Category Archives: Diseases

Biological control with Varroa

Synopsis : Honey bees were eradicated on Santa Cruz Island following the introduction of Varroa. This provides some useful lessons for beekeepers on the importance of controlling Varroa.


Honey bees are not native to North America. They were first introduced in March 1622 at Jamestown, Virginia. The bees did well and spread west, following the settlers. They finally arrived on the west coast, in Santa Clara, California, 231 years later in 1853. Of a dozen hives ordered by Christopher Shelton, a Santa Clara botanist and rancher, only one survived the journey from New York via Panama.

Shelton barely had a chance to enjoy his bees 1 as he was unfortunately killed when the steamboat Jenny Lind exploded in mid-April 1853.

Explosion on the steamboat Jenny Lind near San Francisco, California

His bees survived 2 and three hives derived from the original stock were auctioned for $110 each. This was over 20 times the price of hives on the east coast at that time and equivalent to over $4200 today 3.

Californian Channel Islands map

Bees were in demand and they continued to spread – both as feral swarms and as farmers established apiaries to help pollination and for honey production. Having reached the California coast they were then spread to the nearby islands. Bees were transported to Santa Cruz, the largest of the eight Channel Islands near Los Angeles, in the 1880’s. They flourished, but did not spread to the other Channel Islands.

Field station, nature reserves, pigs and bees

Santa Cruz Island is 250 square kilometres in area and lies ~35 km south of Santa Barbara. It is one of the four Northern Channel islands. There is a long central valley lying approximately east-west and the rocky mountainous land reaches 740 m. It has a marine temperate climate; the average low and high temperatures are 9°C and 21°C respectively and it receives about 0.5 m of rain a year. It is a good environment for bees.

From the 1880’s to 1960’s Santa Cruz Island was farmed – primarily for wine and wool, and from the 1940’s for cattle – but, after period of university geology field trips and the establishment of a field station on the island, in 1973 it became part of the University of California’s Natural Reserve System (UC NRS).

In the late 1970’s the Stanton family sold their ranching business on the island to The Nature Conservancy who subsequently bought additional land on the eastern end of the island.

Santa Cruz Island is now jointly owned by The Nature Conservancy, National Parks Service, UC NRS and the Santa Cruz Island Foundation and much of the island is used for scientific research and education.

But what about the bees?

Good question.

As a nature reserve and research station, the presence of non-native species causes a potential problem. Why go to all the expense of managing a remote island research centre if all the same species are present as on the mainland?

The Nature Conservancy therefore initiated a programme of eradicating non-native species. It took 14 months to eliminate the feral pigs, using a combination of trapping, helicopter-based shooting and the release of sterilised radio-tagged pigs to locate the stragglers 4.

But getting rid of the bees took a bit longer …

Save the bees, or not

Why get rid of the bees? Surely they weren’t doing any harm?

The introduction of any non-native species upsets the balance (if there’s ever balance) in the ecosystem. The introduced species competes directly or indirectly with those native to the area and can lead to local extinctions.

Jonathan Rosen has described 5 how honey bee swarms, through occupying tree cavities previously used for nesting, probably played a major role in the extinction of the Carolina parakeet.

Pining for the fjords … a stuffed Carolina parakeet (nailed to its perch)

Competition between honey bees and native pollinators has been well studied. It is not always detrimental, but it certainly can be. Furthermore, it is probably more likely to be detrimental in a small, isolated, island ecosystem. For example, studies showed that the presence of honey bees dramatically reduced visitation of native pollinator to manzanita blossoms on Santa Cruz Island.

As part of the larger programme of non-native plant and animal eradication on Santa Cruz Island plans were drawn up in the late 1980’s to eliminate European honey bees. The expected benefits were to:

  • eliminate competition with native bee species (and presumably other non-bee pollinators, though these rarely get a mention 🙁 )
  • reduce pollination of weed species (some of which were also non-native to Santa Cruz Island)
  • facilitate recovery of native plant species that were reliant on native bee pollination
  • provide a ‘field laboratory’ free from ‘exotic’ honey bees in which comparative studies of native pollinators would be possible

Killer bees

After the plans to eradicate Apis mellifera were approved an additional potential benefit became apparent.

There were increasing concerns about the spread of Africanised honey bees which had recently reached Santa Barbara County. Although there was reasonably compelling evidence that swarms could not cross from the mainland (e.g. none of the other Northern Channel Islands had been colonised by bees) there were concerns that the Santa Ana winds might help blow drones from the mainland.

Had these drones arrived they might mate with the non-native but nevertheless local queens resulting in the spread of the dominant genes for defensiveness and absconding. The resulting swarmy, aggressive Africanised bees would cause problems for visitors and scientists working on the island (as they have for visitors to Joshua Tree National Park).

Aerial view of Santa Cruz Island

Although the introgression of African honey bee genes was used as further justification for the eradication it’s not clear whether drones could actually cross 30-40 km of open sea 6.

As an aside, there’s a current project – the amusingly named Game of Drones – running on the Isles of Scilly investigating whether drones can cross the sea between St Agnes, Tresco, Bryher, St Mary’s and St Martin’s. These are, at most, 11 km apart (northern most tip of St Martin’s to most southerly point of St Agnes) but the individual islands are only separated by 1-2 km. I would be surprised if drones could not cross that distance (at least with a strong following wind).

Killing bees

Adrian Wenner and colleagues set about exterminating the honey bees on Santa Cruz Island (Wenner et al., 2009). The process started in 1988 and ended in 2007, and was divided into four phases:

  1. 1988-1993 – location and elimination of feral colonies
  2. 1994-1997 – biological control and colony demise
  3. 1998-2004 – monitoring residual honey bee activity
  4. 2005-2007 – confirmation of the absence of honey bees

None of this is ’beekeeping’ – actually it’s the exact opposite – so I don’t intend to dwell in much detail on the work that was conducted. However, the ’94-’97 phase includes some sobering lessons for beekeepers which are worth discussing.

By the end of phase 1 the team had identified the existence (if not the location) of at least 200 colonies and eliminated 153 of them.

Remember, none of these were managed colonies in hives. They were all feral colonies occupying natural cavities in trees or rocks etc. Each colony was found using painstaking bee lining techniques similar to those described in Thomas Seeley’s book Following the Wild Bees.

Once located, nests were destroyed with methyl chloroform and the cavity sealed to prevent it being reoccupied.

Some colonies could not be accessed; in these cases acephate-laced sucrose-honey syrup baits were used. This organophosphate has delayed toxicity for bees, allowing foragers to return to the colony which in due course dies. This approach had been partially successful in eliminating Africanised bees on the mainland (Williams et al., 1989), but baits needed to be be monitored to avoid killing the other insects they attracted.

The scientists also deployed swarm traps (aka bait hives) and destroyed any swarms that moved in.

Together these interventions reduced honey bee numbers significantly – as monitored by regular observations at pollen- or nectar-rich plants – but did not eradicate them.

Let there be mite

Heavy rains in January ’93 washed out roads on Santa Cruz Island, thereby severely limiting travel around the island. In addition, the previous removal of cattle had resulted in the near-uncontrolled growth of fennel which now formed dense, impenetrable thickets.

Bee lining became impossible and the scientists had to invent more devious strategies to eliminate the residual feral colonies.

The approach they chose involved the introduction of Varroa.

Varroa was first detected in the USA in 1987 (in Florida) and became widespread over the next 5-8 years. Up until 1994 the honey bees on Santa Cruz Island were free of the ectoparasitic mite.

It was likely that they would have remained that way … there was no beekeeping on Santa Cruz Island and the location was too remote for bees to cross from the mainland (see above).

Varroa was already known to have a devastating impact on the health of honey bee colonies (Kraus and Page, 1995). It was also known that, other than its native host Apis cerana (the Eastern honey bee), Varroa did not parasitise other bee or wasp species (Kevan et al., 1991).

These two facts – host specificity and damage inflicted – suggested that Varroa could be used for biological control (‘biocontrol’) on Santa Cruz Island.

Biological control

Biological control or biocontrol is a method of controlling pests using natural mechanisms such as predation or parasitism.

The pest could be any living thing – from animals to bacterial plant diseases – present where it’s unwanted.

On Santa Cruz Island the pest was the honey bee.

In other studies (covered in a previous post entitled More from the fungi 7 ) biocontrol of Varroa has been investigated.

Control of the pest involves the introduction or application of a biological control agent. The key requirements of the latter have already been highlighted – specificity and damage.

Biological control works well when the specificity is high and the damage is therefore tightly targeted. It can be an abject failure – or worse, it can damage the ecosystem – if the specificity is low and/or the damage is widespread.

The cane toad was introduced to Australia to control infestations of greenback cane beetle (a pest of sugar cane). Cane toads were introduced in 1935 and rapidly spread. Unfortunately, cane toads can’t jump very high and so singularly failed to control the greenback cane beetle which tends to 8 stay high up the cane stems.

Female cane toad (not jumping)

But it gets worse; cane toads have a very catholic diet and so outcompeted other amphibians. They introduced foreign diseases to the native frogs and toads and – because of the poisons secreted from their skin – harmed or killed predators that attempted to eat them.


Vertebrates are usually poor biological control agents as they tend to be generalist feeders i.e. no specificity.

But Varroa is specific and so the damage it causes is focused. The likelihood of ecosystem damage was considered low and so the mite was introduced to the island.

Introduction of Varroa

In late 1993 Adrian Wenner caught 85 foraging bees and, to each one, added a single Varroa mite. The bees were then released and presumably flew back to their colonies … taking the hitchhiking mite with them.

Adult mites – the dark red ones you see littering the Varroa tray after you treat with Apivar – are mated females.

Due to their incestuous lifestyle a single mite is sufficient to initiate a new infestation.

The mated adult female mite parasitises a honey bee pupa and produces a series of young; the first is male, the remainder are female. You’re probably reading this before the 9 pm watershed so I’ll leave it to your lurid imagination to work out what happens next (or you can read all the sordid details in Know your enemy).

The presence of honey bees – determined by successful swarm trapping or field observation at likely sites – was then regularly monitored over the next four years.

Swarm numbers remained largely unchanged until 1996 and then dramatically decreased.

Numbers of new swarms on Santa Cruz Island 1991 – 2005. Varroa introduction indicated.

It’s worth noting that during ’94-’96 over 70 swarms were found in natural sites or bait hives. There must have been a significant number of established colonies in 1993 to produce this number of swarms.

But, from 1997 it all stopped … only a single swarm was subsequently found, in a natural cavity in 2002.

Monitoring and confirmation of eradication

From 1998 to 2004 the scientists continued to actively monitor the island for honey bees, focusing on 19 areas rich in natural forage. Although honey bees were found – in decreasing numbers – there were too few to attempt bee lining to locate their colonies.

At the sites being monitored, bees were detected 9, 7, 4, 2 and 1 times respectively in the 5 years from 2000 to 2004. After that, despite continued monitoring, no more honey bees were detected.

The final phase of the project (’05-’07) confirmed the absence of honey bees on Santa Cruz Island.

Whilst, as a scientist, I’m a firm believer that ’absence of evidence does not mean evidence of absence’, as a beekeeper I’m well aware that if there are no scout bees, no swarms and no foragers (when I search in likely places) then there are no honey bee colonies.

Lessons for beekeepers

I wouldn’t have recounted this sorry tale – at least from a beekeeping perspective – unless I thought there were some useful lessons for beekeepers.

There are (at least) three.

The first relates to Varroa resistance, the second to Varroa transmission in the environment and the last to ‘safe’ levels of Varroa. All require some ‘arm waving guesstimates’ 9, but have a good grounding in other scientific studies.

Varroa resistance

There wasn’t any.

At a very conservative estimate there were at least 20 colonies remaining on Santa Cruz Island in 1995. I say ‘conservative’ because that assumes each colony generated two swarms that season (see graph above). In studies of other natural colonies only about 75% swarm annually, meaning the actual number of colonies could have been over 50.

The numbers – 20 or 50 – matter as they’re both much higher than the number of colonies most beekeepers manage (which, based upon BBKA quoted statistics, is about 5).

Whether it was 20 or 50, they were all eliminated following the introduction of 85 mites. Colonies did not become resistant to Varroa.

This all took a few years, but – inferring from the swarm numbers above – the vast majority of colonies were killed in just two years, 1994 and 1995. This timing would fit with numerous other studies of colony demise due to mites.

Wenner estimates that only 3 colonies survived until 2001.

Leaving small numbers of colonies 10 untreated with an expectation that resistance – or even tolerance (which is both more likely and not necessarily beneficial) – will arise is a futile exercise.

I’ve discussed this before … it’s a numbers game, and a handful of colonies isn’t enough.

Varroa spread

Wenner doesn’t elaborate on where the foragers were captured before he added the mites. If I was going to attempt this I’d have chosen several sites around the island to ensure as many feral colonies as possible acquired mites … let us assume that’s what he did.

However, with 85 mites piggybacking on returning workers, and somewhere between (my guesstimated) 20 to 50 colonies, I think it’s highly likely that at least some colonies received none of this ’founding’ mite population.

Yet almost all the colonies died within two years, and those that did not subsequently died with no further intervention from the scientists. We don’t know what killed off the last surviving colonies but — and I know I’m sticking my neck out here – I bet it was the mites.

This is compelling evidence for the spread of Varroa throughout the island environment, a process that occurs due to the activities of drifting and robbing.

If a neighbouring apiary to yours has mites some will end up in your hives … unless you are separated by several kilometres 11.

The transmission of mites in the environment is a very good reason to practice coordinated Varroa control.

One mite is all it takes

But, just as I’ve argued that some colonies may have received none of the founding mites, I’m equally sure that others will have acquired very small numbers of mites, perhaps just one.

And one mite is all it takes.

Without exceptional beekeeping skills, resistance in the bee population or rational Varroa control 12 there is no safe level of mites in a colony.

The more you prevent mites entering the colony in the first place, and the more of those that are present you eradicate, the better it is for your bees.

Here endeth the lesson 😉


It’s worth noting that island populations do offer opportunities for the development of Varroa resistant (or tolerant) traits … if you start with enough colonies. Fries et al., (2006) describes the characteristics of the 13 surviving colonies on Gotland after leaving about 180 colonies untreated for several years. I’ve mentioned this previously and will return to it again to cover some related recent studies.


Fries, I., Imdorf, A. and Rosenkranz, P. (2006) ‘Survival of mite infested (Varroa destructor) honey bee (Apis mellifera) colonies in a Nordic climate’, Apidologie, 37(5), pp. 564–570. Available at:

Kevan, P.G., Laverty, T.M. and Denmark, H.A. (1990) ‘Association of Varroa Jacobsoni with Organisms other than Honeybees and Implications for its Dispersal’, Bee World, 71(3), pp. 119–121. Available at:

Kraus, B. and Page, R.E. (1995) ‘Effect of Varroa jacobsoni (Mesostigmata: Varroidae) on feral Apis mellifera (Hymenoptera: Apidae) in California’, Environmental Entomology, 24(6), pp. 1473–1480. Available at:

Wenner, A.M., Thorp, R.W., and Barthell, J.F. (2009) ‘Biological control and eradication of feral honey bee colonies on Santa Cruz Island, California: A summary’, Proceedings of the 7th California Islands Symposium, pp. 327–335. Available as a PDF.

Williams, J.L., Danka, R.G. and Rinderer, T.E. (1989) ‘Baiting system for selective abatement of undesirable honey bees’, Apidologie, 20(2), pp. 175–179. Available at:


Mellow fruitfulness

Synopsis : Final colony inspections and some thoughts on Apivar-contaminated supers, clearing dried supers, feeding fondant and John Keats’ beekeeping.


The title of today’s post comes from the first line of the poem ’To Autumn’ by John Keats:

Season of mists and mellow fruitfulness

The poem was written just over 200 years ago and was the last major work by Keats (1795-1821) before he died of tuberculosis. Although it wasn’t received enthusiastically at the time, To Autumn is now one of the most highly regarded English poems.

The poem praises autumn, using the typically sensuous imagery of the Romantic poets, and describes the abundance of the season and the harvest as it transitions to winter.

That’s as maybe … the last few lines of the first verse raises some doubts about Keats’ beekeeping skills:

And still more, later flowers for the bees,
Until they think warm days will never cease,
 For summer has o’er-brimm’d their clammy cells.

It’s certainly true that there are late summer flowers that the bees can forage on 1. However, he’s probably mistaken in suggesting that the bees think in any sense that involves an appreciation of the future.

And what’s all this about clammy cells?

If there’s damp in the hive in late summer then it certainly doesn’t bode well for the winter ahead.

Clammy is now used mean damp; like vapour, perspiration or mist. The word was first used in this context in the mid-17th Century.

‘Clammy’ honey

But Keats is using an earlier meaning of ’clammy’ … in this case ’soft, moist and sticky; viscous, tenacious, adhesive’, which dates back to the late 14th-Century.

And anyone who has recently completed the honey harvest will be well aware of how apt that definition is 😉 … so maybe Keats was a beekeeper (with a broad vocabulary).

And gathering swallows twitter in the skies

That’s the last line of ’To Autumn’ (don’t worry … you’ve not inadvertently accessed the Poetry Please website). The swallows are gathering and, like most summer migrants, already moving south. Skeins of pink-footed geese have started arriving from Iceland and Greenland.

Skein of geese over Fife

My beekeeping over the last fortnight has been accompanied by the incessant, plaintive mewing of buzzards. These nest near my apiaries and the calling birds are almost certainly the young from this season.

A few nights ago, while hosing the extractor out in the bee-free-but-midge-filled late evening, I was serenaded by tawny owls as the adults evicted their young from the breeding territory in preparation for next season.

These are all signs, together with the early morning mists, that summer is slipping away and the autumn is gently arriving.

Morning mist clearing over the loch

The beekeeping season is effectively over and all that remains is preparing the colonies for winter.


All the supers were off by the 22nd of August. There was still a little bit of nectar being taken in but the majority was ripe and ready. As it turns out there was fresh nectar in all the colonies when I checked on the 10th of September, but in such small amounts – no more than half a frame – that it wouldn’t have been worth waiting for.

At some point you have to say … enough!

Or, this year, more than enough 🙂 .

Most of the honey was extracted by the end of August. It was a bonanza season with a very good spring, and an outstanding summer, crop. By some distance the best year I’ve had since returning to Scotland in 2015.

Of course, that also meant that there were more supers to extract and return and store for the winter ahead.

Lots of lifting, lots of extracting and lots of buckets … and in due course, lots of jarring.

Storing supers wet or dry?

In response to some recent questions on storing supers wet or dry I tested ‘drying’ some.

I’ve stored supers wet for several seasons. I think the bees ‘like’ the heady smell of honey when they are added back to the hives for the spring nectar flow. The supers store well and I’ve not had any problems with wax moth.

However, this year I have over two full carloads of supers, so – not having a trailer or a Toyota Hilux 2 – I have to make multiple trips back to put them in storage 3. These trips were a few days apart.

I added a stack of wet supers to a few hives on the 1st of September and cleared them on the 9th. All these supers were added over an empty super (being used as an eke to accommodate a half block of fondant – see below) topped with a crownboard with a small hole in it (no more than 2.5 cm in diameter, usually less).

Converting wet supers to dry supers – note the crownboard with a small central hole

When I removed the supers on the 10th they had been pretty well cleaned out by the bees. In one case the bottom super had a very small amount of fresh nectar in it.

So, 7-8 days should be sufficient for a strong colony to clean out 3-4 supers and it appears as though you can do it at the same time as feeding fondant … result 🙂 .

Feeding fondant

I only feed my colonies Baker’s fondant. I add this on the same day I remove the honey-laden supers. I’ve discussed fondant extensively here before and don’t intend to rehash the case for its use again.

Oh well, if you insist 😉 .

I can feed a colony in less than two minutes; unpacking the block, slicing it in half and placing it face down over a queen excluder (with an empty super as an eke) takes almost as much time to write as it does to do.

Take care with sharp knives … much easier with a slightly warm block of fondant

But speed isn’t the only advantage; I don’t need to purchase or store any special feeders (an Ashforth feeder costs £66 and will sit unused for 49 weeks of the year). I’ve also not risked slopping syrup about and so have avoided encouraging robbing bees or wasps.

I buy the fondant through my association. We paid £13 a block this year (up from about £11 last year). That’s more expensive than making or buying syrup (though not by much) and I don’t need to have buckets or whatever people use to store, transport and distribute syrup. Fondant has a long shelf life so I buy a quarter of a ton at a time and store what I don’t use.

All gone! 12.5 kg of fondant added on 22/8/22 and photographed on 9/9/22

And, contrary to what the naysayers claim, the bees take it down and store it very well.

What’s the biggest problem I’ve had using fondant?

The grief I get when I forget to return the breadknife I stole from the kitchen … 😉 .

Apivar-contaminated honey and supers

Last season I had to treat a colony with Apivar before the supers came off. This was one of our research colonies and we had to minimise mite levels before harvesting brood.

I’ve had a couple of questions recently on what to do with supers exposed to Apivar … this is what I’ve done/will do.


The Apivar instructions state something like ’do not use when supers are present’ … I don’t have a set of instructions to check the precise wording (and can’t be bothered to search the labyrinthine VMD database).

Of course, you’re free to use Apivar whenever you want.

What those instructions mean is that honey collected if Apivar is in the hive will be ’tainted’ and must not be used for human consumption.

But, it’s OK for the bees 🙂 .

So, I didn’t extract my Apivar-exposed supers but instead I stored them – clearly labelled – protected from wasps, bees and mice.

This August, after removing the honey supers I added fondant to the colonies. In addition, I added an Apivar-exposed super underneath the very strongest colonies – between the floor and the lower brood box.

I’ll leave this super throughout the winter. The bees will either use the honey in situ or will move it up adjacent to the cluster.

In spring – if I get there early enough – the super will be empty.

If I’m late they may already be rearing brood in it 🙁 … not in itself a problem, other than it means I’m flirting with a ridiculous ’double brood and a half’.

Which, of course, is why I added it to the strongest double brood colonies. It’s very unlikely the queen will have laid up two complete boxes (above the nadired super) before I conduct the first inspection.

But what to do with the now-empty-but-Apivar-exposed supers?

It’s not clear from my interpretation of the Apivar instructions (that I currently can’t find) whether empty supers previously exposed to Apivar can be reused.

WARNING … my reading might be wrong. It states Apivar isn’t to be used when honey supers are on but, by inference, you can use and reuse brood frames that have been exposed to Apivar.

Could you extract honey from brood frames that have previously (i.e. distant, not immediate, past) been Apivar-exposed?

Some beekeepers might do this 4.

It’s at this point that some common sense it needed.

Just because re-using the miticide-exposed supers is not specifically outlawed 5 is it a good idea?

I don’t think it is.

Once the bees have emptied those supers I’ll melt the wax out and add fresh foundation before reusing them.

My justification goes something like this:

  • Although amitraz 6 isn’t wax-soluble a formamidine breakdown product of the miticide is. I have assumed that this contaminates the wax in the super.
  • I want to produce the highest quality honey. Of course this means great tasting. It also means things like wings, legs, dog hairs and miticides are excluded. I filter the honey to remove the bee bits, I don’t allow the puppies in the extracting room and I do not reuse supers exposed to miticides.
  • During a strong nectar flow bees draw fresh comb ‘for fun’. They’re desperate to have somewhere to store the stuff, so they’ll draw out comb in a new super very quickly. Yes, drawn comb is precious, but it’s also easy to replace.

Final inspections

I conducted final inspections of all my colonies in Fife last weekend 7.

For many of these colonies this was the first time they’d been opened since late July. By then most had had swarm control, many had been requeened and all were busy piling in the summer nectar.

Why disturb them?

The queen had space to lay, they weren’t likely to think about swarming again 8 and they were strong and healthy.

Midsummer inspections are hard work … lots of supers to lift.

If there’s no need then why do it?

Of course, some colonies were still busy requeening, or were being united or had some other reason that did necessitate a proper inspection … I don’t just abandon them 😉 .

I don’t just abandon them … introducing a queen to a nucleus colony

But now the supers were off it was important to check that the colonies were in a suitable state to go into the winter.

I take a lot of care over these final inspections as I want to be sure that the colony has the very best chance of surviving the winter. 

I check for overt disease, the amount of brood in all stages (BIAS; so determining if they are queenright) and the level of stores.

And, while I’m at it, I also try and avoid crushing the queen 🙁 .


I don’t have to see the queen. In fact, in most hives it’s almost impossible to see the queen because the box is packed with bees. If there are eggs present then the queen is present 9.

But, there might not be a whole lot of eggs to find.

Firstly, the queen is rapidly slowing down her egg laying rate. She’s not producing anything like 1500-2000 eggs per day by early autumn.

A National brood frame has ~3000 cells per side. If you find eggs equivalent in area to one side of a brood frame she’s laying at ~1000/day. By now it’s likely to be much less. At 500 eggs/day you can expect to find no more than half a frame of eggs in the hive.

Remember the steady-state 3:5:13 (or easier 1:2:4) ratio of eggs to larvae to pupae? 10

Several of my colonies had about half a frame of eggs but significantly more than four times that amount of sealed brood … clear evidence that the laying rate is slowing dramatically.

The shrinking brood nest – note the capped stores and a little space to lay in the centre of the frame

Secondly, the colony is rapidly filling the box with stores, so reducing the space she has to lay. They’re busy backfilling brood cells with nectar.

Look and ye shall find …

So I focus carefully on finding eggs. I gently blow onto the centre of the frames to move the bees aside and search for eggs.

In a couple of hives I was so focused on finding eggs that – as I prepared to return the frame to the colony – I only then saw the queen ambling around on the frame. D’oh!

Some colonies had only 3-4 frames of BIAS, others had lots more though guesstimating the precise area of brood is tricky because of the amount of backfilling taking place.

I still need to check my notes to determine whether it’s the younger queens that are still laying most eggs … I’d not be surprised.


Boxes are now heavy but not full. All received (at least) half a block of fondant in late August and more last weekend. There’s also a bit of late nectar. The initial half block was almost finished in a week.

Once the bag is empty I simply peel it away from the queen excluder. If you’re doing this, leave the surrounding super in place. It acts as a ‘funnel’ to keep the thousands of displaced bees in the hive rather than down your boots and all over the floor.

Although the bees were flying well, the bees in and around the super were pretty lethargic. I’ve seen this before and am not concerned. I don’t know whether these are bees gorged with stores, having a kip or perhaps young bees that don’t know their way about yet. However, it does mean that any bees dropped while removing the bag tend to wander aimlessly around on the ground.

I’d prefer they were in the hive, out of the way of my size 10’s.

If you look at many of the frames in the hive they will be partially or completely filled with stores. The outer frames are likely to be capped already. 

An outer frame of capped stores

These frames of stores are heavy. There’s no need to look through the entire box. I simply judge the weight of each frame and inspect any that are lighter than a full frame of stores.

Closer to the brood nest you’ll probably find a frame or two stuffed, wall-to-wall, with pollen. Again, a good sign of a healthy hive with the provisions it needs to rear the winter bees and make it to spring.


The only sign of disease I saw was a small amount of chalkbrood in one or two colonies. This is a perennial situation (it’s not really a problem) with some of my bees. Quite a few of my stocks have some (or a lot of) native Apis mellifera mellifera genes and these often have a bit of chalkbrood.

I also look for signs of overt deformed wing virus (DWV) damage to recently emerged workers. This is the most likely time of the year to see it as mite levels have been building all season and brood levels are decreasing fast. Therefore, developing brood is more likely to become infested and consequently develop symptoms.

Fortunately I didn’t see any signs of DWV damage and the initial impression following the first week or so of miticide treatment is that mite levels are very low this season. I’ll return to this topic once I’ve had a chance to do some proper counts after treating for at least 8-10 weeks (I use Apivar and, since my colonies all have medium to good levels of brood, the strips need to be present for more than the minimum recommended 6 weeks).

Closing up

Although these were the last hive inspections, they weren’t the last time I’ll be rummaging about in the brood box.

At some point during the period of miticide treatment I’ll reposition the strips (adjacent to the ever-shrinking brood nest) having scraped them to maximise their effectiveness.

Apivar scratch and sniff repositioning studies

However, all that will happen in a month or so when I can be reasonably sure the weather will be a lot less benign. Far better to get the inspections out of the way now, just in case.

So, having added the additional fondant (typically half a block) I closed the hives, strapped them up securely and let them get on with making their preparations for the coming winter.

Goodbye and thanks for the memories

There’s a poignancy about the last hive inspections of the season.

The weather was lovely, the colonies were strong and flying well, and the bees were wonderfully placid. It’s been a great season for honey, disease levels are low to negligible and queen rearing has gone well 11.

But it’s all over so soon 🙁 .

Hive #5 (pictured somewhere above … with the empty bag of fondant) was from a swarm control nuc made up on the last day of May (i.e. a 2021 queen). It was promoted to a full hive in mid-June. At the same time, while the hive they came from (#28) was requeening I’d taken more than 20 kg of spring honey from it. The requeening of #28 took longer than expected as the first was almost immediately superseded. Nevertheless, the two hives also produced almost 4 full supers (conservatively at least 40 kg) of summer honey.

Good times 🙂 .

My notes – for once – are comprehensive. Over the long, dark months ahead I’ll be able to sift through them to try and understand better 12 what went wrong.

That’s because – despite what I said in the opening paragraph of this section – there were inevitably any number of minor calamities and a couple of major snafu’s.

Or ’learning opportunities’ as I prefer to call them.

Last light over Rum and Eigg … not a bad view when visiting an out apiary

But that’s all for the future.

For the moment I have a sore back and aching fingers from extracting for days and the memory of a near-perfect final day of proper beekeeping.

It’s probably time I started building some frames 🙁


Shook swarms and miticides

Synopsis : Combining a shook swarm with miticide treatment removes most mites in the colony and dramatically reduces DWV levels. The application of this strategy for practical beekeeping is discussed.


Why does Varroa have such a devastating impact on colony health?

Feeding on haemolymph – or the abdominal fat body – by Varroa is probably detrimental. Furthermore, during feeding the mite induces immunosuppressive responses which make the bee both more susceptible to bacterial infections and compromises its nutritional status (Aronstein et al., 2012 1 ).

But if that wasn’t enough, the real damage is caused by transmission of viruses – in particular deformed wing virus (DWV) – from the mite to the developing pupa (and adult worker, as mites probably also feed on newly eclosed workers during the misnamed phoretic stage of the life cycle).

In the absence of Varroa, DWV is seemingly inconsequential for honey bees. Varroa-free colonies – including mine on the remote west coast of Scotland – carry DWV, but virus levels are very low and there is never any overt disease.

But Varroa infested colonies, particularly at this time of the season, often have very high levels of DWV.

Individual pupae parasitised by Varroa can develop stratospherically high DWV levels – reaching over a million times higher levels than seen in unparasitised bees (which can be similar to those recorded in Varroa-free bees). In the mite-exposed pupae the virus levels can kill the developing bees, or result in the characteristic symptoms (primarily deformed wings but also stunted abdomens and discolouration) that give the virus its name.

Worker bee with DWV symptoms

Worker bee with DWV symptoms

But bees not directly exposed to Varroa also have higher DWV levels in mite-infested colonies, particularly as the season progresses. Presumably this is due to horizontal transmission of the virus during larval feeding or trophallaxis.

What happens to these elevated virus levels after the removal of Varroa using a miticide such as Apivar?

Who cares? … I mean, Why could that matter?

The clue is in the section above.

Here it is again:

But bees not directly exposed to Varroa also have higher DWV levels [ … snip … ] presumably this is due to horizontal transmission of the virus during larval feeding or trophallaxis.

If you remove mites the virus levels in the treated adult bees are often surprisingly high 2. That makes sense because the miticide is only removing the vector for the virus … the bees with high levels of virus infection are unaffected.

If, during larval feeding or trophallaxis, these elevated levels of DWV result in yet more bees acquiring high DWV levels then the health of the colony will remain compromised.

The real reason that DWV is a problem for honey bees is that high levels of the virus result in the reduced longevity of bees. This isn’t an issue for the short-lived summer foragers 3. However, reducing the longevity of the winter bees – the so-called diutinus bees – can be fatal for the colony. These are the bees that support the queen in winter, thermoregulating the hive and that rear the first brood of the following season.

Their importance to successful overwintering cannot be overemphasised.

So, the question remains. What happens to the virus levels in the hive after the removal of Varroa?

Of course, the reason I’m posing this question is that we now know … 😉 .

Two easy-to-understand potential outcomes

It seemed to us that there were at least two likely outcomes.

  1. The virus levels in the hive drop very quickly after mite removal (red dashed line, below) and return to some sort of basal level. How quickly and to what basal level? We didn’t know.
  2. Virus levels remain elevated for a long period after Varroa is removed (red solid line, below). How long and to what elevated level? Yes – you guessed it – we didn’t know 😉 .

Of course, biology isn’t binary. There are any number of alternative outcomes … it’s just that those two seemed the most likely.

Two possible outcomes for virus levels after mite removal (black vertical dashed line)

What’s more, they’re the easiest to understand … and to explain.

Why might virus levels remain high if Varroa are removed?

Surely the short lifespan of adult bees means these would soon be lost from the colony … particularly if they have reduced longevity?

Yes, but …

We published a paper a couple of years ago that clearly demonstrated that honey bee larvae fed high levels of DWV became infected with the fed virus. The latter, which we could distinguish from any DWV already present in the larvae, replicated to similar high levels seen in a mite-infested hive (Gusachenko et al., 2020).

This observation perhaps suggested that the second scenario outlined above could occur. All the mites are slaughtered, but the remaining bees with high levels of DWV feed developing brood which consequently also go on to develop high levels of DWV.

Although it’s always good to remove mites this would not be the best outcome for the colony.

Virus quantification

Before I explain how we tested which, if any, of these two possibilities is correct I need to say a few things about virus ‘levels’.

For a variety of reasons I don’t have time, space or energy to explain, we don’t actually count viruses, instead we count copies of the virus’s genetic material (the genome).

All the magic happens in one of these machines – a Bio-Rad CFX96 Touch Real Time PCR system.

The virus genome is made of ribonucleic acid (RNA) and we can therefore use fantastically expensive sensitive and accurate diagnostic methods to measure how many copies are present in a particular sample – for example, in a worker bee, or a developing pupa.

Still with me?


To complicate things a little, we can’t meaningfully express the number of virus genomes present as an absolute number (like one million, or 2,478) because bees are different sizes; larvae are tiny, pupae are bigger, drones are larger still.

In addition, different workers are different sizes, larvae grow etc.

Therefore we express it as genomes per unit of total RNA extracted from the sample. That’s a bit of a mouthful, so we abbreviate it to GE / μg 4.


And finally, to put some numbers on the low and high levels of DWV I discussed earlier, a bee from a Varroa-free colony contains ~1,000 – 10,000 GE / μg (103 – 104) of DWV whereas a pupa parasitised by Varroa regularly has 10,000,000,000 to 1,000,000,000,000 GE / μg (1010 – 1012).

That’s a lot of virus 🙁 .

The experiments

Experiments plural because we did these studies in both 2018 and 2019. ‘We’ are Luke (a then PhD student and now post-doctoral fellow in my laboratory, and the first author on the paper) together with our friends and collaborators, Craig, Ewan and Alan (in Aberdeen) and Giles (in Newcastle). The work was published a few days ago in the journal Viruses and is ‘open access’ (Woodford et al., 2022). This means that anyone feeling particularly masochistic or suffering from sleep deprivation can read all the gruesome details at their leisure.

Not ‘breaking rocks in the hot sun’ … but it sometimes feels like that

The paper covers more than just the one experiment I’m going to discuss here. We also looked at how the virus population changes when mite-free bees become infested with Varroa.

I’ll save that for another post 5  … it’s a good story in its own right.

Most mites are in capped cells

It’s been known for at least three decades that the majority of the Varroa population in a brood rearing colony are within capped cells, feasting on developing pupae.

Nom, nom, nom!

Precisely what percentage of the population is the majority varies a bit 6, but a figure of 90% is often quoted as typical for midseason.

% of mites in capped cells

The percentage of mites in capped cells (this is predicted, not actual data)

We reasoned that the best way to quickly remove all 7 the Varroa in a colony was to combine treatment of the phoretic mites with removal of all the brood … where the majority of the mites are lurking.

And to remove the brood (and associated mites) we conducted a shook swarm.

The shook swarm

Many beekeepers will be familiar with the technique called a shook swarm.

Shook swarm setup. Note Apivar strips in the open hive. Returning foragers already clustering at the entrance

This involves shaking all the adult bees into a new hive with frames containing fresh foundation. All the old frames and brood from the original hive are discarded.

We modified this by including Apivar strips in the hive into which we shook the adult bees.

Shook swarmed colony strapped up for transport … we wait for all the bees to enter the hive before moving it

The ‘shook swarm and miticide’ experiment – which we conducted in May – therefore involved the following steps (we used three strong double brood hives per season, each containing similar amounts of bees and brood):

  1. We quantified DWV in emerging brood in hives in which no Varroa management was conducted.
  2. The queen was removed, caged and kept safe for a few hours.
  3. All adult bees were shaken into a new brood box containing 11 frames of fresh foundation and two strips of Apivar 8.
  4. The shook swarms were relocated to a quarantine apiary.
  5. The queen was returned to the shook swarmed colonies and they were fed ad libitum with syrup to encourage them to draw fresh comb.
  6. Mite drop was recorded at 5 day intervals, increasing to longer intervals, until October when brood rearing ceased.
  7. DWV levels were quantified on a monthly basis from June to October.

As you can see, a very simple experiment.

The results

The mite levels in the ‘donor’ hives were much higher in 2019 than 2018. It’s not unusual to see this type of year to year variation in mite levels. In this instance the mean temperature in February and March 2018 had been several degrees colder than 2019 (remember the Beast from the East?).

The Beast from the East ...

The Beast from the East …

This almost certainly reduced early season brood rearing and so delayed mite replication. Brood rearing was strong by late Spring, but the mite levels in 2018 had yet to catch up.

The results of the experiment in both years were essentially the same. However, for clarity I’ll just present the 2019 data as the mite infestation numbers were so dramatic.

Mite drop after conducting the shook swarm

The cumulative mite drop from Apivar-treated shook swarms ranged from ~500 to ~3000 in the first 5 days. After that the daily mite drop remained at extremely low levels until recording stopped in October.

Mite drop following shook swarm and Apivar treatment

If you assume that only 10% of mites were phoretic at the time we conducted the shook swarm, this means that the total number of mites in some of these colonies was about 30,000. Even the colony with the lowest mite drop may have been hiding an additional 4,500 mites in capped cells.

Remember … the National Bee Unit guidance states that if mite levels exceed 1,000 then treatment is strongly recommended ’to avoid Varroa causing significant adverse effects to the colony’.

I think this part of the study shows just how effective Apivar is. After the first 5 days of treatment the cumulative drop – the Apivar strips still were left in place for 8 weeks – was extremely low for each fortnightly sampling period.

Of course – other than the very high numbers – none of this was particularly surprising. We know Apivar kills Varroa.

Perhaps you’re thinking ”My hives drop more Varroa during the autumn treatment, and for longer.”

When you treat a colony with brood present the mite drop is high in the first few days, but then often remains significant over the next 2-3 weeks while the mite-infested brood emerges. 

In our case, all the mites were on adult bees. By killing these mites in the first few days before there was new sealed brood in the colony we ensured the majority of the new brood did not become infested.

Virus levels before and after the shook swarm

In each colony we sampled a dozen emerging workers, once before the shook swarm and then on a monthly basis until brood rearing stopped. By testing emerging brood we could be certain they had been reared in the test colony, rather than drifting in from elsewhere. 

Before the shook swarm virus levels ranged from 105 to 1010 per worker, with an average of around 5 x 107 GE / μg. For those of you unfamiliar with scientific notation that is 50 million virus genomes.

Virus quantification in individual workers from colonies before and after the shook swarm and Apivar treatment

Strikingly, from the June sample onwards, virus levels dropped to an average of about 104 GE / μg (10,000 virus genomes, a 5,000-fold reduction). This average obscured a range of individual levels, from about 102 to 106.

These reductions are statistically significant … always reassuring 😉 .

The 2018 data showed a similar marked reduction in virus levels. The pre-treatment levels were marginally lower (remember, it was a ’low Varroa’ season), but the levels dropped to an average of only 1,000 GE / μg, a slightly higher fold-reduction and again highly statistically significant.

If you remove the majority of the Varroa the virus levels drop very fast to levels seen in mite-free colonies, or colonies with very low mite counts.

Tough love?

Some beekeepers consider that a shook swarm is tough on the colony. 

I’m not sure I agree.

How and when the shook swarm is done matters a lot.

It can be tough, but it shouldn’t be.

The bees need to draw new comb. For this they need ample feeding, lots of bees and warm weather. By conducting shook swarms on strong colonies in late May and giving them a few gallons of syrup we achieved all this.

‘I know I put that caged queen down here … somewhere’

Doing a shook swarm on a weak colony, too early (or late) in the season or omitting feeding is a recipe for disaster. The colony will struggle to draw comb, its brood rearing will be limited and it will be playing ’catch up’ for the remainder of the year.

Our shook swarmed colonies were booming by late July and entered the winter very strong. All overwintered successfully.

I’d argue that a shook swarm is a lot less tough on a colony than the disease burden caused by thousands of mites … 🙁 .

Why Apivar?

It’s worth emphasising that this was a scientific experiment to investigate the consequences for the virus population of removing almost all of the Varroa.

It was not designed as an example of how a beekeeper would necessarily choose to manage a honey production colony.

Our choice of Apivar was considered and deliberate. Application is straightforward, toxicity – at the levels we used – is undetectable and, critically for these studies, it remains active for weeks.

Apivar strip on wire hangar

Of course, Apivar cannot be used when there are honey supers on the hive 9. Any supers added for the summer nectar flow were not extracted.

Additionally, feeding gallons of syrup when there are honey supers present is also not recommended 😉 .

What else could we have used?

The two obvious choices were MAQS or oxalic acid. Both are effective against phoretic mites, though perhaps less so than Apivar. However, both are only active for a short period in the hive; the treatment period for MAQS is 7 days and the activity of oxalic acid – trickled or vaporised – is probably less than a week.

Neither could be relied upon to slaughter the maximum number of mites, a necessity to produce an understandable result 10. We were additionally concerned about problems with queens or absconding had we used MAQS (both of which would have invalidated the study), and we were keen to avoid the need for repeat treatments with oxalic acid (not least because this is not an approved application method).

With thousands of mites we wanted to ensure that the majority were killed quickly … and, as important, that any that survived the first few days of miticide treatment were also more than likely to be killed later 11.

Application to practical beekeeping

The main aim of this experiment was to investigate the levels of DWV in the colony after the majority of Varroa are removed. However, we were also mindful that the method may be useful for a beekeeper who discovers his/her colony has damagingly high mite levels mid-season, or for someone who inherits abandoned hives with high mite loads.

In these scenarios, assuming there are sufficient bees, some nice warm weather and lashings of syrup available, the combination of a shook swarm and simultaneous miticide application is probably the fastest way to restore colony health.

I am not suggesting that beekeepers routinely conduct a shook swarm and miticide application mid-season. It might not be tough on the colony, but that doesn’t mean it’s not very disruptive. If it’s not needed (because mite levels are well controlled, for example) then it’s a waste of brood … and syrup.

However, there are times when I could imagine it might be useful.

If your primary crop is heather honey you’ll know that the hives sometimes don’t come back from the hills until late-September. That’s late to be applying miticides to protect the winter bees. In an area with an extended June gap (which often starts in May) it might be possible to effectively rid the hives of Varroa in June and have a strong colony to take to the moors in early August.

This is probably a better approach than using a half dose of Apivar in June (as some do) which probably doesn’t kill all the mites anyway, risks contributing to amitraz resistance in the mite population and may result in Apivar strips being left in the hive during the heather flow 12.


Miticides kill mites … big deal.

However, it’s the viruses – in particular deformed wing virus – that kill colonies.

We have now shown that removing the majority of the mites from a colony (including those associated with sealed brood) results in the levels of DWV in the hive dropping very quickly.

The speed with which this happens – four weeks or less – is probably accounted for by the lifespan of the adult bees in the colony following the shook swarm.

This suggests that high levels of virus are not horizontally transmitted or (and this is subtly different) that horizontal transmission, through feeding, of large amounts of virus does not result in elevated levels of virus replication in the recipient bee (larva or adult).

All sorts of questions remain. Would oxalic acid be a suitable replacement for Apivar? How much virus is transferred from a worker to a larva during brood rearing, or between workers during trophallaxis? Is this below a threshold for efficient infection? Do virus levels drop as dramatically when treating a broodless colony (e.g. after caging the queen for three weeks)?

In the meantime just remember that ”the only good mite is a dead mite” … and, if you kill the mites, you also quickly reduce virus levels to a level at which they do not damage the colony.

And a straightforward way to achieve that is to combine a shook swarm with an effective miticide.



Aronstein, Katherine A., Eduardo Saldivar, Rodrigo Vega, Stephanie Westmiller, and Angela E. Douglas. ‘How Varroa Parasitism Affects the Immunological and Nutritional Status of the Honey Bee, Apis Mellifera’. Insects 3, no. 3 (27 June 2012): 601–15.

Gusachenko, Olesya N., Luke Woodford, Katharin Balbirnie-Cumming, Ewan M. Campbell, Craig R. Christie, Alan S. Bowman, and David J. Evans. ‘Green Bees: Reverse Genetic Analysis of Deformed Wing Virus Transmission, Replication, and Tropism’. Viruses 12, no. 5 (May 2020): 532.

Woodford, Luke, Craig R. Christie, Ewan M. Campbell, Giles E. Budge, Alan S. Bowman, and David J. Evans. ‘Quantitative and Qualitative Changes in the Deformed Wing Virus Population in Honey Bees Associated with the Introduction or Removal of Varroa Destructor’. Viruses 14, no. 8 (August 2022): 1597.



Workers not shirkers

Synopsis : Not all foragers are equal. A small proportion – the elite foragers – make the majority of foraging trips. These are the most experienced foragers. Could pathogens and pesticides that reduce worker longevity compromise nutrition of the hive?


All bees are the same.


No, of course not.

The three castes, from The ABC of Bee Culture, 1895

For a start there are three castes of honey bee; the queen, drones and workers … but we can also sub-divide these castes.


For example, most beekeepers would agree that there are fundamental and important differences between a virgin queen and a mated queen. They behave differently, their physiology is different and so are their their senses.


Similarly, the difference between virgin and mated drones is also pretty fundamental. In fact, it’s literally a matter of life and death 😉 . However, there are also less dramatic – largely physiological – differences between sexually immature and mature drones.


And there are differences in this caste as well.

Any beekeeper who uses Pagden’s artificial swarm for their swarm control has – although perhaps unknowingly – exploited the difference between two broad groups of workers; the hive (or nurse) bees and the flying bees (or foragers). The former are bees that have yet to fly from the hive. They rear the developing brood, look after the queen and perform a range of housekeeping duties.

After about three weeks the maturing worker goes on several orientation flights and eventually becomes a forager, responsible for collecting the pollen, nectar, resin and water the colony needs.

‘Eventually’ because workers undertake additional roles e.g. guard bees, undertaker bees and scouts, as they segue from hive bees to foragers 1. This change in roles during the lifetime of a worker bee is termed temporal polyethism 2.

Elite foragers

In this post I’m going to focus on the last of the roles the worker fulfils, that of foraging.

There is a lot of good observational and experimental science on foraging behaviour; for example, the preference of foragers for certain pollen or nectar sources, or the features of the colony that induces foraging activity. Some of this is briefly reviewed in ’A closer look – Foraging behaviour’ by Clarence Collison in Bee Culture 3.

Instead of rehashing those things I’m instead going to describe the concept of ’elite’ foragers. These are a minority of the forager population that do the majority of the foraging. They are therefore probably a very important cohort of bees for the colony.

The definition of elite foragers was first demonstrated for honey bees in studies conducted by Paul Tenczar working with Gene Robinson in 2014 4.

Gross differences in foraging activity was not a new concept. It had been observed in a wide range of eusocial insects in studies dating back to the 1970’s. Since reproductive fitness of eusocial insects – like bees, wasps and ants – is determined at the colony level, and workers are genetically related, variation in worker performance was neither expected nor had an obvious origin.

However, significant differences in worker performance are observed when suitable technology exists to detect it.

’We have the technology’ 5

Tenczar used RFID tags to label individual worker bees. I’ve described this technology before. It allows the unique identification of individual bees. Foragers were detected leaving or arriving at the hive by monitoring them with two RFID ‘readers’ arranged along the narrow hive entrance tunnel.

Theoretically at least, a bee registered by the inner and then the outer reader should be leaving the hive, whereas one registered first by the outer reader, followed by the inner, would be arriving. If these two pairs of events were separated by a several minutes it should mean the bee has successfully completed a round trip.

Unfortunately, the RFID/reader technology was in its (relative) infancy 6 and trips were missed. They even added two RFID chips to each bee to improve detection rates. Manual observation showed that there was a 76-94% chance of a trip being detected by at least one of the four readers. They therefore used reads rather than trips as a metric for activity level. This is a bit of a fudge but it will do for the purpose of this study.

As I will show shortly, the technology has now improved and there are more accurate ways to measure foraging trips.

Orientation flights and the age of onset of foraging

Tenczar et al., labelled over 1000 day-old workers in five separate experimental colonies and monitored their activity for 5-7 weeks.

Orientation flights occur before foraging flights, and usually take place in the afternoon. To be sure they were only monitoring foraging flights, they defined the first day of foraging activity as the one when there were at least 6 ‘reads’, and with at least 25% of reads occurring before midday.

Age at onset of foraging for tagged bees

The average age of a worker at the onset of foraging was 20.4 days – a figure in agreement with the ‘three weeks in the hive’ statement I made above. However, if you average the five separate lines of hive data (above) it is also clear that a significant proportion of the bees (actually 27% of them) started foraging within the first 10 days.

This is so-called ’precocious foraging’ and had been seen previously in colonies created with a single cohort of bees. The colonies used in these studies were small (~1000 bees in each) and were started with bees of all the same age. The fact that some start foraging precociously is a demonstration of the plasticity in temporal polyethism I referred to in an earlier footnote … and which non-scientists would describe as doing ‘different jobs at different times, that might vary’.

Workers and shirkers

However, just looking at the cumulative count of workers (above) it is clear that there is considerable variation in the timing of the onset of foraging.

In addition, and perhaps more surprisingly, the level of foraging activity also varied greatly.

Some workers made rather few foraging trips, others started early and finished late, making repeated closely-spaced trips throughout the day.

Contributions of individual bees to the total foraging activity in two colonies

These histograms show the relative foraging activity of RFID tagged bees in two representative colonies. The vertical axis records the number of bees making a particular relative foraging effort. The shape of the graph – large bars on the left and much smaller bars on the right – show that the majority of the bees (hence the large bars) do relatively little foraging, whereas a much smaller number of bees (in the shorter bars on the right) do lots.

Lorenz curves

A more informative way to represent this data is to use a Lorenz curve which displays the share of foraging activity (vertical axis) against the percentage of foraging bees (horizontal axis).

Example plot of a typical Lorenz curve of cumulative share of foraging activity for one of five study colonies

If all foragers contributed equally to foraging activity the ‘curve’ would be the diagonal dotted line i.e. 50% of the bees would ‘deliver’ 50% of the foraging activity.

In the colonies studied, the actual contribution to foraging activity is shown by the curved line.

Approximately 20% of the foragers accounted for 50% of the foraging activity (the area I’ve shaded blue). These are the elite foragers. Conversely, the ‘laziest’ 20% of foragers make less than 5% of the foraging trips (shaded red) 7.

These are the shirkers … the less said about them the better 😉 .

Tenczar et al., conducted additional analysis of the pattern of foraging activity per bee per day, and the consequences of removal of elite foragers (in due course other foragers become elite foragers). However, I’ll skip these as I want to move on to a more recent study of elite foragers.

We really do ‘have the technology’

In 2019 Simon Klein and colleagues published another RFID-tagged forager study 8. In the intervening years the RFID tag and reader technology had improved. They used two modified four frame nucleus hives in which bees traversed separate tunnels for entry and exit.

Colony entrance with sensors – bees enter and depart the hive using different tunnels

In addition to using more reliable RFID readers (#3 in the diagram above) they also weighed (#4) the bees as they entered and exited the hive and recorded video (#7) of the returning bees to determine whether they were carrying pollen.

These additional measurements meant that, in addition to the number of trips completed, the authors were also able to measure foraging performance in terms of pollen or nectar collected.

Actually, that’s a bit of an overstatement.

Pollen foragers were identifiable on video by their pollen-filled corbiculae (PDF). In contrast, foragers returning without pollen could have made unsuccessful trips, or may have collected nectar or water. They therefore classified foraging activity into ‘pollen’ or ‘non-pollen’ trips.

In total they monitored 564 foragers who made an average of 19 foraging trips in their lifetime (~10,500 trips in total). Interestingly, the average foraging lifetime was less than 5 days. As with Tenczar et al., they excluded orientation flights from the trips recorded (though in a different way).

Practice makes perfect

None of the bees monitored foraged exclusively for pollen. However, it was noted that the more trips a bee made, the greater the proportion of the trips were for pollen until a maximum was reached, after which pollen collecting declined.

Changes of foraging performance with experience – pollen collection (each line is an individual tagged bee)

Foragers lost weight on pollen foraging trips – presumably using crop contents to ‘fuel’ the flight. Since the weight of the crop contents were unknown, it wasn’t possible to calculate the weight of the pollen collected.

Non-pollen foragers weighed about the same (or a little more) upon return as when they left. Since both the weight of the crop contents and the identity of what was being collected (water or nectar) were unknown, it was not possible to determine how much had been collected. However, as individual bees aged – and so took more non-pollen foraging trips – their gain in weight per trip increased. This suggested that (as with the likelihood of collecting pollen) increased experience resulted in more efficient foraging.

The elite foragers are the best performing foragers

Analysis of the average number of foraging trips per day demonstrated that it increased over the first ten days and then plateaued at about 10 trips per day.

Changes of foraging performance with experience – average trips per day

It’s worth noting a couple of points here; firstly, on average foragers only made 19 trips in their lifetime and secondly, the majority of the foragers never reached maximal pollen foraging activity (in the multicoloured graph above, most lines terminate before they reach a peak and start to decline).

These points, together with the average foraging lifetime being under five days, indicate that there is a very high attrition rate amongst foragers.

Most die young … by which I mean within the first week of leaving the hive 🙁 .

But, some live long enough to become the experienced elite foragers, and these bees were the best performing foragers.

Like the previous study, an average of 19% of the foragers performed 50% of all foraging trips recorded 9. These were the elite bees.

These elite bees were the most likely to collect pollen and – on non-pollen trips – were most likely to show a gain in weight, indicating a greater resource (nectar or water) load was being carried.

Conclusions and consequences

The logical conclusion is that, through experience, bees improve their foraging performance. However, most bees never realise their full potential as they perish long before they achieve the status of elite foragers.

It is known that bees exhibit both learning and memory. With regard to foraging, it’s known that navigation skills improve with experience, and that both flower discrimination and ‘handling’ also get better i.e. they are more likely to find (and re-find) remote flowers, to distinguish them from other flowers in the immediate area and to harvest the pollen or nectar from the flower.

These elite bees must make a significant contribution to resourcing the colony.

Numerically they are a minority of the foraging population, but they collect lots of the pollen, nectar, water and propolis needed by the colony.

However, without knowing the precise number and age/experience distribution of the foragers and the mass of the pollen/nectar loads collected, it is not possible to determine whether they collect the majority of these resources.

For example, do 1000 inexperienced foragers collect more (or less) than 100 elite foragers? Are the massed ranks of young naïve foragers more effective at provisioning the colony than a few dozen of ‘old timers’?

We don’t know … yet.

The experiments needed to determine this are more difficult, and a lot more intrusive. You need to know both the weight of whatever was collected, together – in the case of pollen and nectar – with its value to the colony. For example, we know that bees preferentially forage on particular high protein pollens, or on high-sucrose nectar sources … presumably (though it needs to be shown) elite bees do this better than naïve foragers.

Looking after the elderly

Tenczar et al., showed that depleting the elite bee population had little long-term effect, because younger bees made additional foraging trips in subsequent days. However, this ‘replacement’ was only measured in terms of foraging trips, not foraging efficiency (which Tenczar didn’t measure).

If efficiency comes with experience – as suggested – it may be that additional time would also be needed to turn these extra flights into foraging trips that significantly benefitted the colony.

All of which means that stressors that adversely affect ageing bees, or that shorten the lifespan of foragers, may have a marked impact on colony pollen and nectar collection.

And there are lots of these sorts of stressors … pesticides, pollution, poor nutrition and pathogens – an alliterative gamut of threats to these important, elderly but highly effective bees. For example, increasing cumulative exposure to sub-lethal levels of pesticides may be deleterious to older bees.

Deformed wing virus

Unsurprisingly, being a virologist, it is the pathogens that interest me. In particular, deformed wing virus (DWV).

DWV symptoms

DWV symptoms

DWV is probably responsible for the majority of overwintering colony losses because it reduces the longevity of the (nominally long-lived) winter bees. I’ve discussed this at length elsewhere but the important bits are as follows:

  • winter bees should live for months, not weeks, maintaining the colony through until springtime
  • if there are lots of Varroa present during the early autumn (when winter bees are being reared) the developing winter bees will have high levels of DWV
  • some bees will die before emergence, but those that don’t will instead die in weeks, not months
  • consequently the winter cluster shrinks rapidly in size, becomes unable to thermoregulate and is separated from its stores
  • this doesn’t end well … the colony either dies, or struggles through to the spring and is too weak to expand

But what happens to foragers with high levels of DWV in the summer?

Studies from my lab 10 have shown that pupae injected with DWV – essentially recapitulating what Varroa does when it feeds on a developing pupa – have three potential fates; they either die during development (~15%; see B in the figure below), emerge with developmental abnormalities (~65%; the deformed wing bit which ’does what it says on the tin’) or emerge and appear ‘normal’ (~20%).

Unanswered questions

Interestingly, the small proportion of bees that appear ‘normal’ have indistinguishable levels of DWV to those that have deformed wings (panel A below).

The fate of bees injected with DWV

Do these bees with high-DWV levels live long enough to become foragers?

We don’t know.

If they do become foragers – which , frankly, I doubt – do they learn how to forage well?

Again, we don’t know, though we do know from other studies that high levels of DWV leads to some cognitive impairment, so navigation at least may well be suspect 11.

Finally, if they do become foragers, do they live a long and healthy life, or do they die prematurely?

Unfortunately … we don’t know this either.

Hive inspections

That’s a lot of ‘ifs’ there … and just as many unanswered questions.

Let’s assume that bees with high levels of DWV can mature to become foragers, but that they exhibit reduced longevity. If that is the case then the elite forager population would be reduced, so jeopardising provisioning the colony with nectar and pollen.

I’m sceptical that bees with such high DWV levels can survive long enough to mature into elite foragers. Nevertheless, I’d prefer to test this experimentally in the comfort of my lab 12, rather than in my honey-production or queen-rearing colonies.

Therefore, during hive inspections I look carefully for the signs of overt DWV disease or varroosis – bees with deformed wings, uncapped developing brood or phoretic mites. I also periodically measure mite drop. If I see see problems (and with correct timing and appropriate treatment in autumn and winter you shouldn’t 13 ) I intervene.

Midseason mite management may save the elite foragers … and help prevent the loss of the colony overwinter.

A gradation of DWV levels

However, there’s a related – more subtle – thing to consider.

Studies from our group (and others) have shown that injection of tiny amounts of DWV results in a very rapid replication of DWV to stratospherically high levels. As shown above, these kill or maim ~80% of exposed bees.

But there’s a less well understood feature of colonies with high Varroa levels. During the course of the season the levels of DWV in bees not exposed to Varroa during development rise.

In March or April, DWV levels may be ~103/bee 14. This is about the lowest level we ever see in bees, and is equivalent to the levels of DWV present in colonies from Varroa-free regions like Colonsay.

However, by mid- or late-summer the levels are 100-1000 times higher i.e. 105-106/bee 15. This is still 10,000 times lower than the levels DWV reaches in Varroa-parasitised pupae 16.

As an aside, we don’t formally understand how the DWV levels increase during the season. I suspect it’s through trophallaxis though we have also published some evidence of larval susceptibility to DWV during feeding.

Whatever the mechanism, bees carrying one million copies of DWV look completely normal and, as far as we can tell, behave completely normally.

‘As far as we can tell’, as no one has really done the right experiments …

I think it would be very interesting to carefully investigate the longevity and ability to achieve elite forager status for early season (very low DWV levels) and midsummer (intermediate DWV levels) bees.

Perhaps these intermediate levels of DWV are damaging after all?


The Klein et al., paper has been very poorly proofread 17 and contains several errors, some of which potentially change the meaning of the text.

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 …