flies’ 5, resulting in the colony succumbing by the year end.

But this colony loss is never recorded on any surveys.

The once enthusiastic beekeeper has moved on and is now passionate about growing prize-winning vegetables or cheesemaking or keeping chickens. 

Beekeeping associations train lots of new beekeepers and – although membership numbers are increasing – it’s well below the rate they’re trained at.

Some may not be ‘joiners’ and go their own way.

Many just quietly stop after a year or two.

How many people have you met that say “Oh yes, I used to keep bees”

Did you ask them whether they ever completed a winter losses survey?

I’m not sure any of the surveys listed above do much ‘groundtruthing’ to establish whether the data they collect is truly representative of the population actually surveyed. With large numbers of respondents spread across a wide geographic and climatic range it’s not an easy thing to do.

So, treat these surveys with a healthy degree of scepticism.

Undoubtedly there are high levels of winter losses – at least sometimes – and the overall level of losses varies from year to year.

Losses and costs

The direct financial cost of these colony losses to beekeepers is very high.

Ignoring time invested and ‘consumables’ like food, miticides and foundation these costs in ’16/’17 for just Austria, the Czech Republic and Macedonia were estimated at €56 million 😯  

These figures simply reflect lost honey production and the value of the lost colonies. They do not include the indirect costs resulting from lost pollination.

But, for the small scale beekeeper, these economic losses are irrelevant.

Most of these beekeepers do not rely on bees for their income.

The real cost is emotional 🙁

It still saddens me when I lose a colony, particularly when I think that the loss was avoidable or due to my incompetence, carelessness or stupidity 6.

Little snow, big snow. Big snow, little snow.

Your hives should be quiet in winter, but it hurts when they are silent in spring.

Anatomy of a death

The COLOSS surveys give a breakdown of winter losses in three categories:

  • natural disasters
  • queen problems
  • dead colonies

Natural disasters are things like bears, honey badgers, flooding or falling trees.

We can probably safely ignore honey badgers in the UK, but climate change is increasing the weather extremes that causes flooding and falling trees.

Moving to higher ground ...

Moving to higher ground …

Don’t assume that poly hives are the answer to potential flooding.

They do float, though not necessarily the right way up 🙁

Queen problems cover a variety of issues ranging from reduced fecundity to poor mating (and consequent drone laying) to very early or late – and failed – supersedure 7.

Beekeepers with a lot more experience than me report that queen problems are increasing.

Drone laying queen ...

Drone laying queen …

Perhaps the issues with fecundity and drone laying are related to toxic levels of miticides in commercial foundation? It’s certainly known that these residues reduce drone sperm fertility significantly. I intend to return to this topic sometime during the approaching winter … perhaps in time to encourage the use of some foundationless frames for (fertile) drone production 😉

In the ’16/’17 COLOSS data, natural disasters accounted for 1.6% of all overwintered colonies (so ~7.5% of losses), queen problems resulted in the loss of 5.1% of colonies (i.e. ~24% of losses) and the remainder (14.1% of colonies, ~68% of losses) just died.

Just died?

We’ll return to natural disasters (but not bears or honey badgers) and queen problems shortly. What about the majority of losses in which the colony ‘just died’?

If you discuss colony post-mortems with beekeepers they sometimes divide the ‘just died’ category (i.e. those not readily attributable to failed queens, marauding grizzlies or tsunamis) into four groups:

  • disease
  • isolation starvation
  • starvation
  • don’t know 

The most important disease associated with overwintering colony losses is high levels of Deformed wing virus (DWV). This results from uncontrolled or inadequately controlled Varroa infestation. For any new readers of this site, please refer back to many of the articles I’ve already written on Varroa management 8.

I strongly suspect that a significant proportion of the reported isolation starvation is actually also due to disease, rather than isolation per se.

A consequence of high levels of DWV is that winter bees die prematurely. Consequently, the colony shrinks faster than it would otherwise do. It starts the size of a basketball but (too) rapidly ends up the size of a grapefruit … or an orange.

Isolation starvation and disease

The small cluster is then unable to remain in contact with stores, and so starves. 

Yes, the colony died from ‘isolation starvation’, but the cause was the high levels of Varroa and the viruses it transmits.

Isolation starvation ...

Isolation starvation …

What about regular starvation?

Not because the cluster became isolated from the stores, but simply because they had insufficient stores to get through the winter.

Whose fault was that?

And the last category, the “don’t knows”?

I bet most of these are due to high levels of Varroa and DWV as well 🙁

Yes, there will be other reasons … but probably not a huge number. 

What’s more … if you don’t know the reason for the colony loss there’s very little you can do to mitigate against it in future seasons.

And, other than wild and increasingly vague speculation, there’s little I can write about if the reason for the loss remains unknown 9.

Avoiding winter losses

So, let’s rationalise those earlier lists into the probable (known) major causes of overwintering colony losses:

  • natural disasters
  • queen problems
  • starvation
  • disease (but probably mainly DWV and Varroa

As the long, hot days of summer gradually shorten and cool as early autumn approaches, you should be thinking about each of these potential causes of overwintering colony loss … and doing what you can to ensure it doesn’t happen to you (or, more correctly, your bees).

Ardnamurchan autumn

Ardnamurchan autumn

Some are easier to deal with than others.

Here’s a whistle-stop tour of some more specific problems and some practical solutions 10. Some, all or none may apply to your bees – it depends upon your location, your climate, your experience and future plans as a beekeeper. 

Natural disasters

These fall into two broad groups:

  • things you can do almost nothing about (but might be able to avoid)
  • things you can relatively easily solve

Flooding, falling trees, lightning, landslides, earthquakes, volcanoes, meteor strikes etc. all fall into the first group.

If you can avoid them, do. 

Your local council will have information on areas at risk from flooding. There are also searchable maps available from SEPA. Do not underestimate the severity of some of the recent flooding. Some parts of Scotland and Northern England had 600 mm of rain in two days in 2015.

You might be surprised (and from an insurance aspect, devastated) at the classification of some areas now ‘at risk’. 

Where did Noah keep his bees? In his Ark hive.

Where did Noah keep his bees? In his Ark hive.

Consider moving hives to higher ground before the winter rains start. One consequence of climate change is that heavy rainfall is now ~20% heavier than it was a few decades ago. This means that floods occur more frequently, are more extensive and the water levels rise faster. You might not have a chance to move the hives if flooding does occur,

More rain and stronger winds (particularly before leaf fall) mean more trees will come down. You might be able to identify trees potentially at risk from falling. It makes sense to remove them (or site your hives elsewhere). 

No risk of this larch tree falling on my hives

Lightning, earthquakes, volcanoes, meteor strikes … all a possibility though I would 11 probably worry about Varroa and woodpeckers first 😉

Solvable natural disasters

The ‘solvable’ natural disasters include preventing your colonies being robbed by other bees or wasps. Or ransacked by mice or woodpeckers after the first hard frosts start. A solution to many of these are ‘reduced size entrances’ which either enable the colony to better defend itself, or physically restricts access to critters.

The L-shaped ‘kewl floors‘ I use prevent mice from accessing the brood box. They are also easier for the colony to defend from bees/wasps, but can also easily be reduced in size with a narrow piece of hardwood. If you don’t use these types of floor you should probably use a mouseguard.

Polyhives and polythene

Polyhives and polythene …

Woodpeckers 12 need to cling onto the outside of the hive to hammer their way through the side. You can either place a wire mesh cage around the hive, or wrap the box in something like damp proof membrane (or polythene) to prevent them gaining purchase on the side walls.

Keep off Woody

Keep off Woody

Doing both is probably overkill though 😉

Strong colonies

Before we move onto queen problems – though it is related – it’s worth emphasising that an even better solution to prevent robbing by bees or wasps is to maintain really strong colonies.

Strong colonies with a well balanced population of bees can almost always defend themselves successfully against wasps and robbing bees.

Nucs, that are both weaker and – at least shortly after being made up – unbalanced, are far less able to defend themselves and need some sort of access restriction.

By ‘balanced’ I mean that the numbers and proportions of bees fulfilling the various roles in the nucleus colony are reflective of a full hive e.g. nurse bees, foragers, guard bees. 

Reduced entrance ...

Reduced entrance …

But the benefits of strong colonies are far greater than just being able to prevent wasps or robbing bees. There is compelling scientific evidence that strong colonies overwinter better

I don’t mean strong summer colonies, I mean colonies that are strong in the late autumn when they are fully populated with the winter bees.

Almost the entire complement of bees in the hive are replaced between late summer and late autumn. Remember that a really strong summer colony may not be strong in the winter if Varroa and virus levels have not been controlled.

How do you ensure your colonies are strong?

  1. Minimise disease by controlling Varroa levels in early autumn to guarantee the all-important winter bees are reared without being exposed to high levels of DWV.
  2. Try and use a miticide treatment that does not reduce the laying rate of the queen.
  3. Avoid blocking the brood nest with stores where the queen should be laying eggs.
  4. Requeen your colonies regularly. Young queens lay more eggs later into the autumn. As a consequence the colonies have increased populations of winter bees.
  5. Unite weak colonies (assuming they are disease-free) with stronger colonies. The former may well not survive anyway, and the latter will have a better chance of surviving if it is even stronger – see below. 
  6. Use local bees. There’s good evidence that local bees (i.e. reared locally, not imported from elsewhere) overwinter better, not least because they produce stronger colonies.

Uniting – take your losses in the autumn

My regular colony inspections every 7-10 days during May and June are pretty much abandoned by July. The risk of swarming is very much reduced after the ‘June gap’ in my experience. 

I still check the colonies periodically and I’m usually still rearing queens. However, the rigour with which I check for queen cells is much reduced. By July my colonies are usually committed to single-mindedly filling the supers with summer nectar.

They are already making their own preparations for the long winter ahead.

Although the inspections are less rigorous, I do keep a careful watch on the strength of each colony. Often this is directly related to the number of supers I’ve had to pile on top.

Colonies that are underperforming, and – more specifically – understrength are almost always united with a stronger colony.

An Abelo/Swienty hybrid hive ...

An Abelo/Swienty hybrid hive …

Experience has taught me that an understrength colony is usually more trouble than it’s worth. If it’s disease-free it may well overwinter reasonably well. However, it’s likely to start brood rearing more slowly and build up less well. It may also need more mollycoddling 13 in the autumn e.g. protection from wasps or robbing bees.

However, a colony that is not flourishing in the summer is much more likely to struggle and fail during the winter. Perhaps the queen is not quite ‘firing on all cylinders’ and laying at a really good rate, or she might be poorly mated.

Far better that the workforce contributes to strengthening another hive, rather than collect an underwhelming amount of honey before entering the winter and eventually becoming a statistic.

My winter losses are low and, over the last decade, reducing.

That’s partly because my Varroa management is reasonably thorough.

However, it’s probably mainly due to ensuring only strong colonies go into the winter in the first place.


I’ve dealt with uniting in several previous posts.

It’s a two minute job. 

You remove the queen from the weak colony, stack one brood box over the other separated by a sheet or two of newspaper with a very small (~3mm) hole in the middle. Add the roof and leave them to get on with things.

I don’t think it makes any difference whether the strong colony goes on the top or the bottom.

I place the colony I’m moving above the box I’m uniting it with. My – wildly unscientific – rationale being that the bees in the top box will have to negotiate the route to the hive entrance and, in doing so, will help them orientate to the new location faster 14.

If you unite colonies early or late in the day most foragers will be ‘at home’ so not too many bees will return to find their hive missing.

If there are supers on one or both hives you can separate them with newspaper as well. Alternatively, use a clearer the day before to empty the supers prior to uniting the colonies. You can then add back the supers you want and redistribute the remainder to other hives in the apiary.

Successful uniting ...

Successful uniting …

Don’t be in too much of a hurry to check for successful uniting.

Leave them a week. The last thing you want is for the queen to get killed in an unseemly melee caused by you disturbing them before they have properly settled.

Done properly, uniting is almost foolproof. I reckon over 95% of colonies I unite are successful.

That’s all folks … more on ‘Cutting your losses’ next week 🙂


At just over 3000 words this post got a bit out of control … I’ll deal with more significant queen problems, feeding colonies, the weather and some miscellaneous ‘odds and sods’ next week.

Irrational Varroa control

About a month ago I wrote a post on rational Varroa control. I define this as choosing a miticide appropriate for your colony 1 and environment, administering it properly and at the right time so providing the maximum benefit to your bees. The long term goal of rational Varroa control is reduced colony losses due to mite-transmitted viruses.

Primarily this means reduced overwintering losses due to Deformed wing virus.

I regularly give talks on this topic. In these I provide a few brief examples of the misuse of miticides. Some of these examples originate – anonymised – in questions I’ve received in previous talks or by email. Some are from surveys of miticide usage.

Winter is coming – miticide treatments and fondant on

A few (a very few) are from direct personal experience 🙁

I’m aware that these examples qualify as ‘misuse’ because I’ve read up about the active ingredient in miticides and have a reasonable understanding of how, when and why they work.

Misuse could probably be broken down into three divisions:

  • Incorrect usage that does not reduce the efficacy of the treatment. 
  • Using the miticide in a manner that significantly reduces its efficacy, but otherwise does little or no harm.
  • Incorrect usage that has no impact upon the mite population and/or that significantly harms the colony and/or spoils the honey.

After a short 2 preamble, I’ll outline some of the more glaring examples. Keep these in mind when planning your mite control strategy and you should see improved winter survival, better spring build up, stronger summer colonies … and more honey 🙂

What can I use?

The Veterinary Medicines Directorate maintain a list of miticides approved for use on honey bees (and other animals). It’s a large database and I find the easiest way to see the current product list – which changes quite regularly as things are added and removed – is to use the search facility.

VMD database search … bees, budgies and bearded dragons

Simply select ‘Bees’ from species list (they’re between ‘Bearded dragons’ and ‘Budgies’!) and then hit the big Run search button at the bottom.

There are currently 31 approved products though this contains a number of duplicates as Great Britain and Northern Ireland are listed as separate territories.

In my view, the two most important useful columns in the returned table are the ‘Active substance‘ and the ‘Aligned product‘. 

The active substance is the chemical in the miticide that is responsible for killing mites. 

Look down the list. There are relatively few different active substances, present alone or in combination.

I currently count just five.

And then there were five … VMD approved miticides classified by active ingredient

Thymol, amitraz, pyrethroids, formic acid and oxalic acid.

I’m ignoring the minor components like eucalyptus oil that, individually, do not have high levels of miticidal activity.

Flumethrin and tau-fluvalinate (the active ingredients of Bayvarol and Apistan respectively) are both pyrethroids and have the same mode of action (and, more importantly, resistance to one usually confers resistance to the other).

What about management methods to control mites?

The VMD database lists the miticidal products approved for treating honey bees. 

They don’t list management techniques like drone brood uncapping or small cell foundation, or the application of non-toxic compounds like dusting with icing sugar.

There’s nothing to stop you using these approaches.

However, first conduct some sort of cost-benefit analysis to determine if they are worthwhile.

Is the disruption to the colony, or the potential tainting of honey stores, justified by the reduction of Varroa numbers?

For example, Randy Oliver has done some analysis of the impact of sugar dusting and shows that – at best and with weekly applications – it might be able to hold Varroa levels steady. You can download his Excel calculations and play with some of the assumptions.

If you increase the percentage of mites capped in cells from 60% (which is probably rather conservative) and decrease the percentage of mites dislodged by sugar dusting from 40% (which is probably rather aggressive) then mite replication rapidly outruns the control method applied.

That’s a questionable ‘benefit’ in return for the disruption of blowing 120 g of icing sugar into the hive every 7 days 3 … but there’s nothing to stop you using it as a control method.

Though it might not do much controlling … 🙁

I choose miticides that kill at least 90% of mites when used properly. I prefer to use miticides once or twice during the season, rather than dabbling every other week or month.

What can’t or shouldn’t I use?

You cannot use things like fenpyroximate, spirotetramat or spirodiclofen.


These are used for mite or tick treatment of other animals or plants 4. They have been shown to be effective against Varroa (though at high levels they may also kill bees) but are not approved for use. 

Remember also that VMD approval involves both the compound and the mode by which it is administered.

Amitraz is approved for use as the active ingredient in Apivar strips. However, in Great Britain and Northern Ireland you cannot fumigate colonies with amitraz … which, as Apiwarol, is a popular treatment method in Poland.

There are some (perhaps surprising) ‘restrictions’ in terms of approved usage. Api-Bioxal can be used to trickle treat twice per season, but only used once for vaporisation.


This is one of the many oddities buried in the depths of the VMD database.

Api-Bioxal is not approved for spray administration, but Oxuvar is. Both have the same active ingredient. 

No wonder beekeepers find this confusing … 🙁

Read the instructions and other documentation

Miticides approved by the VMD come with documentation. This takes the form of the instructions written (often in really tiny print) on the packet. The other thing written on the packet is the ‘use by’ date. 

Apivar instructions – duration of treatment

Keep the packaging 🙂

Read the instructions for use.

It may seem like an obvious thing to suggest but at least a third of the questions I get asked are answered in the documentation.

Part of the problem is the wording that’s often used or the apparent (and sometimes real) contradictions between the instructions provided for two miticides that have the same active ingredient and formulation 5

Api-Bioxal additional documentation

For more legible documentation you could refer again to the VMD database of approved miticides. If you following the Aligned product’ link you will have access to several additional pieces of information, most useful of which are the Summary of product characteristics and the Product literature. The latter is a copy of the literature in a very easy-to-read format, without any fancy colours or tiny fonts.

Keep records

You should keep records of when you treat and what you treat with, including batch numbers. I simply keep the packet the miticide was supplied in. I know when I treated as my copious 6 notes record the dates … and allow me to work out when the period of treatment is finished.

Apivar (but the batch number is on the back of the packet)

Even easier … just take a photograph of the empty packet, but make sure you include the batch number in the shot.

Examples of miticide misuse

It’s not possible to provide a comprehensive set of examples of miticide misuse (or Irrational Varroa control) … at least not in ~2000 words 😉 – after all, there might be one or two ways to use a miticide properly, but thousands of ways it could be used improperly.

Here is a non-exclusive and far-from-comprehensive list of miticide misuse or bad practice. 

Don’t do this at home … or in the out apiary 😉

Too little

Use the correct dose. Using less than the recommended dose ensures that some mites will survive. This may lead to the development of mite resistance.

Some beekeepers have been known to add a single strip of Apivar to colonies in preparation for going to the heather moors.

This is bad practice.

Even worse … some have been found leaving the strip in the hive while they were at the heather. Is that forgetfulness or just reckless? The honey will be tainted  and the long-term exposure to low levels of amitraz may contribute to the development of resistance 7.

Too late

Remember that the goal of the late summer treatment is to prevent the developing winter bees from being exposed to Varroa and the viruses that the mite transmits.

If you treat too late in the season you may well kill lots of mites, but the winter bees will already have been exposed.

Mite levels (solid lines) when treated July to November and timing of winter bee production.

In the graph above, treating in mid-October will kill significantly more mites than treating in mid-August.

But that’s not really the goal.

An October treatment will kill more mites because they’ve been breeding like rabbits throughout September.

And, what have they been reproducing on? Your developing winter bee pupae 🙁

Winter bee production is not an all or nothing event. The colony does not switch from producing summer bees to winter bees on a particular date. As late summer segues into early autumn an increasing proportion of the developing brood will be winter bees.

It’s your responsibility to ensure that enough of them are protected from Varroa so that they can lead a long and protective life, getting the colony through until February or March next year.

Too much

Many miticides are reasonably well tolerated by bees. Nevertheless, overdosing is also to be avoided. If you read the product characteristics for Api-Bioxal for example it states that:

Significantly higher bee mortality was observed in hives that received double (by sublimation) or triple (by trickling) dosages of product. In addition, when overdosed, the over-wintering  capacity of colonies was diminished and there may be detrimental effects on colony development in the future 8.

Remember that nucleus colonies are smaller than full sized colonies. It’s not unusual for a  beekeeper to administer a full dose to what is essentially a half-sized colony. 

In the case of Amitraz, ‘overdosing’ is well tolerated and two strips in a nucleus colony is unlikely to do the bees any harm.

However, the same cannot be said of MAQS. The product characteristics for MAQS specifically state that it should not be used for colonies with less than 6 frames of brood.

Too high

The formic acid-containing MAQS is poorly tolerated by the colony at high ambient temperatures. The literature suggests it should not be used when the peak daily temperature might exceed 29.5°C.

I’ve never used MAQS. I’m told by other beekeepers that they’ve had problems with queen losses at temperatures as ‘low’ as 25°C.

Remember, when you add the MAQS strips they need to be in the hive for 7 days. You therefore need to check the forecast for the week ahead before starting treatment.

Too low

Alison Gray and Magnus Peterson (both at Strathclyde University) have conducted surveys of Scottish beekeepers for about the last 15 years, summaries of which appear annually in The Scottish Beekeeper. Results are also collated into the COLOSS analysis. 

The surveys have evolved over the years, but have included questions on the type and timing of treatment.

I read some of these carefully when I returned to live in Scotland (in 2015). I was interested to see what other beekeepers were using. One thing that surprised me was the amount and timing of thymol (Apiguard) treatment.

For example, in the 2014 report (PDF), ~25% of treatments used were thymol, with 60% of these being applied in September, October and November.

A quick check of the Apiguard Product characteristics turns up the following statement:

Do not use the product when the maximum daily temperature expected during the treatment is lower than 15°C or when the colony activity is very low or when temperature is above 40°C.

I don’t know anywhere in Scotland in where the maximum daily temperature exceeds 15°C for four weeks (the duration of treatment) between September and November. 

I checked a personal weather station a couple of miles from my apiaries in Fife … in October 2013 the average temperature was 11.5°C, but the maximum failed to reach 15°C on 10 days through the month.

Inevitably, the efficacy of treatment would be reduced.

West coast temperatures 26/7/21 to 26/8/21 … 7 days fail to reach 15°C making Apiguard a very poor choice here.

You need good long range weather forecasting skills. Prior experience of what might be expected can really help your planning in these circumstances. 

Too long

Do not leave Apivar strips in the colony longer than stated in the instructions. 

How long is that?

Again, from the Product characteristics documentation:

If brood is not present or at its lowest level, the strips can be removed after 6 weeks of treatment. If brood is present, leave the strips in place for 10 weeks and remove the strips at the end of treatment.

Strips added to a colony with a young queen (and therefore laying well, and late into the year) on the day this article appears 9 should probably be removed in the first week of November.

Used and removed Apivar strips

That’s not a great time of year to be lifting roofs and prising up crownboards.

However, the alternative is worse. If you leave the strips in situ you ensure that any surviving mites (and there will be some) are continuously exposed to a low level of amitraz … perfect conditions to help select for resistance 🙁

Too short

This is much the same as too little (see above). If you remove the miticide before the correct period of time has elapsed then some mites will escape treatment.

No miticide is 100% effective. However, why store up problems for the future by unnecessarily reducing the efficacy of the treatment?

Remember … the only good mite is a dead mite

Too old

Miticides are not inexpensive. It is therefore very tempting to save and re-use opened packets. 

For example, beekeepers with just a couple of hives still have to purchase a packet of Apivar 10 for an eye-watering £31. It must be very tempting to tuck the unused portion away in the shed for the next time.

And, when they do, how many do a before and after count of phoretic mites to confirm that the treatment worked?

I suspect very few. 

If they did they might be in for a bit of a disappointment.

Amitraz, the active ingredient in Apivar strips, is quite unstable and rapidly degrades. I’ve used strips from previously opened packets and observed that they were significantly less effective 11.

What I should have done was read the Product characteristics documentation where it clearly states:

Shelf life after first opening the immediate packaging: use immediately and discard
any unused product.

Similar problems (short or non-existent shelf life) apply to some of the oxalic acid-containing solutions, in this case because they degrade to product hydroxymethylfurfural which is toxic to bees at high concentrations (see the notes on this at the end of the post of preparing Api-Bioxal for trickle treating).

One and two hive owners should coordinate their purchases and treatment of colonies to avoid wasting money.

Coordinated treatment has additional benefits, which neatly takes me to the final topic … 

Too few

OK, these titles are getting a bit contrived now, but this is the last one 😉

Treat all the colonies in the apiary simultaneously. I’ve written extensively about drifting and robbing. Both activities redistribute adult bees and the mites piggybacking on them around the apiary (or the wider environment).

If you treat just one colony it will soon 12 become reinfested with mites from neighbouring colonies.

If you don’t treat one colony and it develops high mite levels in autumn, perhaps due to a late surge in brood rearing, it will shed mites (hitching a ride on young bees going on orientation flights) to your adjacent treated colonies.

A final note on Apistan and pyrethroid-containing miticides

Resistance to Apistan is widespread. It’s so widespread that the National Bee Unit apparently stopped keeping records a decade or so ago.

Apistan is a very effective miticide … against mites that are sensitive. 

If you intend to use Apistan (or any of the approved pyrethroid-containing miticides) I would strongly suggest determining the level of infestation before treatment, and confirming a 90+% reduction in mite levels after treatment.

Unfortunately, just counting the dead mites on the Varroa tray is not evidence that the treatment has been effective.

If you find 1000 dead mites on the tray it just tells you is that you have 1000 mites less in the colony.

There may still be 9000 left if the treatment was only 10% effective … 

That’s not going to end well 🙁


Rational Varroa control

It’s the end of July … in the next two to three weeks the first eggs will be laid that will develop into the winter bees that get your colonies through to next spring. Protecting these winter bees is necessary to prevent overwintering colony losses.

I’ve written and lectured extensively on Varroa control and related topics for at least 5 years. The following article is published in August’s BBKA Newsletter and The Scottish Beekeeper. It provides an overview of what I term rational Varroa control.

I define this as effective mite management based upon our current understanding of the biology of bees and Varroa. The goal of this control is to minimise winter losses due to Varroa and viruses.

It is not a recipe with easy to follow if this, then that instructions. Neither does it provide a calendar-based guide of what to do and when to do it.

It does not even tell you what you should use for mite control.

Instead it focuses on the principles … understanding these will enable you to implement control strategies that help your bees, in your environment, survive.

This version is hyperlinked to additional, more expansive, posts on particular topics, is slightly better illustrated than those that appeared in print and contains some additional footnotes with caveats and exceptions.


Despite almost 30 years experience of Varroa in the UK, this ectoparasitic mite of honey bees remains the greatest threat to bees and beekeeping. With the exception of those fortunate to live in mite-free regions, all beekeepers must manage the mite population in their hives or risk losing the colony to the viruses transmitted when Varroa feeds on developing pupae. 

Fortunately, Varroa control is relatively straightforward; there are a range of approved and effective miticides that – used appropriately – reduce mite infestation levels significantly. The key words in that last sentence are ‘approved’, ‘effective’ and ‘used appropriately’. In reality annual colony losses, primarily occurring in the winter, often exceed 20% (Figure 1) and may be significantly higher in long or harsh winters 1. Many of these losses are attributable to Varroa and viruses. It is therefore clear that many beekeepers are not successful in managing Varroa; either they are not treating at all, or they are treating inappropriately.

Figure 1. BBKA winter survival survey – larger studies (COLOSS and BIP) often show much higher losses

This article is primarily aimed at relatively inexperienced beekeepers, but may also help the more experienced who still suffer with high levels of winter losses. It emphasises the importance of two, correctly timed, appropriate miticide treatments per season that should ensure colony survival. It is not going to deal with treatments of questionable or minor efficacy. These include the use of small cell foundation, drone brood culling or sugar dusting. These may reduce mite levels, but insufficiently to benefit colony health. Nor will it discuss the use of any miticides (or application methods) that are not approved by the Veterinary Medicines Directorate. I will also not discuss treatment-free beekeeping, selection of mite-resistant bees or advanced colony manipulations like queen trapping. In my view any or all of these could or should be tried … but only once a beekeeper can routinely successfully overwinter colonies using strategies similar to those described here.

The problem

Varroa is an ectoparasitic mite that feeds on developing honey bee pupae. During feeding it transmits a range of honey bee viruses, the most important of which is deformed wing virus (DWV). DWV is present in honey bees in the absence of Varroa. In our studies, using sensitive PCR-based detection methods, we never detect bees – even those from mite-free regions of Scotland – without DWV. The virus is transmitted horizontally between bees during trophallaxis, and vertically from drones or the queen through sperm or eggs. These routes of transmission are rarely if ever associated with any significant levels of disease and virus only replicates to modest levels (perhaps 1-10 thousand viruses per bee). However, when Varroa transmits DWV the virus bypasses the bee’s natural defence mechanisms and replicates to very high levels in recipient pupae (billions per pupa, 1 – 10 million times higher than in unparasitised pupae). Studies from our laboratory have shown that ~75% of pupae with these high virus loads either do not emerge, or emerge exhibiting the characteristic “deformed wings” that give the virus its name (Figure 2; Gusachenko et al., Viruses 2020, 12, 532; doi:10.3390/v12050532). The ~25% of bees that do emerge and appear ‘normal’ exhibit a range of symptoms including reduced fitness, impaired learning and reduced foraging. However, most importantly they also exhibit reduced longevity. During the summer this is probably not critical; the lifespan of a worker is only ~6 weeks and, assuming the queen is laying well, there are thousands of half-sisters around with more being produced every day.

DWV symptoms

Figure 2. DWV symptoms

But during the winter, brood rearing either stops completely or drops to a very low level. The bees reared from late summer onwards are physiologically very different. These are the ‘winter bees’, also termed the diutinus bees (from the Latin meaning long-lived). Physiologically these bees resemble juvenile workers and they can survive for many months. And they need to … it is these bees that get the colony through the autumn, winter and into the following spring. They protect the queen, they thermoregulate the hive and, usually around the winter solstice, they start to rear small amounts of new brood for the season ahead.

The longevity of the bees in the hive in winter is critical to colony survival. If the winter bees have high DWV levels their longevity is reduced (in addition to the reduced numbers due to overt disease or non-viability). This means that the winter cluster shrinks in size faster than it would do otherwise. With reduced numbers of bees it cannot keep brood warm enough and so the colony fails to expand early the following season. In cold spells it may be unable to reach the food stores resulting in the colony perishing from ‘isolation starvation’. It may not be able to maintain sufficient warmth to protect the queen, or may simply freeze to death.

The goal of rational Varroa control

Successful overwintering requires lots of winter bees. The size of the winter cluster is directly related to its survival chances. Therefore the goal of rational Varroa control is to prevent the winter bees from being exposed to mites and mite-transmitted viruses during their development. Winter bee production is induced by a range of factors including photoperiod, nectar and pollen availability, brood and forager pheromones. Together these induce slowed behavioural maturation of the winter bees. This is not like flicking a switch, instead it is a seamless transition occurring as late summer segues into early autumn (Figure 3). Winter bee production is also influenced by the queen. Young queens lay later into the autumn, so increasing the numbers of winter bees. 

Figure 3. Colony age structure from August to December.

It is important to note that these events are environment-driven, not calendar-driven. It will not happen at precisely the same time each year, or at the same time in different locations (or latitudes) each year.

To protect these winter bees the colony needs to be treated with an effective miticide before the majority of the winter bees are produced. This ensures that the developing winter bee pupae are not parasitised by virus-laden mites and so do not suffer from reduced longevity. 

When are winter bees produced in the UK? 

Unfortunately, I’m not aware of any direct studies of this. Scientists in Bern (49.9°N) in 2007/08, where the average temperatures in November and December were ~3°C, showed that the Varroa- and virus-reduced longevity of bees was first measurable in mid-November, 50 days after emergence. By extrapolation, the eggs must have been laid in the first week of September. 

Doing large scale experiments of Varroa control is time-consuming and subject to the vagaries of the climate (and, as a molecular virologist, beyond me in terms of the resources needed). I have therefore used the well-established BEEHAVE program of colony development (from scientists in the University of Exeter; to model the numbers of developing and adult bees, and the mite numbers in a colony. BEEHAVE by default uses environmental parameters (climate and forage) based upon data from Rothamsted (51.8°N). Using results from this model system, the bees present in the hive at the end of December – by definition the diutinus winter bees – were produced from eggs laid from early/mid August (Figure 4).

Whatever the precise date – and it will vary from season to season as indicated above – at some point in September the adult bee population starts to be entirely replaced with winter bees. Large numbers of these need to live until the following February or March to ensure the colony survives and is able to build up again once the queen starts laying.

When to treat – late summer

The numbers of pupae and adult bees present in the colony are plotted in Figure 4 using dashed lines. Adult bee number decrease in early spring until new brood is reared. The influence of the ‘June gap’ on pupal numbers is obvious. Brood rearing gradually tails off from early July and stops altogether sometime in late October or early November. The shaded area represents the period of winter bee production – from early/mid August until brood rearing stops. 

Figure 4. Winter bee production and mite levels – see key and text for further details

Mite levels are indicated using solid lines. The impact on the mite population of treating in the middle of each month from July to November is shown (arrowed and labelled J, A, S, O and N) using the colours green, blue, red, cyan and black respectively. The absolute numbers of bees or mites is irrelevant, but bees (pupae and adults) are plotted on the left, and mites on the right hand axis, so they cannot be directly compared. The miticide treatment modelled was ‘applied’ for one month and was 95% effective, reproducing many licensed and approved products.

Mite levels peak in the colony in late September to October. If treatment does not occur until this time of the season then the majority of winter bees will have been reared in the presence of large amounts of mites. Unsurprisingly, the earlier the treatment is applied, the lower the mite levels during the period of winter bee production. 

Rational Varroa control therefore involves treatment soon after the summer blossom honey is removed from the hive, so maximising the winter bees produced in the presence of low mite numbers. If you leave treatment until mid-September, you risk exposing the majority of winter bees to high levels of Varroa in the hive. If your primary crop is heather honey, which is not harvested until September, you may need to consider treating earlier in the summer – for example during the brood break when requeening or during swarm control.

Why treat in midwinter?

A key point to notice from Figure 4 is that, paradoxically, the earlier the miticide is applied, the higher the mite levels are at the end of the year. Compare the August (blue) and October (cyan) lines at year end for example. This is because mites that survive treatment – and some always do – subsequently reproduce in the small amount of brood reared late in the season. This is what necessitates a ‘midwinter’ treatment. Without it, mite levels increase inexorably year upon year, and cannot be controlled by a single late-summer treatment. Beekeepers bragging on social media that their mite drop after the winter treatment was zero probably applied the summer treatment too late to effectively protect their winter bees.

And when is midwinter?

Historically beekeepers apply the ‘midwinter’ treatment between Christmas and New Year. This is probably too late. The usual miticide used at this time is oxalic acid, a ‘one shot’ treatment that is ineffective against mites in capped cells. For maximum efficacy this must be applied when the colony is broodless. Brood rearing usually starts (if it ends at all, again this is climate-dependent) around the winter solstice. By delaying treatment until a lull in the Christmas festivities or even early January some mites will already be inaccessible in capped cells. 

Figure 5. Biscuit coloured (or a bit darker) cappings indicating brood rearing in this colony

I check my colonies for brood – either by looking for biscuit-coloured cappings on the Varroa tray (Figure 5) or by quickly inspecting frames in the centre of the cluster – and usually treat in November or very early December. If I cannot check visually I apply the treatment during the first extended cold spell of the winter. By treating when the colony is broodless I can be certain my intervention will have maximal effect.

What to treat with?

I have deliberately avoided – other than mentioning oxalic acid – specific miticides. Rational Varroa control involves the choice of an appropriate miticide and its correct application. Examples of incorrect or inappropriate miticide choice include; use of Apistan when resistance is known to be very widespread, use of Apiguard when the average ambient temperature is below 15°C (which makes Apiguard of little use for effective control in much of Scotland) or the use of Api-Bioxal when there is capped brood present. In addition, use of a half-dose or a reduced period of application will both reduce efficacy and potentially lead to the selection of resistance in the mite population. Used correctly – the right dose at the right time and for the right duration – the majority of the currently licensed miticides are be capable of reducing mite levels by over 90%. If they do not, use one that does. Miticide choice should be dictated by your environment and the state of the colony.

All together now

Most beekeepers grossly underestimate the movement of bees (and their phoretic mites) between colonies. Numerous studies have shown that drifting and (to an even greater extent) robbing can result in the transfer of large numbers of mites from adjacent and, in the case of robbing, more distant colonies. 

Gaffer tape apiary

Figure 6. Gaffer tape apiary …

Rational Varroa control therefore involves treating all colonies within an apiary, and ideally the wider landscape, in a coordinated manner. In communal association apiaries (Figure 6), where beekeeping experience and therefore colony management and health can vary significantly, this is particularly important. Coordinated treatment is only relevant in late summer when bees are freely flying.


Swarms originating from unmanaged or poorly managed colonies will have high mite levels. The bee population in a swarm is biased towards younger bees; these are the bees that phoretic mites preferentially associate with. Studies have shown that ~35% of the mite population of a colony leaves with the swarm.

Figure 7. Varroa treatment of a new swarm in a bait hive…

Since swarms contain no sealed brood until ~9 days after they are hived oxalic acid is the most appropriate treatment. I usually treat them using vaporised oxalic acid late in the evening soon after they are hived (Figure 7). Even casts get this treatment and I have not experienced any issues with the queen not subsequently mating successfully. I’d prefer to have a queenless low-mite colony than a queenright one potentially riddled with Varroa.

Midseason mite treatment

The text above describes the mite management strategies I have used for several years. I apply Apivar immediately the summer honey is removed and treat with oxalic acid when broodless before the end of the year. Doing this has almost never required any additional midseason treatments; if mite levels are sufficiently low at the beginning of the season they cannot rise to dangerous levels before the late summer treatment. I still get winter colony losses, but they are almost always due to poor queen mating and rarely due to Varroa and viruses.

Figure 8. Queenright splits and the window(s) of opportunity

However, if midseason treatments are required – either because there are signs of overt infestation, because regular mite counts have shown there is a problem, or to have low mite colonies after the heather honey is collected – then there are two choices. Treat with MAQS which is approved for use when there are supers on the hive and, more importantly, is effective against mites in capped cells 2. Alternatively, treat during swarm control. With care, the majority of splits (e.g. the Pagden artificial swarm or the nucleus method) can be performed to give a broodless period for both the queenright (Figure 8) and queenless partitions. That being the case, a single application of an oxalic acid-containing miticide can be very effective in controlling the mite population.


Many beekeepers complain about the cost of licensed and approved miticides. However, some perspective is needed. A colony with low levels of mites will be more likely to survive overwinter, so reducing the costs of replacement bees. In addition, a healthy colony will be a stronger colony, and therefore much more likely to produce a good crop of honey (and potentially an additional nuc). Over the last 5-6 years my miticide costs are equivalent to one jar of honey per colony per year. This is an insignificant amount to pay for healthy colonies.


Rational Varroa control requires an understanding of the goals of treatment – protecting the winter bees and minimising mite levels for the beginning of the following season – and an appreciation of how this can best be achieved using miticides appropriate for the environment and the state of the colony. Like so much of beekeeping, it involves judgement of the colony and will vary from season to season and your location. I’ve applied my midwinter treatment as early as the end of October or as late as mid-December, reflecting variation in timing of the broodless period. Rational Varroa control also involves an understanding of the biology of bees and an awareness of the influence of beekeeping (e.g. crowding colonies in apiaries which increases mite and disease transmission) on our bees. However, none of this is difficult, expensive or time consuming … and the benefits in terms of strong, healthy, productive colonies are considerable.


A version of this post appeared in the BBKA Newsletter, August 2021

A version of this post appeared in The Scottish Beekeeper, August 2021

Superinfection exclusion





The majority of readers will identify these as the current circulating variant strains of Covid 1. The World Health Organisation decided upon this naming system as being easier and more practical to discussed (sic) by non-scientific audiences 2.

All viruses vary, and viruses with genomes made from ribonucleic acid (RNA) vary more than those which have deoxyribonucleic acid (DNA) genomes. This is because the enzymes that replicate RNA virus genomes do not have an error correction facility. 

The virus that causes Covid-19 is an RNA virus and so is Deformed wing virus (DWV), probably the most significant virus to bees and beekeepers (other than those who have Covid that is).

Virus variation

Why does virus variation matter? 

Or, asking the same question in a more roundabout way, why don’t RNA viruses evolve error correcting enzymes (after all, the DNA viruses have these … can it be that difficult?).

If the enzyme makes errors then that’s surely a bad thing?

Actually … for the particular ‘lifestyle’ that these viruses practice, errors and variation are a good thing.

They benefit the virus.

But you know all this already, even if you don’t think you do.

Covid cases caused by the delta variant

The SARS-Cov2 delta variant now accounts for at least 99% of cases of Covid-19 in the UK. It accounted for just 0.1% of cases in late February.

The delta variant is much more transmissible. It carries errors, or mutations as they’re more correctly known, that – for whatever reason – means it can be passed from person to person much more efficiently 3.

These mutations benefit the virus and allow it to spread further and faster 4.

If the mutations (‘errors’) that the virus acquires are beneficial – by increasing transmission, by expanding the cell, tissue or host range, by helping evade the immune response in a partially vaccinated population (!) for example – then the virus will successfully replicate and produce more viruses carrying the same mutation.

And a bunch of additional ones as well … acquired during the last round of replication.

Strains and types

At some point a virus acquires sufficient mutations from an earlier incarnation that it’s identified as a distinct strain.

For example, SARS-Cov2 is ~82% identical at the genome level to SARS-Cov1 which caused the SARS pandemic in 2003 5. SARS-Cov2 did not evolve from SARS-Cov1, but they share a common ancestor. They are different strains or types of coronavirus.

There are no hard and fast rules that define when a virus is considered a different strain or type. Often it’s historical, reflecting the geographic origin, or the source from which the virus was isolated. Different strains may exhibit different phenotypes – host range, transmission, disease etc. – but don’t have to.

For example (before we get back to honey bee viruses) there are three ‘types’ of poliovirus that are about 80% identical at the genetic level. They all cause exactly the same disease (poliomyelitis) and they replicate in an identical manner – same cell, tissue and host range for example. However, to the human immune system they ‘look’ different. The immune response to poliovirus type 1 will not protect you from infection with poliovirus type 3. That’s why the poliovirus vaccine contains a mixture of all three types, to protect you from all polioviruses.

You can even get infected with two types of poliovirus simultaneously, and the virus can replicate in the same cells in your gut … or, if you’re unfortunate, your brain.

I’ll return to dual infections shortly as it’s an important topic … and related to the study I’m going to discuss.

Deformed wing virus

There are two types of DWV, designated type A and type B 6.

Originally these had names that reflected their original isolation.

DWV type A was termed Deformed wing virus and was isolated from honey bees displaying the characteristic symptoms of developmental deformities shown in the image below.

DWV type B was termed Varroa destructor virus type 1 and was isolated from the ectoparasitic mite Varroa destructor.

It “does what is says on the tin” … DWV symptoms in a recently emerged worker

These viruses are very similar to each other. They are something like 85% identical at the level of the RNA genome. More importantly than this genetic identity (or perhaps similarity would be a better term to use here) is the fact that they appear to cause very similar diseases in honey bees.

Although early studies suggested there were some differences in their virulence, more recent work from Prof. Rob Paxton in Germany, from my lab, and from Dr. Eugene Ryabov and colleagues in the USA suggests these two types of DWV are actually very similar, at least in pathogenesis.

There do appear to be some differences, with the suggestion that type A does not replicate in Varroa whereas (full disclosure, the following study is from my lab) type B does. Undoubtedly, the other genetic differences between the types will confer some subtle variation in phenotype (effectively what they ‘do’), but – as far as beekeeping is concerned – they should probably be considered the same.

A protective, non-lethal type A DWV? 

All of which made a 2015 colony-level study 7 of DWV infection rather intriguing.

This reported the survival of colonies that were infected with a “non-lethal” type B strain which were protected from infection with the “lethal” type A strain 8. The authors summarised the significance of this study like this:

We propose that this novel stable host-pathogen relationship prevents the accumulation of lethal variants, suggesting that this interaction could be exploited for the development of an effective treatment that minimises colony losses in the future.

At the time there was a flurry of excitement and discussion about this 9

Superinfection exclusion

They proposed that the mechanism that prevented the infection with the type A strain was superinfection exclusion.

Virologists love mechanisms … 🙂 

Which brings me back to virus variation. 

Imagine a population of variant viruses trying to infect a new host … like a bee.

Survival of the fittest – selection for better replicating viruses from a mixed population

In mixed infections, a virus that has an advantage over the others in the population ends up ‘winning’ the competition for the resources of the host. They therefore make more progeny viruses.

One of the advantages could be that the virus simply replicates faster

Another – more subtle, but the same outcome – is that the virus prevents other viruses from infecting the same cell (and, by extension, host).

By excluding competing viruses it effectively ‘wins’ the competition.

Superinfection exclusion – one virus (type) can prevent infection by related but different viruses

And some viruses do exactly this using a variety of cellular mechanisms.

For example, many viruses turn off the expression of the cellular receptor (think of this as the door) they use to enter the cell. If there’s no receptor (no door) then another virus cannot enter.

With no other viruses to compete with in the same cell there can only be one type of virus produced 10.

There are other mechanisms as well, but we’ll stick with the receptor one as it’s easy to comprehend.

Superinfection of a cell containing a virus that has the ability to turn off the cellular receptor it uses will effectively exclude the second virus from replicating … hence superinfection exclusion.

What don’t we know about DWV?

A lot 🙁

We don’t know how it gets into cells. We don’t know a huge amount about its replication and we know precious little about the way it interacts with the cellular machinery (the ‘stuff’ in the cell that the virus hijacks to make more viruses) of the host. 

However, since 2015 we do know that all the types of DWV that have been carefully studied appear to be more or less equally virulent. None appear to be ‘non-lethal’ as claimed for the type A virus in the superinfection exclusion paper.

This prompted us to look in a bit more detail at the consequences of dual or sequential infections with DWV in the laboratory. 

Is there a precedence at work?

In mixed infections, does one virus always ‘win’ and predominate in the new virus population?

In sequential infections, does it matter the order in which the viruses are acquired?

And mixed infections are pretty much the norm for DWV infection. All bees, whether previously exposed to Varroa or in Varroa-free regions, appear to have low levels of DWV already present. If parasitised by the mite, these bees must experience a mixed infection.

In addition, studies we published several years ago 11 showed that recombinant viruses – essentially hybrids between type A and type B DWV – often predominated in heavily Varroa-infested colonies 12.

If mixed infections cannot occur, how do such hybrids form?

Some of the answers to these questions are in our recently paper published in the ISME Journal. Gusachenko, O. et al., (2021) First come, first served: superinfection exclusion in Deformed wing virus is dependent upon sequence identity and not the order of virus acquisition. ISME J (2021).

Mixed DWV infections

I don’t propose to give a pupa-by-pupa account of the studies we conducted. You can read the paper – it’s open access and (because Olesya ‘Alex’ Gusachenko, the lead author, did most of the writing) relatively easy to comprehend 😉

But here are a few highlights.

Over the last few years we have produced reagents that allow us to produce almost ‘pure’ stocks of type A, type B or hybrid type A/B 13 strains of DWV.

At least 99.99% of these stocks are of one DWV type. Note that there will still be variation within this population as the replication errors probably generate one mutation per virus in the population. We therefore refer to these virus stocks as near clonal.

Injection of honey bee pupae with any of these viruses resulted in very similar levels and kinetics of replication – all the viruses replicate as far and as fast as each other.

In mixed injections, when two viruses were administered simultaneously, both replicated to equivalent levels. 

We therefore found no evidence for the dominance of one strain over another.

Sequential DWV infections

But it got more interesting when we did sequential injections. We did these by injecting with one virus, waiting 24 hours and then injecting with a second virus.

Using type A and type B DWV both viruses had replicated to similar high levels (billions of virus per pupa) within 48 hours, irrespective of the order of addition. 

If superinfection exclusion was operating we would have perhaps expected type A to have prevented or reduced the replication of type B. However, that didn’t appear to be the case.

Competition between sequential infecting DWV isolates. VVV is type B, VDD is type A and VVD is a hybrid between them.

But, when we looked at sequential infections between type A and a type A/B hybrid we did see that replication of the second virus was delayed.

Delayed, but not stopped altogether.

It would take a complete post to describe the figure above 🙁 . We’ve quantified the virus present 5-7 days after sequential injections with type A (VDD), type B (VVV) or a hybrid virus (VVD) 14.

The columns labelled VDD→VVV or VVV→VDD show the viruses and order of addition. The dots represent the amount of virus present at 5 or 7 days post injection. When the viruses were more similar to each other – for example, the VVV→VVD or VVD→VVV pairs on the right – there was a greater impact on the replication of the virus added second.

The same but different

We extended these studies to look at sequential infections with two viruses that only differed by 4 nucleotides (the building blocks) of the 10,140 nucleotides in the RNA genome of DWV i.e. 99.6% identical.

Cunningly, these four differences allowed us to unambiguously identify which virus was replicating.

In this part of the study the virus added second did not replicate to detectable levels. 

So … our data clearly demonstrates that viruses that were more similar to each other were more likely to inhibit replication during sequential infections.

In addition, no individual virus type was dominant over any others. 

This didn’t look much like classic superinfection exclusion to us.

Red or green viruses

Not content with generating graphs and tables we went on to take photographs of virus infected pupae. 

You can’t beat a nice colour image when trying to impress the peer reviewers 😉 .

Remember that DWV is too small to see with even the most powerful light microscope. You could fit several billion on the head of a pin.

We therefore engineered the virus genome to ‘show’ us where it was replicating.

Green bees

We did this by introducing an additional gene that fluoresced green or red when under UV light. I’ve discussed green viruses before … the red version uses similar technology, but using a different fluorescent reporter gene.

DWV replication (showed by green fluorescent signal) in the head, wing and abdomen of honey bee pupae

Using the red or green viruses we showed very similar results to those described above. When we superinfected with a genetically similar virus, its replication was inhibited. When it was genetically more divergent it could replicate (and we could visualise it as red or green foci of infection in a variety of tissues of the developing pupa).

Red and green viruses

We also infected bee with the red and green viruses simultaneously. Most of the fluorescent foci of infection were red or green, but a small number were both red and green. 

Green (EGFP) and red (mCherry) expressing DWV coinfecting a honey bee pupa. Arrow indicates dually infected cells.

The most likely explanation for having both colours overlapping in the photograph was because the virus were replicating in the same cells in the honey bee pupa.

Since this was exactly the sort of situation that was needed to generate recombinants (hybrids or chimeras) between the two different DWV viruses we specifically looked for them 15.

And there were lots and lots of recombinants …

Recombination between DWV viruses. The size and position of ‘bubbles’ indicate the location and number of junctions.

The bubble plot above shows the location and frequency of junctions. One virus is plotted on the horizontal and one on the vertical axis. It’s a sort of two-dimensional map of the virus. Think of a junction as where one virus ends and the other starts. They are located throughout the DWV genome – hundreds of them.

This suggests that pupae infected with both type A and type B DWV will act as ‘factories’ for the production of thousands more different hybrid variants between the two viruses.

Most of these hybrids will grow poorly.

Many will be uncompetitive.

But some – like the delta variant of SARS-Cov2 – might be more transmissible.

And some could be more pathogenic.

Or – the nightmare scenario – both 😯 .

What’s this got to do with practical beekeeping?

Every time a beekeeper moves bees about s/he is also moving viruses about.

This happens when you move bees to out apiary, when selling a nuc or when importing a queen.

Double brood ...

Moving viruses (and bees) to a new apiary …

This will contribute to the constant mixing of DWV variants that occurs when bees drift between hives, when drones mate with queens, when phoretic Varroa jumps onto a bee that is robbing a collapsing colony.

There’s a difference of course.

All those bee-driven mixing events are local and small scale … a few bees and a few miles.

But if you import a nuc from Greece via Northern Ireland both the distance and number of bees (and hence number of viruses) is much greater.

Of course, most of this mixing will just generate more mixtures of viruses.

It will also generate more recombinants.

But there’s always the possibility it might throw up a highly virulent, highly transmissible variant.

Which would not be a ‘good thing’.

And if it does, a ‘non-lethal type A strain’ (should such a thing actually exist) is not going to help prevent infection by the mechanism of superinfection exclusion I’m afraid 🙁 .

Without doubt the best way to prevent infection is to minimise the mite numbers in your colonies. This is a subject I’ll be tackling again in a couple of weeks.

But, before I go, do we understand how the more closely related strains of DWV prevent superinfection? 

Yes … probably, and it’s all to do with the immune response of the bee

I’ll discuss this in the future as it’s a mechanism that could be exploited to produce bees immune to the ravages of DWV.


More from the fun guy

Great fleas have little fleas upon their backs to bite ’em,
And little fleas have lesser fleas, and so ad infinitum.

Augustus de Morgan’s quote from A Budget of Paradoxes (1872) 1 really means that everything is preyed upon by something, which in turn has something preying on it.

The Flea, engraving from Robert Hooke’s Micrographia (1665)

As a virologist I’m well aware of this.

There are viruses that parasitise every living thing.

Whales have viruses and so do unicellular diatoms. All the ~30,000 named bacteria have viruses. It’s likely that the remaining 95% of bacteria that are unnamed also have viruses.

There are even viruses that parasitise viruses. The huge Mimivirus that infects amoebae 2 is itself parasitised by a small virophage (a fancy name for a virus that infects viruses) termed Sputnik.

Whether these interactions are detrimental depends upon your perspective.

The host may suffer deleterious effects while the parasite flourishes.

It’s good for the latter, but not the former.

Whether these interactions are detrimental for humans 3 also depends upon your perspective.

The deliberate introduction of rabbit haemorrhagic disease virus to Australia benefitted sheep farmers who were plagued with rabbits … but it was bad news for rabbit farmers 4.


Beneficial parasitism, particularly when humans use a pathogen to control an unwanted pest, is often termed biocontrol, a convenient abbreviation for biological pest control.

There are numerous examples; one of the first and best known is control of greenhouse whitefly infestations with the parasitoid wasp Encarsia formosa.

Tomato leaf with whitefly nymphs (white) parasitized by E. formosa (black).

One of the benefits of biocontrol is its self-limiting nature. The wasp will stop replicating once it runs out of whitefly to parasitise.

A second benefit is the specificity of the interaction between the host and whatever is administered to control it; by careful selection of the biocontrol agent you can target what you want to eradicate without lots of collateral damage.

Finally, unlike toxic chemicals such as DDT, the parasitoid wasp – and, more generally, other biocontrol agents – do not accumulate in the environment and cause problems for the future.

And, with all those benefits, it’s unsurprising to discover that scientists have investigated biocontrol strategies to reduce Varroa mite infestation of honey bee colonies.

It’s too early for an aside, but I’ll make one anyway … I’ve discussed the potential antiviral activity of certain fungi a couple of years ago. That wasn’t really biocontrol. It was a fungal extract that appeared to show some activity against the virus. Although that story has gone a bit quiet, one of the authors – Paul Stamets – is also a co-author of the Varroa control paper discussed below.

Biocontrol of Varroa using entomopathogenic fungi

Entomopathogenic means insect killing 5. There are several studies on the use of insect killing fungi to control Varroa 6, with the most promising results obtained with a variety of species belonging to the genus Metarhizium

Metarhizium produces asexual spores termed mitospores. The miticidal activity is due to the adhesion of these mitospores to Varroa, germination of the spore and penetration by fungal hyphae 7 through the exoskeleton of the mite and proliferation within the internal tissues.

A gruesome end no doubt.

And thoroughly deserved 🙂

Although Metarhizium is entomopathogenic it has a much greater impact on Varroa than it does on honey bees. This is the specificity issue discussed earlier.

It is for this reason that scientists have continued to explore ways in which Metarhizium could be used for biocontrol of Varroa.

But there’s a problem …

Although dozens of strains of Metarhizium have been screened, the viability – and therefore activity – of the mitospores is significantly reduced by the relatively high temperatures within the colony.

The spores would be administered, they’d show some activity and some Varroa would be slaughtered. However, over time treatment efficacy would reduce as spores – either administered at the start of the study, or resulting from subsequent replication and sporulation of Metarhizium on Varroa – were inactivated.

As beekeepers you’ll be familiar with the limitation this would impose on effective control of mites.

Varroa spend well over half of their life cycle capped in a cell while it feeds on developing pupae. Anything added to kill mites must be present for extended periods to ensure emerging mites are also exposed and killed.

This is why Apiguard involves two sequential treatments of a fortnight each, or why Apivar strips must be left in a hive for more than 6 weeks.

In an attempt to overcome these limitations, scientists are using directed evolution and repetitive selection to derive strains of Metarhizium that are better able to survive within the hive, and so better able to control Varroa than the strains they were derived from.

Good news and bad news

Like many scientific papers on honey bees 8 those with even a whiff of ‘saving the bees’ get a lot of positive press coverage.

This often implies that the Varroa ‘problem’ is now almost solved, that whatever tiny, incremental advance is described in the paper represents a new paradigm in bee health.

This is both understandable and disappointing in equal measure.

It’s understandable because people (not just beekeepers) like bees. News publishers want ‘good news’ stories to intersperse with the usual never-ending menu of woe they serve up.

It’s disappointing because it’s a variant of “crying wolf”. We want the good news story to describe how the impact of Varroa can now be easily mitigated.

It gets our hopes up.

Unfortunately, reality suggests most of these ‘magic bullets’ are a decade away from any sort of commercial product.

They will probably get mired in licensing problems.

And they may not be any better than what we currently use.

You finally end up as cynical as I am. This might even force you to read the original manuscript, rather than the Gung ho press release or the same thing regurgitated on a news website.

And, if you do that, you’ll better understand some of the clever approaches that scientists are applying to the development of effective biocontrol for Varroa.

We’re not there yet, but progress is being made.

V e r y   s l o w l y.

The paper I’m going to discuss below is Han, J.O., et al. (2021) Directed evolution of Metarhizium fungus improves its biocontrol efficacy against Varroa mites in honey bee colonies. Sci Rep 11, 10582.

It’s freely available should you want to read the bits I get wrong 😉

Solving the temperature-sensitivity problem of mitospores

The strain of Metarhizium chosen for these studies was M. brunneum F52. This had previously been demonstrated to have some efficacy against Varroa. Almost as important, it can be genetically manipulated and there was some preliminary evidence that its pathogenicity for Varroa – and hence control potential – could be improved.

Genetic manipulation covers a multitude of sins. It could mean anything from selection of pre-existing variants from a population to engineered introduction of a toxin gene for destruction of the parasitised host.

In this study the authors used directed evolution of a population of Metarhizium to select for strains with more heat tolerant spores.

Directed evolution of Metarhizium to select mitospores with increased thermotolerance

This is not genetic engineering. They grew spores under stressful conditions and increasing temperatures. Hydrogen peroxide (H2O2) , a mild mutagen, was added in some cases. Nutritional stress also increases population variation. Spores selected using nutritional stress are better able to withstand UV and heat stress.

The optimal growth temperature for the strain of Metarhizium they started with was 27°C. By repeated selection cycles at increasing temperatures they derived spores that grew at 35°C, the temperature within a colony.

Ladders and snakes

A well known phenomena of repeated selection in vitro (i.e. in a test tube in the laboratory, though you actually grow Metarhizium on agar plates) is that a pathogen becomes less pathogenic.

It was therefore unsurprising that – when they eventually tested the thermotolerant spores – only about 3% of the Varroa that died did so due to Metarhizium infection.

Field selection after directed evolution of Metarhizium in the laboratory

They therefore modified the repetitive selection, but this time did it on Varroa-infested colonies in the apiary. Mites that died from Metarhizium mycoses 9 were used as a source to cultivate more Metarhizium.

They were therefore selecting for both thermotolerant (because the experiments were being conducted in hives at 35°C) and pathogenic fungi, because they only cultivated mitospores from Varroa that had died from mycoses.

And it worked …

Amplification of Varroa mycoses by Metarhizium. Black arrows indicate the treatment dates.

After four rounds of selection over 60% of the mites that died did so because they were infected with Metarhizium.

All very encouraging … but note I was very careful with my choice of words in that last sentence. I’ll return to this point shortly.

Before that, here’s the ‘proof’ that the strain selected by directed evolution (which they termed JH1078) possessed more thermostable spores.

Thermostable spores

They measured this by recording the percentage that germinated. At 35°C ~70% of JH11078 spores germinated compared to only ~45% of the M. brunneum F52 strain they started with.

But it’s not all good news

My carefully chosen “60% of the mites that died” neatly obscures the fact that you could get a significant increase in mites dying of Metarhizium, but still have almost all the mites in the hive surviving unscathed.

The authors continued repeated Metarhizium monthly treatments for a full season after the selection experiments described above. The apiary contained 48 colonies, 24 received Metarhizium JH1078 and the remainder received no treatment.

Did Metarhizium treatment stop the well documented increase in Varroa levels observed in colonies not treated with miticides?

Varroa levels in Metarhizium treated and untreated (control) colonies.

Er … no.

They describe this data (above) as showing a ‘delay’ in the exponential increase in Varroa … but acknowledge that it ‘did not totally prevent it’.

Hmmm … looking at the error bars in the last few timepoints I’d be hard pressed to make the case that there was any significant difference in Varroa increase caused by treatment.

And while we’re here look at the mite infestation rate … 10-25 mites per 100 bees.

These are catastrophically high numbers and, unsurprisingly, 42 (~88%) of the 48 colonies – whether treated or untreated – died by the end of 2018, succumbing to “Varroa, pathogen pressure and intense yellow jacket predation”

There was some evidence that colonies receiving Metarhizium treatment survived a bit longer than the untreated controls, but the end results were the same.

Almost every colony perished.

Metarhizium vs. oxalic acid

Typically a paper on a potential improved biocontrol method for Varroa would do a side-by-side comparison with a widely used, currently licensed treatment.

There’s only one comparative experiment between Metarhizium and dribbled oxalic acid treatment. It’s buried at the end of the Supplementary Data 10. In it they show ‘no significant difference’ between the two treatments.

Frankly this was a pretty meaningless experiment … it was conducted in June 2020 when colonies would have been bulging with brood. Consequently 90% of the mites would have been hidden under the cappings. They assayed mite levels only 18 days after a single application of Metarhizium or oxalic acid.

Although it showed ‘no significant difference’ – like the “60% of the mites that died” quote – it obscures the fact that most mites were almost certainly completely untouched by either treatment.

What does this study show?

This study involved a large amount of work.

The directed evolution in the laboratory is a very nice example of how the combination of phenotypic selection and natural variation can rapidly yield new strains with desirable characteristics.

Combination of this with in vivo selection for enhanced pathogenesis successfully produced a novel strain of Metarhizium with some of the features desirable for biocontrol of Varroa.

However, in the apiary-based studies the majority of the colonies, whether treated or not, died.

This shows that, although scientists might have made a promising start, they are still a very long way from having an effective biocontrol solution for Varroa.

Unmanaged Varroa replicates to unmanageable levels

One of the Supplementary Data figures illustrated the Varroa drop per month from colonies in the research apiary.

Cumulative mite drop per colony for the month of July 2018

This is relatively late in the study, July 2018. These hives were established from commercial packages of bees in April 2017. They were either treated with the experimental Metarhizium spores or were untreated controls for the 15 months between April 2017 and July 2018 11.

Look at those Varroa numbers!

This is the mite drop just after the peak of the season. Brood levels would be close to maximum in their short, warm summer 12. The majority of the mite population would have been safely tucked away feasting on developing brood.

This is not the mite drop after miticide treatment … it’s just the drop due to bee grooming, natural mite mortality and the general ‘friction’ in the hive.

The average is 2866 mites dropped per month per hive 😥

Maybe nothing could have saved hives as heavily infested as these? 13

Don’t wait for Metarhizium … be vigilant now

These numbers – of mites and dead colonies – are a stark warning of the replication potential of Varroa and the damage is causes our bees.

Left untreated, Varroa will replicate to very high levels.

Colony mortality – either directly due to the mite and viruses, or indirectly due to the weakened colonies succumbing to robbing – is a near-inevitable consequence.

I’ve discussed the importance of Varroa management repeatedly over the years. It’s a topic I’ll be returning to again – probably in August when it starts to become a necessity.

In the meantime, keep an eye on the mite levels in your own colonies as they get stronger during the season.

While you’re doing that think of the scientists who are looking for practical, effective and environmentally-friendly strategies to control Varroa. Understand that these studies are time-consuming, progress is glacial incremental … and they might not work anyway.

Of course, if we finally manage to develop a suitable Metarhizium-based mite control strategy then bees and beekeepers will not be the only beneficiaries.

Metarhizium has its own parasites. Some of the best characterised of these are small RNA viruses.

If beekeepers are sprinkling billions of Metarhizium spores over their colonies every year then these viruses will be having a great time 😉


Winter losses

I lost 10% of my colonies this winter.

It’s always disappointing losing colonies, but it’s sometimes unavoidable.

I suspect the two I lost were unavoidable … though, as you’ll see, they weren’t completely lost.

April showers frosts

Late April may seem like mid-season for many beekeepers based in southern England. While they were adding their second super, the bees here in Scotland were only just starting to take their first few tentative flights of the year.

This April has been significantly cooler in Fife 1 than any year ‘since records began’.

However, the records I’m referring to are from the excellent Auchtermuchty weather report 2 which only date back to about 2013 … I like it because it’s local, not because it’s historically comprehensive 😉

The average April temperate has only been 5.5°C with 15 nights with frost in the first three weeks of the month. In contrast, the same month in 2019 and 2020 averaged over 9°C with only 3-4 nights with frosts 3

In both 2019 and 2020 swarming started at the end of April. Several colonies had queen cells when I first inspected them and I hived my first swarm (not lost from one of my colonies 😉 ) on the last day of the month.

First inspections and winter losses

Unsurprisingly, with appreciably lower temperatures, things are less well advanced this season. None of the colonies I inspected on the 19th were making swarm preparations. Instead, most were 2-4 frames of brood down on the strength I’d expect them to have before they started thinking about swarming.

Nevertheless, most were busy on a lovely spring day … lots of pollen (mainly gorse and some late willow by the looks of things) being delivered by heavily-laden foragers, and fresh nectar in some of the brood frames.

Fresh nectar glistening in a brood frame

The first inspection of the season is an opportunity to not only check on the strength and behaviour of the colony, but also to do some ‘housekeeping’. This includes:

  • swapping out old, dark brood frames (now emptied of stores) and replacing them with new foundationless frames
  • removing excess stores to make space for brood rearing
  • removing the first sealed drone brood in the colony to help hold back Varroa replication

And, as the winter is now clearly over, it’s the time at which the overall number of winter losses can be finally assessed.

Winter losses

Winter losses generally occur for one of four reasons:

  • disease – in particular caused by deformed wing virus (DWV) vectored by high levels of Varroa in the hive. DWV reduces the longevity of the diutinus winter bees, meaning the colony shrinks in size and falls below a threshold for viability. There are too few bees to thermoregulate the colony and too few bees to help the queen rear new larvae. The colony either freezes to death, dwindles to the size of an orange, or starves to death because the cluster cannot reach the stores 4.
  • queen failure – for a variety of reasons queens can fail. They stop laying altogether or they only lay drone brood. Whatever the reason, a queen that doesn’t lay means the colony is doomed.
  • natural disasters – this is a bit of a catch-all category. It includes things like flooded apiaries, falling trees and stampeding livestock. Although these things might be avoidable – don’t site apiaries in flood risk areas, under trees or on grazing land – these lessons are often learnt the hard way 5.
  • unnatural disasters – these are avoidable and generally result from inexperienced, or bad 6 , beekeeping. I’d include providing insufficient stores for winter in this category, or leaving the queen excluder in place resulting in the isolation of the queen, or allowing the entrance to be blocked. These are the things that the beekeeper alone has control over. 

The BBKA run an annual survey of winter losses in the UK. This is usually published in midsummer, so the graph below is from 2020.

BBKA winter survival survey

Over the 13 years of the survey the average losses were 18.2% 7. Long or particularly hard winters result in higher levels of losses.

Lies, damn lies and statistics

I’ve no idea how accurate these winter loss surveys are.

About 10% of the BBKA membership reported their losses, and the BBKA membership is probably a bit over 50% of UK beekeepers. 

I would expect, with precious little evidence to back it up, that the BBKA generally represents the more ‘engaged’ beekeepers in the UK 8. It also probably represents a significant proportion of new beekeepers who were encouraged to join while training.

So, like Amazon reviews, I treat the results of the survey with quite a bit of caution. I suspect beekeepers who have low losses complete it enthusiastically to ‘brag’ about their success (despite its anonymity), while those with large losses either keep quiet or are happy to share their grief. 

Unlike Amazon reviews, I’d be surprised if there are many fake submissions to the BBKA and I’m not aware there’s a living to be made from selling fake colony survival reviews in bulk online.

For comparison, the Bee Informed Partnership in the USA runs a similar survey every year.

Bee Informed Partnership loss and management survey

This survey covers about 10% of the colonies in the USA. Again it is voluntary and likely subject to the same inherent biases that may affect the BBKA survey.

The USA winter colony losses average ~28% over the same 13 year survey period.

Are US beekeepers less good at keeping their colonies alive than beekeepers in the UK?

Perhaps the US climate is less suited to honey bees?

Or, possibly, US beekeepers are simply more honest than their UK counterparts?

I doubt it 9.

Running on empty

My two colony losses were due to queen failures.

Old winter bees and no brood

In the first colony there was no evidence the queen had laid any brood since the previous autumn. There were about 6 seams of bees in the hive, but the outer 2-3 frames were solid with untouched winter stores.

Unused winter stores

This is usually a dead giveaway … literally. The colony hasn’t used the stores because they’ve not had any hungry mouths to feed. With no new brood the colony is doomed.

This queen appears to have simply run out of sperm and stopped laying. She was present (a 2019 marked queen and the same one I’d seen in August last year) and ambling around the frame, but she wasn’t even going through the pretence of inspecting cells before laying.

I removed the queen and united what remained of the colony over a nearby strong colony.

Strong colony ready for uniting

Assuming the queen stopped laying at the end of year all the bees in the hive – and there were a good number – were old, winter bees. These won’t survive long, but will provide a temporary boost to the colony I united them with. 

Every little bit helps 🙂

Even more valuable than the bees were the frames they were on.

Most of the comb in the colony with the failed queen was relatively new. By uniting them I can quickly swap out the old comb (from the stronger hive #34) when I next inspect the hive. At the same time I’ll rescue the frames of sealed stores for use when making up nucs during queen rearing.

Drone laying queen

The second failure was a drone laying queen (DLQ).

These are usually unmistakeable … the brood is clustered, with drone pupae occupying worker brood cells. If the queen has been drone laying for some time there may be lots of undersized ‘runt’ drones present in the hive as well.

Drone laying queen ...

Drone laying queen …

Again, this colony was doomed. With no new queens available and a lot of pretty old bees in the hive they could not be restored to a functioning colony.

However, many of the bees could be saved …

The colony wasn’t overrun with drones. Going by the amount of stores consumed it had probably been rearing worker brood since the winter solstice.

The queen was unmarked and unclipped. I strongly suspect she was a late-season supersedure queen who was very poorly mated.

The 3-4 weeks of drone brood rearing 10 had wrecked quite a few of the frames, but the bees were worth saving.

Under these circumstances I decided to shake the colony out.

When I do this I like to move the original hive and the stand it’s on. If you don’t move the stand the displaced bees tend to cluster near the original hive entrance, festooned from the hive stand. 

In poor weather, or late in the afternoon, this can lead to lots of bees unnecessarily perishing.

However, the stand was shared with two other colonies, so couldn’t be removed. It was also late morning and the weather was excellent.

I moved the hive away and shook the bees out. 

Sure enough … they returned to their original location.

They then marched along the hive stand to the entrance of the adjacent hive.

This way sisters!

And, by the time I left the apiary in mid-afternoon there were only a few diehard bees clustered near where the original hive entrance was.

Why didn’t I just unite them as I’d done with the other failed queen?

Drone brood is a Varroa magnet

Varroa replicate when feeding on developing pupae. The longer development time of drone pupae (when compared with worker pupae) means that you get ~50% more Varroa from drone brood 11

Unsurprisingly perhaps (or not, because that’s the way evolution works) Varroa have therefore evolved to preferentially infest drone brood. When given the choice between a drone or worker pupa to infest, Varroa choose the drone about 10 times more frequently than the worker.

And that ~10:1 ‘preference ratio’ increases when drone brood is limiting … as it is early in the season.

What this means is that the first burst of drone brood production in a colony is very attractive to Varroa.

Unless there are compelling reasons to keep this very early drone brood – for example, a colony with stellar genetics I’d like to contribute as much as possible to the local gene pool – I often try and remove it.


Drone-worker-drone …

If you use foundationless frames this is often as easy as simply cutting out a single panel of drone brood.

But, in the case of this drone laying queen, it meant that the logical action was to discard all of the drone brood to ensure I discarded the majority of the Varroa also present in the colony 12.

Which is why I shook the colony out, rather than uniting them 🙂

Boxes of bees

Several colonies in one apiary went into the winter on double brood colonies. Inevitably, with the loss of bees during the winter months, the colony contracts and the queen almost invariably ends up laying in the upper box.

The first inspection of the season is often a good time to remove the lower box. It can be removed altogether, or replaced (above the other box) for a Bailey comb change if the weather is suitable.

At this stage of the year the lower box is often reasonably empty of bees and totally empty of brood. 

Emptying a box of bees

If the comb in the lower box is old and dark (see the picture above) I place the upper box on the original floor and add an empty super on top. I then go through the lower box, shaking the bees into the empty super. Good frames are retained, the rest are destined for the wax extractor and firelighters.

Using an empty super helps ‘funnel’ the bees into the brood box.

Sometimes the queen has already laid up a frame or so in the lower box. Under these circumstances – particularly if the comb is relatively new – I’ll simply reverse the boxes, placing the lower box on top of the upper one. This results in the queen quite quickly moving up and laying up the space in the upper brood chamber.

It’s then time to add a queen excluder and the first super.

The beekeeping season has definitely started 🙂


I commented a fortnight ago about the apparent lateness of the 2021 spring. I’m adding this final note on the afternoon of the 23rd and have still yet to see or hear either cuckoo or chiffchaff on the west coast. Last year they were here in the middle of the month. This, combined with the temperature data (see above) show that everything is a week or two behind events last year.

Which means I can expect to start doing some sort of swarm prevention and control in the next fortnight.

Brexit and beekeeping

The ‘oven ready’ deal the government struck with the EU in the dying hours of 2020 was a bit less à la carte and a bit more table d’hôte.

The worst of the predictions of empty supermarket shelves and the conversion of Essex into a 3500 km2 lorry park have not materialised 1.

But there are other things that haven’t or won’t appear.

And one of those things is bees.

Bee imports

There is a long history of bee imports into the UK, dating back at least a century. In recent years the number of imports has markedly increased, at least partially reflecting the increasing popularity of beekeeping. 

Going up! Imports of queens, nucs and packages to the UK, 2007-2020 (National Bee Unit data)

Queens are imported in cages, usually with a few attendant workers to keep them company. Nucs are small sized colonies, containing a queen, bees and brood on frames. 

Packages are the ‘new kid on the block’ (in the UK) with up to 2500 per year being imported after 2013. Packages are queenless boxes of bees, containing no frames or brood.

Empty boxes after installing packages of bees

They are usually supplied in a mesh-sided box together with a queen. The bees are placed into a hive with frames of foundation and the queen is added in an introduction cage. They are fed with a gallon to two of syrup to encourage them to draw comb.

Installing a package of bees

It’s a very convenient way to purchase bees and avoids at least some of the risk of importing diseases 2. It’s also less expensive. This presumably reflects both the absence of frame/foundation and the need for a box to contain the frames.

But, post-Brexit, importation of packages or nucs from EU countries is no longer allowed. You are also not allowed to import full colonies (small numbers of these were imported each year, but insufficient to justify adding them to the graph above).

Queen imports are still allowed.

Why are were so many bees imported?

The simple answer is ‘demand’.

Bees can be reared inexpensively in warmer climates, such as southern Italy or Greece. The earlier start to the season in these regions means that queens, nucs or packages can be ready in March to meet the early season demand by UK beekeepers.

If you want a nuc with a laying queen in March or April in the UK you have two choices; a) buy imported bees, or b) prepare or purchase an overwintered nuc.

I don’t have data for the month by month breakdown of queen imports. I suspect many of these are also to meet the early season demand, either by adding them to an imported package (see above) or for adding to workers/brood reared and overwintered in a UK hive that’s split early in the season to create nucleus colonies.

Some importers would sell the latter on as ‘locally reared bees’. They are … sort of. Except for the queen who of course determines the properties of all the bees in the subsequent brood 🙁

An example of being “economical with the truth” perhaps?

Imported queens were also available throughout the season to replace those lost for any number of reasons (swarming, poor mating, failed supersedure, DLQ’s, or – my speciality – ham-fisted beekeeping) or to make increase.

And to put these imports into numerical context … there are about 45,000 ‘hobby’ beekeepers in the UK and perhaps 200+ bee farmers. Of the ~250,000 hives in the UK, about 40,000 are managed by bee farmers.

What are the likely consequences of the import ban?

I think there are likely to be at least four consequences from the ban on the importation of nucs and packages to the UK from the EU:

  1. Early season nucs (whatever the source) will be more expensive than in previous years. At the very least there will be a shortfall of ~2000 nucs or packages. Assuming demand remains the same – and there seems no reason that it won’t, and a realistic chance that it will actually increase – then this will push up the price of overwintered nucs, and the price of nucs assembled from an imported queen and some ‘local’ bees. I’ve seen lots of nucs offered in the £250-300 range already this year.
  2. An increase in imports from New Zealand. KBS (and perhaps others) have imported New Zealand queens for several years. If economically viable this trade could increase 3.
  3. Some importers may try and bypass the ban by importing to Northern Ireland, ‘staging’ the bees there and then importing them onwards to the UK. The legality of this appears dubious, though the fact it was being considered reflects that this part of the ‘oven ready’ Brexit deal was not even table d’hôte and more like good old-fashioned fudge.
  4. Potentially, a post-Covid increase in bee smuggling. This has probably always gone on in a limited way. Presumably, with contacts in France or Italy, it would be easy enough to smuggle across a couple of nucs in the boot of the car. However, with increased border checks and potential delays, I (thankfully) don’t see a way that this could be economically viable on a large scale.

Is that all?

There may be other consequences, but those are the ones that first came to mind.

Of the four, I expect #1 is a nailed-on certainty, #2 is a possibility, #3 is an outside possibility but is already banned under the terms of the Northern Ireland Protocol which specifically prohibits using Northern Ireland as a backdoor from Europe, and #4 happens and will continue, but is small-scale.

Of course, some, all or none of this ban may be revised as the EU and UK continue to wrangle over the details of the post-Withdrawal Agreement. Even as I write this the UK has extended the grace period for Irish sea border checks (or ‘broken international law’ according to the EU). 

This website is supposed to be a politics-free zone 4 … so let’s get back to safer territory.

Why is early season demand so high?

It seems likely that there are three reasons for this early season demand:

  1. Commercial beekeepers needing to increase colony numbers to provide pollination services or for honey production. Despite commercials comprising only ~0.4% of UK beekeepers, they manage ~16% of UK hives. On average a commercial operation runs 200 hives in comparison to less than 5 for hobby beekeepers. For some, their business model may have relied upon the (relatively) inexpensive supply of early-season bees.
  2. Replacing winter losses by either commercial or amateur beekeepers. The three hives you had in the autumn have been slashed to one, through poor Varroa management, lousy queen mating or a flood of biblical proportions. With just one remaining hive you need lots of things to go right to repopulate your apiary. Or you could just buy them in.
  3. New beekeepers, desperate to start beekeeping after attending training courses through the long, dark, cold, wet winter. And who can blame them? 

For the rest of the post I’m going to focus on amateur or hobby beekeeping. I don’t know enough about how commercial operations work. Whilst I have considerable sympathy if this change in the law prevents bee farmers fulfilling pollination or honey production contracts, I also question how sensible it is to depend upon imports as the UK extricates itself from the European Union.

Whatever arrangement we finally reached it was always going to be somewhere in between the Armageddon predicted by ‘Project Fear’ and the ‘Unicorns and sunlit uplands’ promised by the Brexiteers.

Where are those sunlit uplands?

And that had been obvious for years.

I have less sympathy for those who sell on imported bees to meet demand from existing or new beekeepers. This is because I think beekeeping (at least at the hobbyist level) can, and should, be sustainable.

Sustainable beekeeping

I would define sustainable beekeeping as the self-sufficiency that is achieved by:

  • Managing your stocks in a way to minimise winter losses
  • Rearing queens during the season to requeen your own colonies when needed (because colonies with young queens produce brood later into the autumn, so maximising winter bee production) and to …
  • Overwinter nucleus colonies to make up for any winter losses, or for sale in the following spring

All of these things make sound economic sense. 

More importantly, I think achieving this level of self-sufficiency involves learning a few basic skills as a beekeeper that not only improve your beekeeping but are also interesting and enjoyable.

I’ve previously discussed the Goldilocks Principle and beekeeping, the optimum number of colonies to keep considering your interest and enthusiasm for bees and the time you have available for your beekeeping.

It’s somewhere between 2 and a very large number. 

For me, it’s a dozen or so, though for years I’ve run up to double that number for our research, and for spares, and because I’ve reached the point where it’s easy to generate more colonies (and because I’m a lousy judge of the limited time I have available 🙁 ).

Two is better than one, because one colony can dwindle, can misbehave or can go awry, and without a colony to compare it with you might be none the wiser that nothing is wrong. Two colonies also means you can always use larvae from one to rescue the other if it goes queenless.

And with just two colonies you can easily practise sustainable beekeeping. You are no longer dependent on an importer having a £30 mass-produced queen spare.

What’s wrong with imported bees?

The usual reason given by beekeepers opposed to imports is the risk of also importing pathogens.

Varroa is cited as an example of what has happened. 

Tropilaelaps or small hive beetle are given as reasons for what might happen.

And then there are usually some vague statements about ‘viruses’. 

There’s good scientific evidence that the current global distribution of DWV is a result of beekeepers moving colonies about.

More recently, we have collaborated on a study that has demonstrated an association between honey bee queen imports and outbreaks of chronic bee paralysis virus (CBPV). An important point to emphasise here is that the direction of CBPV transmission is not yet clear from our studies. The imported queens might be bringing CBPV in with them. Alternatively, the ‘clean’ imported queens (and their progeny) may be very susceptible to CBPV circulating in ‘dirty’ UK bees. Time will tell.

However, whilst the international trade in plants and animals has regularly, albeit inadvertently, introduced devastating diseases e.g. Hymenoscyphus fraxineus (ash dieback), I think there are two even more compelling reasons why importation of bees is detrimental.

  1. Local bees are better adapted to the environment in which they were reared and consequently have increased overwintering success rates.
  2. I believe that inexpensive imported bees are detrimental to the quality of UK beekeeping.

I’ve discussed both these topics previously. However, I intend to return to them again this year. This is partly because in this brave new post-Brexit world we now inhabit the landscape has changed.

At least some imports are no longer allowed. The price of nucs will increase. Some/many of these available early in the season will be thrown together from overwintered UK colonies and an imported queen.

These are not local bees and they will not provide the benefits that local bees should bring.

Bad beekeeping and bee imports

If imported queens cost £500 each 5 there would be hundreds of reasons to learn how to rear your own queens. 

But most beekeepers don’t …

Although many beekeepers practise ‘passive’ queen rearing e.g. during swarm control, it offers little flexibility or opportunity to rear queens outside the normal swarming season, or to improve your stocks.

In contrast, ‘active’ queen rearing i.e. selection of the best colonies to rear several queens from, is probably practised by less than 20% of beekeepers.

This does not need to involve grafting, instrumental insemination or rows of brightly coloured mini-nucs. It does not need any large financial outlay, or huge numbers of colonies to start with.

But it does need attention to detail, an understanding of – or a willingness to learn – the development cycle of queens, and an ability to judge the qualities of your bees.

Essentially what it involves is slightly better beekeeping.

But, the availability of Italian, Greek or Maltese queens for £20 each acts as a disincentive.

Why learn all that difficult ‘stuff’ if you can simply enter your credit card details and wait for the postie?

Overwintering 5 frame poly nuc

Overwintering 5 frame poly nuc

And similar arguments apply to overwintering nucleus colonies. This requires careful judgement of colony strength through late summer, and the weight of the nuc over the winter.

It’s not rocket science or brain surgery or Fermat’s Last Theorem … but it does require a little application and attention.

But, why bother if you can simply wield your “flexible friend” 6 in March and replace any lost colonies with imported packages for £125 each?

Rant over

Actually, it wasn’t really a rant. 

My own beekeeping has been sustainable for a decade. I’ve bought in queens or nucs of dark native or near-native bees from specialist UK breeders a few times. I have used these to improve my stocks and sold or gifted spare/excess nucs to beginners.

I’ve caught a lot of swarms in bait hives and used the best to improve my bees, and the remainder to strengthen other colonies.

The photographs of packages (above) are of colonies we have used for relatively short-term scientific research. 

I’m going to be doing a lot of queen rearing this season. Assuming that goes well, I then expect to overwinter more nucs than usual next winter. 

I then hope that the bee import ban remains in place for long enough until I can sell all these nucs for an obscene profit which I will use to purchase a queen rearing operation in Malta. 😉

And I’m going to write about it here.


BBKA statement made a day or two after this post appeared. The BBKA and other national associations are concerned about the potential import of Small Hive Beetle (SHB) into the UK via Northern Ireland. Whilst I still think this breaches the Northern Ireland Protocol, it doesn’t mean it won’t be attempted (and there’s at least one importer offering bees via this route). It’s not clear that the NI authorities have the manpower to inspect thousands of packages.

It’s worth noting that SHB was introduced to southern Italy in 2014 and remains established there. The most recent epidemiological report shows that it was detected as late as October 2020 in sentinel apiaries and is also established in natural colonies.

With a single exception – see below – every country into which SHB has been imported has failed to eradicate it. As I wrote in November 2014:

“Once here it is unlikely that we will be able to eradicate SHB. The USA failed, Hawaii failed, Australia failed, Canada failed and it looks almost certain that Italy has failed.”

And Italy has failed.

The one exception was a single import to a single apiary in the Portugal. Notably, the illegal import was of queens, not nucs or packages. Eradication involved the destruction of the colonies, the ploughing up of the apiary and the entire area being drenched in insecticide.

The Beekeepers Quarterly

This post also appeared in the summer 2021 edition of The Beekeepers Quarterly published by Northern Bee Books.

Oxalic acid (Api Bioxal) preparation

This post updates and replaces one published three years ago (which has now been archived). The registered readership of this site has increased >200% since then and so it will be new to the majority of visitors.

It’s also particularly timely.

I will be treating my own colonies with oxalic acid in the next week or so.

Mites and viruses

Varroa levels in the hive must be controlled for successful overwintering of colonies. If you do not control the mites – and by ‘control’ I mean slaughter 😉 – the viruses they transmit to the overwintering bees will limit the chances of the colony surviving.

The most important virus transmitted by Varroa is deformed wing virus (DWV). At high levels, DWV reduces the lifespan of worker bees.

This is irrelevant in late May – there are huge numbers of workers and they’re only going to live for about 6 weeks anyway.

In contrast, reduced longevity is very significant in the winter where more limited numbers of overwintering bees must survive for months to maintain the colony through to the Spring. If these bees die early (e.g. in weeks, not months), the colony will dwindle to a pathetic little cluster and likely freeze to death on a cold winter night.

Game over. You are now an ex-beekeeper 🙁

To protect the overwintering bees you must reduce mite levels in late summer by applying an appropriate miticide. I’ve discussed this at length previously in When to treat? – the most-read post on this site.

I’d argue that the timing of this late summer treatment is the most important decision about Varroa control that a beekeeper has to make.

However, although the time for that decision is now long-gone, there are still important opportunities for mite control in the coming weeks.

In the bleak midwinter

Miticides are not 100% effective. A proportion of the mites will survive this late summer treatment 1. It’s a percentages game, and the maximum percentage you can hope to kill is 90-95%.

If left unchecked, the surviving mites will replicate in the reducing brood reared between October and the beginning of the following year. That means that your colony will potentially contain more mites in January than it did at the end of the late summer treatment.

Mid September

Late summer mite treatment and no midwinter treatment.

Over several years this is a recipe for disaster. The graph above shows modelled data that indicates the consequences of only treating in late summer. Look at the mite levels (in red, right hand vertical axis) that increase year upon year.

The National Bee Unit states that if mite levels exceed 1000 then immediate treatment is needed to protect the colony. In the modelled data above that’s in the second year 2.

In contrast, here is what happens when you also treat in “midwinter” (I’ll discuss what “midwinter” means shortly).

Two optimal treatments

Two optimal treatments

Mite numbers remain below 1000. This is what you are aiming for.

For the moment ignore the specific timing of the treatment – midwinter, late December etc.

Instead concentrate on the principle that determines when the second treatment should be applied.

During the winter the colony is likely to go through a broodless period 3.

When broodless all the mites in the colony must, by definition, be phoretic.

There’s no brood, so any mites in the colony must be riding around on the backs of workers.

A phoretic mite is an easy mite to kill 4.

A “midwinter” double whammy

A single oxalic acid based treatment applied during the winter broodless period is an ideal way to minimise the mite levels before the start of the following season.

Oxalic acid is easy to administer, relatively inexpensive and well-tolerated by the bees.

The combination – a double whammy – of a late summer treatment with an appropriate miticide and a “midwinter” treatment with oxalic acid should be all that is needed to control mites for the entire season.

However, “midwinter” does not mean midwinter, or shouldn’t.

Historically, winter mite treatments were applied between Christmas and New Year. It’s a convenient time of the year, most beekeepers are on holiday and it’s a good excuse to avoid spending the afternoon scoffing mince pies in front of the TV.

Or with the outlaws inlaws 😉

But by that time of year many colonies will have started brooding again.

With sealed brood, mites have somewhere to hide, so the treatment will be less effective than it might otherwise have been 5.

Why go to all the trouble of treating if it’s going to be less effective than it could be?

The key point is not the timing … it’s the broodlessness of the colony.

If the colony is broodless then it’s an appropriate time to treat. My Fife colonies were broodless this year by mid-October. This is earlier than previous seasons where I usually have waited until the first protracted cold period in the winter – typically the last week in November until the first week in December.

If they remain broodless this week I’ll be treating them. There’s nothing to be gained by waiting.

Oxalic acid (OA) treatment options

In the UK there are several approved oxalic acid-containing treatments. The only one I have experience of is Api-Bioxal, so that’s the only one I’ll discuss.

I also give an overview of the historical method of preparing oxalic acid as it has a bearing on the amount of Api-Bioxal used and will help you (and me) understand the maths.

OA can be delivered by vaporisation (sublimation), or by tricking (dribbling) or spraying a solution of the chemical.

I’ve discussed vaporisation before so won’t rehash things again here.

Trickling has a lot to commend it. It is easy to do, very quick 6 and requires almost no specialised equipment, either for delivery or personal protection (safety).

Trickling is what I always recommend for beginners. It’s what I did for years and is a method I still regularly use.

The process for trickling is very straightforward. You simply trickle a specific strength oxalic acid solution in thin syrup over the bees in the hive.

Beekeepers have used oxalic acid for years as a ‘hive cleaner’, as recommended by the BBKA and a range of other official and semi-official organisations. All that changed when Api-Bioxal was licensed for use by the Veterinary Medicines Directorate (VMD).

Oxalic acid and Api-Bioxal, the same but different

Api-Bioxal is the VMD-approved powdered oxalic acid-containing miticide. It is widely available, relatively inexpensive (when compared to other VMD-approved miticides) and very easy to use.

Spot the difference ...

Spot the difference …

It’s very expensive when compared to oxalic acid purchased in bulk.

Both work equally well as both contain exactly the same active ingredient.

Oxalic acid.

Api-Bioxal has other stuff in it (meaning the oxalic acid content is a fraction below 90% by weight) and these additives make it much less suitable for sublimation. I’ll return to these additives in a minute or two. These additives make the maths a bit more tricky when preparing small volumes at the correct concentration – this is the purpose of this post.

How much and how strong?

To trickle or dribble oxalic acid-containing solutions you’ll need to prepare it at home, store it appropriately and administer it correctly.

I’ve dealt with how to administer OA by trickling previously. This is all about preparation and storage.

The how much is easy.

You’ll need 5ml of oxalic acid-containing solution per seam of bees. In cold weather the colony will be reasonably well clustered and its likely there will be a maximum of no more than 8 or 9 seams of bees, even in a very strong colony.

Hold on … what’s a seam of bees?

Three seams of bees

Looking down on the colony from above, a seam of bees is the row visible between the top bars of the frames.

So, for every hive you need 5ml per seam, perhaps 45ml in total … with an extra 10% to cover inevitable spillages. It’s not that expensive, so don’t risk running out.

And the strength?

The recommended concentration to use oxalic acid at in the UK has – for many years – been 3.2% w/v (weight per volume) in 1:1 syrup. This is less concentrated than is recommended in continental Europe (see comments below on Api-Bioxal).

My advice 7 – as it’s the only concentration I’ve used – is to stick to 3.2%.

Calculators at the ready!

The oxalic acid in Api Bioxal is actually oxalic acid dihydrate. Almost all the powdered oxalic acid you can buy is oxalic acid dihydrate.

The molecular formula of oxalic acid is C2H2O4. This has a molecular weight of 90.03. The dihydrated form of oxalic acid has the formula C2H2O4.2H2O 8 which has a molecular weight of 126.07.

Therefore, in one gram of oxalic acid dihydrate powder (NOT Api Bioxal … I’ll get to Api Bioxal in a minute! Have patience Grasshopper) there is:

90.03/126.07 = 0.714 g of oxalic acid.

Therefore, to make up a 3.2% oxalic acid solution in 1:1 syrup you need to use the following recipe, or scale it up as needed.

  • 100 g tap water
  • 100 g white granulated sugar
  • Mix well
  • 7.5 g of oxalic acid dihydrate

The final volume will be 167 ml i.e. sufficient to treat over 30 seams of bees, or between 3 and 4 strong colonies (including the 10% ‘just in case’).

The final concentration is 3.2% w/v oxalic acid

(7.5 * 0.714)/167 * 100 = 3.2% 9.

Check my maths 😉

Recipe to prepare Api-Bioxal solution for trickling

Warning – the recipe on the side of a packet of Api-Bioxal makes up a much stronger solution of oxalic acid than has historically been used in the UK. Stronger isn’t necessarily better. The recipe provided is 35 g Api-Bioxal to 500 ml of 1:1 syrup. By my calculations this recipe makes sufficient solution at a concentration of 4.4% w/v to treat 11 hives. 

There’s an additional complication when preparing an Api-Bioxal solution for trickling. This is because Api-Bioxal contains two additional ingredients – glucose and powdered silica. These cutting agents account for 11.4% of the weight of the Api-Bioxal. The remaining 88.6% is oxalic acid dihydrate.

Using the same logic as above, 1g of Api-Bioxal therefore contains:

(90.03/126.07) * 0.886 = 0.633 g of oxalic acid.

Therefore, to make up 167 ml of a 3.2% Api-Bioxal solution you need to use the following recipe, or scale up/down appropriately:

  • 100 g tap water
  • 100 g white granulated sugar
  • Mix well
  • 8.46 g of Api-Bioxal

Again, check my maths … you need to add (7.5 / 0.886 = 8.46) grams of Api-Bioxal as only 88.6% of the Api-Bioxal is oxalic acid dihydrate.

Scaling up and down

8.46 g is not straightforward to weigh – though see below – and 167 ml may be too much for the number of hives you have. Here’s a handy table showing the amounts of Api-Bioxal to add to 1:1 syrup to make up the amount required.

Api-Bioxal recipes for 3.2% trickling in 1:1 syrup

The Api-Bioxal powder weights shown in bold represent the three packet sizes that can be purchased.

I don’t indicate the amounts of sugar and water to mix to make the syrup up. I’ll leave that as an exercise for the reader … remember that 100 g of sugar and 100 ml of water make 167 ml of 1:1 (w/v) syrup.

Weighing small amounts of Api-Bioxal

The amount of Api-Bioxal used is important. A few grams here or there matter.

If you are making the mix up for a limited number of hives you will have to weigh just a few grams of Api-Bioxal. You cannot do this on standard digital kitchen scales which work in 5 g increments.

Buy a set of these instead.

Digital scales … perfect for Api-Bioxal (and yeast)

These cost about a tenner and are perfect to weigh out small amounts 10 of Api-Bioxal … or yeast for making pizza dough.

A few words of caution

I don’t want to spoil your fun but please remember to take care when handling or using oxalic acid, either as a powder or when made up as a solution.

Oxalic acid is toxic

  • The lethal dose for humans is reported to be between 15 and 30 g. It causes kidney failure due to precipitation of solid calcium oxalate.
  • Clean up spills of powder or solution immediately.
  • Take care not to inhale the powder.
  • Store in a clearly labelled container out of reach of children.
  • Wear gloves.
  • Do not use containers or utensils you use for food preparation. A well rinsed plastic milk bottle, very clearly labelled, is a good way to store the solution prior to use.


Storage of oxalic acid syrup at ambient temperatures rapidly results in the acid-mediated breakdown of sugars (particularly fructose) to generate hydroxymethylfurfural (HMF). As this happens the colour of the oxalic acid-containing solution darkens significantly.

This breakdown happens whether you use oxalic acid or Api-Bioxal.

Stored OA solution and colour change

Stored OA solution and colour change …

HMF is toxic to honey bees at high concentrations. Studies from ~40 years ago showed that HMF concentrations below 30 mg/l were safe, but above 150 mg/l were toxic 11.

At 15°C HMF levels in OA solution can reach 150 mg/l in a little over a week. At room temperature this happens much faster, with HMF levels exceeding 150 mg/l in only 2-3 days. In the dark HMF levels build up slightly less quickly … but only slightly 12.

Therefore only make up OA solutions when you need them.

If you must store your oxalic acid-containing syrup for any length of time it should be in the fridge (4°C). Under these conditions HMF levels should remain well below toxic levels for at least one year. However, don’t store it for this long … use it and discard the excess.

Or prepare excess and share it with colleagues in your beekeeping association.

Don’t use discoloured oxalic acid solutions as they’ve been stored incorrectly and may well harm your bees.

Another final few words of caution

I assume you don’t have a fridge dedicated to beekeeping? That being the case please ensure that the bottle containing stored oxalic acid is labelled clearly and kept well out of the reach of children.


A quick trawl through the Veterinary Medicines Directorate database turns up several oxalic acid-containing solutions for managing Varroa. These include:

  • Oxuvar – approved for trickling or spraying, an aqueous solution of oxalic acid to which you add glucose if you intend to use it for trickling.
  • Oxybee – approved for trickling (and possibly other routes, but the paperwork was a minefield!), contains oxalic acid, glycerol and essential oils and is promoted as having a long shelf life.
  • VarroMed – approved for trickling, contains oxalic acid and formic acid and can be used throughout the year in one way or another.

I’ve not read the documentation provided with these and so don’t know the precise concentration of oxalic acid they contain. It will be listed as an active ingredient. I have not used these products. As with everything else on this site, I only write about methods or products I am familiar with. I therefore cannot comment on their relative efficacy compared to Api-Bioxal, to Apivar or to careful siting of your hives in relation to ley lines … or 5G phone masts.