Category Archives: Science

A no competition, competition

Unless you’re in an unseasonably warm part of the country, mid-April is usually early enough to put out your bait hives. This year, because of the unusually cold snap in the last week or so, it might still be a bit early. However, colonies are developing well and as soon as the weather properly warms up they will start thinking about swarming.

Regular readers, look away now

I’ve written a lot about bait hives in previous years. Anyone who assiduously follows this site and – unlike me 😉 – remembers what’s been written before can skip ahead to the next section.

But for those who need an aide memoire

The purpose of a bait hive is to attract a swarm that you (surely not?) or someone else has temporarily misplaced i.e. lost 1. When a colony swarms it settles in a temporary bivouac from which the scout bees fly to survey the area for a suitable new nest site.

The two stage process of swarming

The scout bees have very particular requirements.

They’re looking for cavities of about 40 litres volume with a small, clearly visible, south facing, entrance near the base.

Evolution and fussy house hunters

Bees have evolved to nest in trees – not quaint cartoon churches as shown above – and cavities in trees come in all shapes and sizes.

This is why they don’t care what shape the cavity is. However, cavities with small entrances are easier to defend, which is why they prefer them 2.

In addition, bees favour cavities situated more than 5 metres above ground level. Again, this makes evolutionary sense. It’s not just Winnie the Pooh that likes honey. The higher up a tree they nest, the less likely they would be detected by a bear 3 on the ground. And if the nest remains undetected (or unreachable by a climbing bear) there’s a chance the colony will thrive and reproduce (swarm) to pass on the ‘high altitude’ nest site preference gene.

And if you’ve evolved to nest in a tree cavity in a wood filled with other trees, it again makes evolutionary sense for the entrance to be clearly visible. If it wasn’t, bees on their orientation flights would inevitably get confused (and therefore lost).

Finally, bees have a strong preference for cavities that smell … of bees.

A cavity that’s already heavily propolised, or contains used drawn comb, offers distinct advantages to the incoming swarm. They will have less work to do and so more chance of building up before winter arrives.

The ideal bait hive

And you can reproduce these requirements by offering a used single brood box National hive with a solid floor and an entrance reducing block in place … facing south and situated well off the ground.

I discussed the evolutionary selection pressures that have shaped the preference for a 40 litre box rather than that convenient spare nuc box I’m repeatedly asked about a smaller box a few weeks ago.

In that post I also discussed why I ignore the preference for bait hives located 5 metres above the ground:

  • I want to be able to watch scout bee activity. This is tricky if they’re a long way off the ground 4.
  • It’s a lot safer retrieving a bait hive from a hive stand at knee level than it is when climbing a ladder. I usually move occupied bait hives late in the evening (when the bees are all in residence) and prefer not to do this balanced precariously on top of a ladder.
  • And – though not listed last time – my knee level bait hives are sufficiently successful I don’t need to increase their attractiveness. I don’t doubt they’d be more efficient located at altitude 5 but they work well enough that I don’t  feel the need to risk altitude sickness or a broken leg …

But, what I’ve not really discussed before is the location where bait hives should be sited and the importance of appreciating the ‘competitive‘ aspects of bait hives.

Natural competition

When you place a bait hive in the environment, whether it’s in your garden or the corner of a field or 5 metres up an oak tree 6, you are providing a potential nest site that will be judged in competition with other natural sites in the area.

And ‘the area’ is probably about 25 square kilometres.

If you struggle to visualize that then it’s the area covered by this circle centred on the roof of Fortnum & Mason’s, where there are some hives. London Zoo to Battersea Power Station and the Round Pond to Southwark Bridge … a large area 7.

Fortnum & Mason, 181 Piccadilly, London … scout bee range (in theory at least)

Scout bees survey over 3 km from their nest site, though swarms rarely relocate that far 8.

Why don’t they move ‘that far’?

Again, evolution may have selected bees that choose not to move away from the environment in which the swarming colony has flourished and built up strongly enough to be able to swarm.

Scout bees find nests, they don’t survey the available forage around those nests. So it makes sense to stay in the general area where forage is proven to be good enough (to allow swarming).

However, I suspect a compelling reason that swarms don’t move far from their original nest site is that there are plenty of alternative nest sites available.

Church towers 9, roof spaces, chimneys, tree cavities 10, compost bins, abandoned sheds etc.

Choices, choices

Think about the environment near your hives. Whether urban or rural, there are bound to be thousands of potential cavities within 3 km.

Some will be too small, some will be poorly defendable 11 and some will be unsuitable for other reasons.

But there are very likely to be some that are ideal, or pretty close to it.

Mature woodland and older man made environments are likely to have ample choices.

Occupied bait hive

Occupied bait hive …

And then there’s your lonely bait hive.

Chance in a million?

How can it possibly compete with all those natural cavities in the environment?

Bait hive ...

Bait hive …

The first thing to do is to ensure it adheres as close as is practically possible (and safely achievable) to the idealised requirements determined by Martin Lindauer, Thomas Seeley and others.

  • a 40 litre cavity = National brood box 
  • a small entrance of 10-15cm2 = entrance block, solid floor
  • south facing
  • shaded but in full view
  • over 5m above ground level 12
  • smelling of bees = one old, dark comb against the sidewall (no stores!)

Secondly, locate it within ~500 metres of your own apiary (to hopefully re-capture your own ‘lost’ swarms 13 ) or, more speculatively, anywhere in an environment in which there are other managed or feral colonies.

Which does not mean over the fence from another beekeeper’s apiary!

Be courteous … don’t poach 🙂

The density of bees throughout much of the UK is very high. Look at Beebase to see the numbers of apiaries within 10 km of your own. When I lived in Warwickshire it was ~180-220, in Fife it was ~35-40 14.

In the talks I’ve given on bait hives this winter – where I customise the presentation to the audience location – few areas with active BKAs have under 100 apiaries within 10 km of their teaching apiary.

In both Fife or Warwickshire I never failed to attract swarms to bait hives in my garden every single year … and in several years up to three swarms to a single bait hive location.

And, with one or two exceptions, these weren’t swarms I had lost 15.

The density of managed colonies in the UK means that a suitable bait hive just about anywhere stands a chance of being occupied.

So, that’s how to win the competition with the natural nest sites that are available.

No competition

But do not put out multiple bait hives in one area.

I have recently re-read an old paper by Thomas Seeley and Kirk Visscher on quorum sensing by scout bees. Quorum sensing is a term for a decision making process where enough bees agree on the same choice, rather than the majority.

Seeley and Visscher (2004) Quorum sensing experiment

Like many good experiments it has an elegant simplicity.

They reasoned that if you provided a bivouacked swarm with a choice of suitable nest sites it would reduce the numbers of scouts that favoured each nest site, and in doing so, would increase the time to reach a decision as to which was best.

And it does.

More potential nest sites leads to an increase in time taken to reach a decision

Unsurprisingly, with more nest box sites to choose between, the scout bees per box were reduced in number (top panel), dancing to advertise preferred nest sites was delayed (second panel), and piping – the ‘prepare for take-off’ signal (third panel) for the bivouacked swarm – was also delayed.

I’ll discuss how this favours a quorum sensing mechanism (and some other aspects of the study) if and when I get time in the future.

For the moment the key take home message is ‘more choice = slower decision making’ by the swarm.

And, if you delay the decision making, there’s a chance it’ll start raining, or the swarm will be collected by another beekeeper … or they’ll opt to move into the old tower of that quaint cartoon church.

One area?

I started the last subsection with the sentence ‘But don’t put out multiple bait hives in one area’.

What is one area?

I was being deliberately vague because I don’t know the answer.

Since I don’t know where the bees might come from 16, I don’t know what’s within range of the bivouacked swarm.

Widely separated bait hives (black) are likely to be within reach of more swarms than clustered bait hives (white)

In practical terms this means I space my bait hives at least 500 metres apart. Widely separated bait hives are likely to be within reach of more swarms than clustered bait hives.

More importantly, clustered bait hives are likely to lead to competition between scout bees from the same swarm, resulting in reduced scout bee attention..

Until recently I’ve not kept bees in my garden. I would always place a bait hive in the garden and one near my out apiaries. With permission, I’d locate them in other places as well.

Having moved, I now have much more space and have bees in the ‘garden’. When my bait hives go out 17 they will be placed in likely spots on opposite sides of our bit of scrubby wooded hillside 18 .

But what’s a likely spot?

Ley lines

And if you thought that last bit was slightly vague … brace yourself.

Over the years I’ve noticed that some bait hive locations are much more successful than others.

Under offer ...

Under offer …

My tiny courtyard garden in Fife was a magnet for swarms. I placed a bait hive in a warm corner of the garden on the day we moved in, and within 10 days a swarm had arrived.

Planting tray roof …

Every year, without fail, multiple swarms would occupy bait hives 19 in that corner of the garden. I even had two swarms competing for one bait hive in 2019.

A sheltered south-east facing hedgerow in Warwickshire was equally effective.

Bait hive

Smelling faintly of propolis and unmet promises

As was a south-west facing spot sheltered next to my greenhouse in a previous garden.

Des Res?

Des Res?

But other locations have been far less successful.

Of course, this is a positive reinforcement exercise. I’m more likely to site a bait hive in a location I’ve previously been successful in.

But what else might account for this differential success rate? And can it be exploited in the rational location of bait hives?

Is it, as some suggest, that bait hives work best when they are located at the intersection of ley lines? This, and the possibility of creating Varroa-resistant bees by exploiting geopathic stress lines, surely deserves a post of its own 20.

Call me sceptical

However, as a scientist – and knowing others have been more than a little sceptical about the existence of ley lines – I think there’s a more prosaic explanation.

Without exception, my most successful sites for bait hives have been well sheltered to the north, and – in most cases – to the north-east and north-west directions as well.

For example, the bait hive is situated on the south face of a wall running east-west (Under offer, above), or in a corner sheltered to the north and east (Planting tray roof, above), or facing south-east in a very dense hedgerow running north-east to south-west (Smelling faintly etc., above), or sheltered by surrounding walls or outhouses but with a clear entrance facing south-west (Des res?, above).

And … since I’ve been aware of this for at least five years, and probably subconsciously aware of it for much longer, that’s exactly the type of location I choose to site my bait hives.

Which is, of course, another example of positive reinforcement 🙂

However, it works for me. I choose sites that are well sheltered to the sides and back of the bait hive, and I try and orientate the bait hive to face south (ish).

At knee level 😉

Give it a try.


 

Quick thinking & second thoughts

I gave my last talk of the winter season on Tuesday to a lovely group at Chalfont Beekeepers Society. The talk 1 was all about nest site selection and how we can exploit it when setting out bait hives to capture swarms.

It’s an enjoyable talk 2 as it includes a mix of science, DIY and practical beekeeping.

Nest sites, bait hives and evolution

The science would be familiar to anyone who has read Honeybee Democracy by Thomas Seeley. This describes his studies of the features considered important by the scout bees in their search for a new nest site 3.

Under offer ...

Under offer …

The most important of these are:

  • a 40 litre cavity (shape unimportant)
  • a small entrance of 10-15cm2
  • south facing
  • shaded but in full view
  • over 5m above ground level
  • smelling of bees

All of which can easily be replicated using a National brood box with a solid floor. Or two stacked supers.

And – before you ask – a spare nuc box is too small to be optimal.

That doesn’t mean it won’t work as a bait hive, just that it won’t work as well as one with a volume of 40 litres 4.

Evolution has shaped the nest site selection process of honey bees. They have evolved to preferentially occupy cavities of about 40 litres.

Presumably, colonies choosing to occupy a smaller space (or those that didn’t choose a larger space 5 ) were restricted in the amount of brood they could raise, the consequent strength of the colony and the weight of stores they could lay down for the winter.

Get these things wrong and it doesn’t end well 🙁

A swarm occupying a nuc box-sized cavity would either outgrow it before the end of the season, potentially triggering another round of swarming, or fail to store sufficient honey.

Or both.

Over thousands of colonies and thousands of years, swarms from colonies with genetics that chose smaller cavities would tend to do less well. In good years they might do OK, but in bad winters they would inevitably perish.

Bait hive compromises

If you set out a nuc box as a bait hive, you’re probably not intending to leave the swarm in that box.

But the bees don’t know that. Their choices have been crafted over millenia to give them the best chance of survival.

All other things being equal they are less likely to occupy a nuc box than a National brood box.

Another day, another bait hive, another swarm …

For this reason I don’t use nuc boxes as bait hives.

However, I don’t recapitulate all the features the scout bees look for in a ‘des res’.

I studiously ignore the fact that bees prefer to occupy nest sites that are more than 5 metres above ground level.

This is a pragmatic compromise I’m prepared to make for reasons of convenience, safety and enjoyment.

Bees have probably evolved to favour nest sites more than 5 metres above ground level to avoid attention from bears. The fact that there are no bears in Britain, and haven’t been since the Middle Ages 6, is irrelevant.

The preference for high altitude nest sites was ‘baked into’ the genetics of honey bees over the millenia before we hunted bears 7 to extinction.

However, I ignore it for the following reasons:

  • convenience – I usually move occupied bait hives within 48 hours of a swarm arriving. It’s easier to do this from a knee height hive stand than from a roof ladder.
  • safety – I often move the bait hive late in the evening. Rather than risk disturbing a virgin queen on her mating or orientation flights (assuming it’s a cast that has occupied the bait hive) I move them late in the day. In the ‘bad old days’ when I often didn’t return from the office until late, this was sometimes in the semi-dark. Easy and safe to do at knee height … appreciably less so at the top of a ladder.
  • enjoyment – I can see the scout bees going about their business at a hive near ground level without having to get the binoculars out. Their behaviour is fascinating. If you’ve not watched them I thoroughly recommend it.

Scout bee activity

The swarming of honey bees is a biphasic process. In the first phase the colony swarms and forms a temporary bivouac nearby to the original nest site.

The two stage process of swarming

The scout bees search an area ~25 km2 around the bivouacked swarm for suitable nest sites. They communicate the quality and location of new nest sites by performing a waggle dance on the surface of the bivouac.

Once sufficient scouts have been convinced of the suitability of one of the identified nest sites the second phase of swarming – the relocation of the swarm – takes place.

Swarm of bees

Swarm of bees

However, logic dictates that the scout bees are likely to have already identified several potential new nest sites, even before the colony swarms and clusters in a bivouac.

There are only a few hundred scout bees in the swarmed colony, perhaps 2-3% of the swarm.

Could just a few hundred scouts both survey the area and reach a quorum decision on the best location within a reasonable length of time?

What’s a reasonable length of time?

The bivouacked swarm contains a significant amount of honey stores (40% by weight) but does not forage. It’s also exposed to the elements. If finding sites and reaching a decision on the best nest site isn’t completed within a few days the swarm may perish.

Which is why I think that scout bees are active well before the colony actually swarms.

Early warning systems

If scout bees are active before a colony swarms they could be expected to find and scrutinize my bait hive(s).

If I see them doing this I’m forewarned that a colony within ~3 km (the radius over which scout bees operate) is potentially making swarm preparations.

Since I’ll always have a bait hive or two within 3 km of my own apiaries I’ll check these hives at the earliest opportunity, looking for recently started queen cells.

Whether they’re my colonies or not, it’s always worth knowing that swarming activity has started. Within a particular geographic area, with similar weather and forage, there’s usually a distinct swarming period.

If it’s not one of my colonies then it soon might be 😉

So, in addition to just having the enjoyment of watching the scout bees at work, a clearly visible – ground level – bait hive provides a useful early warning system that swarming activity has, or soon will, start.

Questions and answers

Although talking about swarms and bait hives is enjoyable, as I’ve written before, the part of the talk I enjoy the most is the question and answer session.

And Tuesday was no exception.

I explained previously that the Q&A sessions are enjoyable and helpful:

Enjoyable, because I’m directly answering a question that was presumably asked because someone wanted or needed to know the answer 8.

Helpful, because over time these will drive the evolution of the talk so that it better explains things for more of the audience.

Actually, there’s another reason in addition to these … it’s a challenge.

A caffeine-fueled Q&A Zoom session

It’s fun to be ‘put on the spot’ and have to come up with a reasonable answer.

Many questions are rather predictable.

That’s not a criticism. It simply reflects the normal range of topics that the audience either feels comfortable asking about, or are interested in. Sometimes even a seemingly ‘left field’ question, when re-phrased, is one for which there is a standard answer. The skill in this instance is deciphering the question and doing the re-phrasing.

But sometimes there are questions that make you think afresh about a topic, or they force you to think about something you’ve never considered before.

And there was one of those on Tuesday which involved biphasic swarming and scout bee activity.

Do all swarms bivouac?

That wasn’t the question, but it’s an abbreviated form of the question.

I think the original wording was something like:

Do all swarms cluster in a bivouac or do some go directly from the original hive/location to the new nest site?

And I didn’t know the answer.

I could have made a trite joke 9 about not observing this because my own colonies swarm so infrequently 🙄

I could have simply answered “I don’t know”.

Brutally honest, 100% accurate and unchallengeable 10.

But it’s an interesting question and it deserved better than that.

So, thinking about it, I gave the following answer.

I didn’t know, but thought it would be unlikely. For a swarm to relocate directly from the original nest site the scout bees would need to have already reached a quorum decision on the best location. To do this they would need to have found the new nest site (which wouldn’t be a problem) and then communicate it to other scout bees, so that they could – in turn – find the site. Since this communication involves the waggle dance it would, by definition, occur within the original hive. Lots of foragers will also be waggle dancing about good patches of pollen and nectar so I thought there would be confusion … perhaps they always need to form a bivouac on which the scout bees can dance? Which explains why I think it’s unlikely.

In a Zoom talk you can’t ponder too long before giving an answer or the audience will assume the internet has crashed and they’ll drift off to make tea 11.

An attentive beekeeping audience … I’d better think fast or look stupid

You therefore tend to mentally throw together a few relevant facts and assemble a reasonable answer quite quickly.

And then you spend the rest of the week thinking about it in more detail …

Second thoughts

I still don’t know the answer to the question Do all swarms bivouac?”, but I now realise my answer made some assumptions which might be wrong.

I’ll come to these in a minute, but first let me address the question again with the help of the people who actually did the work.

I’ve briefly looked back through the relevant literature by Seeley and Lindauer and cannot find any mention of swarms relocating without going via a bivouac. I may well have missed something, it wouldn’t be the first time 12.

However, their studies are a little self-selecting and may have overlooked swarms that behaved like this.

Both were primarily interested in the waggle dance and the decision making process, they therefore needed to be able to observe it … most easily this is on the surface of the bivouac.

Martin Lindauer mainly studied colonies that had naturally swarmed, naming them after the location of the bivouac, and then studied the waggle dancing on the surface of the clustered swarm. In contrast, Tom Seeley created swarms by caging the queen and adding thousands of very well fed bees.

Absence of evidence is not evidence of absence.

So, what were the assumptions I made?

There were two and they both relate to confusion between waggle dancing foragers and scout bees.

  1. Swarming usually occurs during a strong nectar flow. Therefore there are likely to be lots of waggle dancing foragers in the hive at the same time the scouts are trying to persuade each other – using their own fundamentally similar – waggle dances.
  2. Bees ‘watching’ are unable to distinguish between scouts bees and foragers.

So, what’s wrong with these assumptions?

A noisy, smelly dance floor

Foragers perform the waggle dance on the ‘dance floor’. This is an area of vertical comb near the hive entrance. It’s position is not fixed and can move – further into the hive if the weather is cold, or even out onto a landing board (outside the hive) in very hot weather 13.

So, although the dance floor occupied by foragers isn’t immovable, it is defined. There’s lots of other regions of the comb that scouts could use for their communication i.e. there could be spatial separation between the forager and scout bee waggle dances.

Secondly, foragers provide both directional and olfactory clues about the identity and location of good sources of pollen and nectar. In addition to two alkanes and two alkenes produced by dancing foragers 14 they also carry back scents “acquired from the environment at or en route to the floral food source” which are presumed to aid foragers recruited by the waggle dancer to pinpoint the food source.

Importantly, non-dancing returning foragers do not produce these alkanes and alkenes. Perhaps the dancing scouts don’t either?

A dancing scout would also lack specific scents from a food source.

Therefore, at least theoretically, there’s probably a good chance that scout bees could communicate within the hive. Using spatially distant dances and a unique combination of olfactory clues (or their absence) scouts may well be able to recruit other scouts to check likely new nest sites.

All of which would support my view that bait hives provide a useful early warning system for colonies that are in the very earliest stages of swarm preparations … rather than just an indicator that there’s a bivouacked swarm in the vicinity.

But?

All this of course then begs the question … if the scout bees can communicate within the hive, why does the swarm need to bivouac at all?

The bivouac must be a risky stage in the already precarious process of swarming. 80% of wild swarms perish. At the very least it’s subject to the vagaries of the weather. Surely it would be advantageous to stay within the warm, dry hive until a new nest site is identified?

Apple blossom ...

Apple blossom … and signs that a bivouacked swarm perished here

This suggests to me that the bivouac serves additional purposes within the swarming process. A couple of possibilities come to mind:

  • the gravity-independent, sun-orientated waggle dancing 15 on the surface of the bivouac may be a key part of the decision making process, not possible (for reasons that are unclear to me) within the confines of the hive.
  • the bivouac acts to temporally coordinate the swarm. A swarm takes quite a long time to settle at the bivouac. Many bees leave the hive during the excitement of swarming but not all settle in the bivouac. Perhaps it acts as a sorting mechanism to bring together all the bees that are going to relocate, separate from those remaining in the swarmed colony?

Clearly this requires a bit more thought and research.

If your association invites me to discuss swarms and bait hives next winter I might even have an answer.

But, as with so many things to do with bees, knowing that answer will only spawn additional questions 😉


 

Frequently asked questions

The 2020/21 winter has been very busy with online talks to beekeeping associations. I’m averaging about five a month, with only the fortnight over Christmas and New Year being a bit quieter. 

When chatting to the organisers of these talks it’s clear that they are getting increasingly successful 1. Audience numbers are encouragingly high as people become more familiar with online presentations.

Beekeepers know they can lounge around in their pyjamas drinking wine, chat with their friends before and after the talk 2, and listen to a beekeeping presentation … a sort of lockdown multitasking.

Some of you that spend hours each day on Zoom will know exactly what I’m talking about 😉

I still lament the absence of homemade cakes, but I suspect the online format is here to stay. At least for some associations, or at least some of the winter programme each year. 

Talking to myself

There’s little point in doing science unless you tell others about it and, as as a scientist, I have presented at invited seminars and conferences for my entire career. 

Some readers will be familiar with public speaking in one form or another. They’ll be familiar with the frisson of excitement that precedes stepping up to the podium in a large auditorium. 

Assuming there’s a large audience filling the large auditorium of course 😉

Those with little experience of speaking might wish the audience was a bit smaller, or a lot smaller … or not there at all.

But the reality is that the audience is a really important part of a presentation. At least, they are once the speaker has sufficient confidence to calm down, to stop worrying they’ll say something stupid, and to ‘read’ the audience. 

An attentive beekeeping audience

By observing the audience the speaker can determine whether they’re still interested and attentive. Not just in the topic (after all, they’re sitting there rather than disappearing to the coffee shop), but in particular parts of the presentation. 

Are you going too fast?

Have you lost their attention?

Was that fancy animated slide you spent 20 minutes on a dismal failure?

Did that last witty aside work … or did it crash and burn? 3

Almost none of which can be determined when delivering a Zoom-type online presentation 🙁

You can ‘see’ the audience.

Or parts of it.

Postage stamp-sized headshots, with poor lighting, distracting backgrounds 4 and enough pixelation to make nuanced judgements about boredom or even species sometimes tricky.

Is that a Labradoodle in the audience … or just another lockdown haircut?

Has the internet frozen … or has everyone simply fallen asleep?

It’s not ideal, but it’s the best we’ve got for now.

Which makes the question and answer sessions even more important than usual.

Mixed abilities

My talks usually include a 5 minute intermission. Talking for an hour uninterrupted is actually quite tiring 5 and it’s good to make a cup of tea and gather my thoughts for ’round two’.

It also allows the audience to raise questions about subjects mentioned in the first half that left them confused.

Fortunately these ‘half time’ questions tend to be reassuringly limited in number 6.

Have a break, have a Kit Kat

However, at the end of the talk there is usually a much more extensive Q&A session. This often covers both the topic of the talk and other beekeeping issues. 

A typical audience contains beekeepers with a wide range of beekeeping experience. Enthusiastic beginners 7 jostle for screen space with ‘been there, done that, bought the T-shirt’ types who have forgotten more than I’ll ever know.

Inevitably this means the talk might miss critical explanations for beginners and omit some of the nuanced details appreciated by the more experienced. As the poet John Lydgate said:

You can please some of the people all of the time, you can please all of the people some of the time, but you can’t please all of the people all of the time 8.

Think about this simple statement:

Varroa feed on the haemolymph of developing pupae.”

The beginner might not know what haemolymph is … or, possibly, even what Varroa is.

The intermediate beekeeper might be left wondering whether the mite also feeds on nurse bees when ‘crowdsurfing’ around the colony during the phoretic stage of the life cycle.

And the experienced beekeeper is questioning whether I know anything about the subject at all as I’ve not mentioned fat bodies and their apparently critical role in mite nourishment.

So I encourage questions … to help please a few more of the people 😉

You’re on mute!

In my experience these are best submitted via the ‘chat’ function. The host – an officer of the BKA or a technically-savvy member press ganged into hosting the talk – can then read them out to me.

Or I can … if I can find my glasses.

One or two beekeeping associations have a Zoom ‘add in’ that allows the audience to ‘upvote’ written questions, so that the most popular appear at the top of the list 9. This works really well and helps ‘please more of the people more of the time’.

The alternative, of asking the audience member to unmute their mic and ask the question is somewhat less satisfactory. It’s not unusual to watch someone wordlessly ‘mouthing’ the question while the host (or I) try and explain how to turn the microphone on.

Finally, it’s worth emphasising that the Q&A session is – as far as I’m concerned – one of the most helpful and enjoyable parts of the evening.

Enjoyable, because I’m directly answering a question that was presumably asked because someone wanted or needed to know the answer 10

Helpful, because over time these will drive the evolution of the talk so that it better explains things for more of the audience.

Anyway – that was a longer introduction than I intended – what sort of questions have been asked frequently this winter (and the talks they usually appeared in).

What do you define as a strong colony? (Preparing for winter)

Strong colonies overwinter better than weak colonies. They contain more bees. This means that the natural attrition rate of bees during the winter shouldn’t reduce the colony size so much that it struggles to thermoregulate the cluster

Midwinter cluster

A strong colony in midwinter

I also think large winter clusters retain better ‘contact’ with their stores, so reducing the chances of overwinter isolation starvation.

Strong colonies are also likely to be healthy colonies. Since the major cause of overwintering colony losses is Varroa and the viruses it transmits, a strong healthy colony should overwinter better than a weak unhealthy colony. 

Colony age structure from August to December.

However, you cannot necessarily judge the strength of a colony in June/July as an indicator of colony strength in the late autumn and winter.

This is because the entire population of bees has turned over during that period. 

A hive bulging with bees in summer might look severely depleted by November if the mite levels have not been controlled in the intervening period.

The phrase ‘a strong colony’ is also relative … and influenced by the strain of bees. Native black bees rarely need more than a single brood box. Compare them to a prolific carniolan strain and they’re likely to look ‘weak’, but if they’re filling the single brood box then they’re doing just fine.

When should I do X? (Rational Varroa control and others)

When usually means ‘what date?’

X can be anything … adding Apivar strips, uniting colonies, adding supers, dribbling oxalic acid.

This is one of the least satisfactory questions to answer but the most important beekeeping lesson to learn.

A calendar is essentially irrelevant in beekeeping.

Due to geographic/climatic differences and variation in the weather from year to year, there’s almost nothing that can be planned using a calendar.

Only three things matter, the:

  1. state of the colony
  2. local environment – an early spring, a strong nectar flow, late season forage etc.
  3. development cycle of queens, workers and drones

By judging the first of these, with knowledge of the second and a good appreciation of the third, you can usually work out whether treatments are needed, colonies united or supers added etc.

This isn’t easy, but it’s well worth investing time and effort in.

Honey bee development

Honey bee development

The last of these three things is particularly important during swarm control and when trying to judge whether (or when) a colony will be broodless or not. The development cycle of bees is effectively invariant 11, so understanding this allows you to make all sorts of judgements about when to do things. 

For example, knowing the numbers of days a developing worker is an egg, larva and pupa allows you to determine whether the colony is building up (more eggs being laid than pupae emerging) or winding down for autumn (or due to lack of forage or a failing queen).

Likewise, understanding queen cell development means you know the day she will emerge, from which you can predict (with a little bit of weather-awareness) when she will mate and start laying.

How frequently should you monitor Varroa? (Rational Varroa control)

This question regularly occurs after discussion of problematically high Varroa loads, particularly when considering whether midseason mite treatment is needed. 

Do you need to formally count the mite dropped between every visit to the apiary?

Absolutely not.

If you are the sort that does then be aware it’s taking valuable time away from your trainspotting 😉 12

The phoretic mite drop is no more than a guide to the Varroa load in the hive. 

Think about the things that could influence it:

  • A colony trapped in the hive by bad weather has probably got more time to groom, so resulting in an increased mite drop.
  • An expanding colony has excess late stage larvae so reducing the time mites spend living phoretically.
  • A shrinking colony will have fewer young bees, so forcing mites to parasitise older workers. Some of these will lost ‘in the field’ and more may be lost through grooming.
  • Strong colonies could have a much lower percentage infestation, but a higher mite drop than an infested weak colony. You need to act on the latter but perhaps not the former.
  • And a multitude of other things that really deserve a more complete post …

So don’t bother counting Varroa every week … or even every month.

Does what it says on the tin.

I think checking a couple of times a season – towards the end of spring and in mid/late summer – should be sufficient. You can do this by inserting a Varroa tray for a week, by uncapping drone brood and looking for mites, or by doing an alcohol wash on a cupful of workers (but these methods aren’t comparable with each other as they measure different things with different efficiencies). 

But you must also look for the damaging effects of Varroa and viruses at every inspection.

If there are significant numbers of bees with deformed wings – characteristic of high levels of deformed wing virus (DWV) – then intervention will probably be needed. 

DWV symptoms

DWV symptoms

And if there are increasing numbers of afflicted bees since your last regular inspection it’s almost certain that intervention will be needed sooner rather than later.

I should add that I also count mite drop during treatment. This helps me understand the overall mite load in the colony. By reference to the late summer count I can be sure that the treatment worked. 

What do you mean by a quarantine apiary? (Bait hives for profit and pleasure)

This question has popped up a few times when I discuss moving an occupied bait hive and checking the health of the colony. 

A swarm that moves into a bait hive brings lots of things with it …

Up to 40% by weight is honey which is very welcome as they will use it to draw new comb. If there’s good forage available as well it’s unlikely the swarm will need additional feeding.

However, the swarm also brings with it ~35% of the mites that were present in the colony that swarmed. These are less welcome.

I always treat swarms with oxalic acid to give them the best possible start in their new home.

Varroa treatment of a new swarm in a bait hive…

More worrisome is the potential presence of either American or European foul broods. Both can be spread with swarms. The last things you want is to introduce these brood diseases into your main apiary.

For this reason it is important to isolate swarms of unknown provenance. The logical way to do this is to re-site the occupied bait hive to a quarantine apiary some distance away from other bees. Leave it there for 1-2 brood cycles and observe the health and quality of the bees.

What is ‘some distance?’

Ideally further than bees routinely forage, drift or rob. Realistically this is unlikely to be achievable in many parts of the country. However, even a few hundred yards away is better than sharing the same hive stand. 

If you keep bees in areas where foul broods are prevalent then I would argue that this type of precautionary measure is essential … or that the risk of collecting swarms is too great.

And how do you know if foul broods are prevalent in your area?

Register with the National Bee Unit’s Beebase. If there is an outbreak near your apiary a bee inspector will contact you.

Remember also that the presence of foul broods in an area may mean that the movement of colonies is prohibited.

‘Asking for a friend’ type questions

These are great.

These are the sort of questions that all beekeepers are likely to need to ask at sometime in their beekeeping ‘career’.

Typically they take the form of two parts:

  1. a description of a gross beekeeping error
  2. an attempt to make it clear that the error was by someone (anyone) other than the person asking the question 😉

Here are a couple of more or less typical ones 13.

  • My friend (who isn’t here tonight) forgot to remove the queen excluder and three full supers from their colony in August. Should I, oops, she remove them now?
  • Here’s an an entirely hypothetical scenario … what would you recommend treating a colony with in March if the autumn and midwinter mite treatments were overlooked?
  • Should my friend remove the Apiguard trays he a) added in November, or b) placed in his colonies before taking them to the heather?
  • I’d been advised by an expert beekeeper to squish every queen cell a few days after discovering my colony had swarmed in June. It’s now late September … how much longer should I wait for the colony to be queenright?

These are very good questions because they illustrate the sorts of mistakes that many beginners, and some more experienced beekeepers, make. 

There’s absolutely nothing wrong with making mistakes. The problem comes if you don’t learn from them.

I’ve made some cataclysmically stupid beekeeping errors. 

I still do … though fewer now than a decade ago, largely because I’ve managed to learn from some of them.

Partly I learned from thinking things through and partly from asking someone else … “A friend has asked me why his colony died. Was it the piezoelectric vibrations from the mite ‘zapper’ bought from eBay or was the hive he bought not suitable?


 

Going the distance

I’m going to continue with a topic related to the waggle dance this week.

This is partly so I can write about the science of how bees measure distance to a food source.

But it’s also to encourage those who didn’t read the waggle dance post to visit it. Weirdly it was only read by about 50% of the usual Friday/weekend readership and I suspect (from a couple of emails I received) that the weekly post to subscribers ended up in spam folders 1.

If you remember, the duration of the waggle phase of the dance – the straight-line abdomen-wiggling sashay across the ‘dance floor’ – indicates the distance from the nest to the desirable food source 2. The vigour of the wiggle indicates the quality of the source.

How do bees measure distance?

Karl von Frisch, the first to decode the waggle dance, favoured the so-called ‘energy hypothesis’. In this, the distance to a food source was determined by the amount of energy used on the outbound flight.

Does that seem logical?

Foragers forage randomly, but usually return directly

If correct, foragers would only be able to determine the energy used after their second trip to a food source. This presumes their first trip was longer as they searched the environment for something worth dancing about 3.

This would be an easy thing to test, though I’m not sure it was ever investigated 4.

As it happens, far better brains determined that the energy hypothesis was probably incorrect. Many of these studies explored how gravity influences the distances reported by dancing foragers.

Going up!

Bees use more energy when flying up. For example, when flying from ground level to the top of a tall building, when compared to level flight. Similarly, they use more energy flying if they have small weights attached to them 5.

A series of experiments, nicely reviewed by Harald Esch and John Burns 6, failed to provide good support for the energy hypothesis. There were lots of these studies, involving steep mountains, tall buildings or balloons, between the 1950’s and mid-80’s.

Interesting science, and no doubt it was a lot of fun doing the experiments.

For example, bees flying to a sugar feeder situated on top of a tall building dance to ‘report’ the same distance as bees from the same hive flying to a feeder at ground level adjacent to the same building.

Similarly, foragers loaded with weights do not overestimate the distance to a food source, as would be expected if the energy expended to reach it was being measured 7.

Interesting and entertaining science certainly, but none of it providing compelling support for the energy hypothesis

It’s notable that there is a rather telling sentence from the Esch & Burns review that states “While reading the original papers, one gains the impression that evidence supporting the energy hypothesis was favored over arguments against it”.

Ouch!

Splash landing

Although Von Frisch was a supporter of the energy hypothesis 8 he also published a study that provided evidence for our current understanding of how bees measure distance.

Bees generally don’t like flying long distances over water. Von Frisch provided two equidistant nectar sources, one of which was situated on the other side of a lake.

Bees flying over calm water underestimate distances

On very calm days the bees that flew across the lake under-reported the distance to the feeder. This underestimate was by 20-25% when compared to bees flying to an equidistant feeder overland.

Von Frisch commented “the bee’s estimation of distance is not determined through optical examination of the surface beneath her”.

He assumed that the mirror-like water surface provided no optical input as it contained no visual ‘clues’. After all, one calm patch of water looks much like any other. Von Frisch used this as an argument for the energy hypothesis.

He also noted that the bees generally flew very low over the water surface, often so low that they drowned 🙁

Perhaps these bees were flying dangerously low to try and find optical clues.

Such as their height above the surface?

Or perhaps the distance travelled?

Going with the flow

Having debunked the energy hypothesis, Esch & Burns proposed instead the optic flow hypothesis. This states that “foragers use the retinal image flow of ground motion to gauge feeder distance”.

Imagine optic flow as tripping a little odometer in the bee brain that records distance as her eyes observe the environment flashing past during flight. The clever thing about that is that the environment is variable. It’s not like counting off regularly spaced telegraph poles from a train window.

When flying, environmental objects that are nearby will move across her vision much faster than distant objects. Bees don’t have stereo vision, but instead use this speed of image motion to infer range.

Optic flow – the arrow size indicates the speed with which the object apparently moves, and hence its range

Esch & Burns returned again to tall buildings to provide supporting evidence for their optic flow hypothesis. They trained bees to fly between two tall buildings with 228 metres separating the hive and the feeder 9.

Returning foragers reported that the food source was only 125 metres away.

However, the bees didn’t make a direct flight. Instead they flew at altitude for 30-50 metres, descended to fly much lower, then ascended again to approach the feeder again at altitude.

Esch & Burns experiment to support the optic flow hypothesis

The interpretation here was that the high altitude flight provided insufficient optic flow to measure distance. The bees descend to get the visual input needed to judge distance, but it’s only for part of the flight … hence leading to under-reporting the distance separating the hive and feeder.

Tunnel vision

Jurgen Tautz 10 and colleagues trained bees to forage in a short, narrow tunnel 11. This elegant experiment provided compelling support for the optic flow hypothesis.

The tunnel was ~6 m long and with a cross sectional area of ~200 cm2 – big enough for a bee to fly along, but sufficiently narrow so that the bee would be closer to the ‘walls’ than in normal free flight. The walls and floor of the tunnel had a random visual texture. Only the end of the tunnel facing the hive was open.

The tunnel experiment.

These studies were conducted when the terms round and waggle were used to distinguish the dance induced by food sources <50 m and >50 m respectively from the hive 12. Rather than emphasise the shape of the dance I’ll just describe it as a >50 m or <50 m waggle dance.

‘Tunneling’ bees misreport distances

In the first tunnel experiment (1) the feeder was 35 m from the hive. 85% of dances indicated the feeder was <50 m away. However, when the feeder was moved to the opposite end of the tunnel (2) – still only 41 m from the hive – 90% of the dances indicated the feeder was >50 m away.

To test how the random pattern influenced the perceived distance the scientists used a third tunnel (3) lined with lengthwise stripes. In this instance – despite the feeder position being unchanged from experiment 2 – 90% of the dances indicated the feeder was <50 m away.

The stripes were predicted to ‘work’ in the same way as the smooth lake surface, providing no visual clues.

In the fourth experiment (4) the feeder was 6 m along a randomly patterned tunnel, which was placed just 6 m from the hive. Over 87% of dances indicated that the feeder was >50 m away.

Interpreting the waggle run

In open flight 13 there is usually an excellent correlation between the duration of the waggle run and the distance to a feeder (see the graph below 14 ). By extrapolation, the bees in experiments 2 and 4 ‘thought’ they had flown 230 m and 184 m respectively. In reality they had flown only 41 m and 12 m in these experiments.

Determining distances from waggle dance observation

How could the bees get it so wrong?

Increased optic flow

Tunnel-traversing bees fly just a few centimeters away from the visible ‘environment’.

As a consequence, at the same flight speed, they experience greater optic flow.

If, instead of driving around in your lumbering old van, you pack your hive tool in a Caterham 7 for the trip to the apiary you’d be well aware of what I mean.

Caterham 7 … check out that optic flow … then make another trip to collect the smoker

30 mph in a Toyota Hilux feels very much slower than 30 mph in a Caterham 7. This is largely because visual reference points, like the broken white lines between lanes in the road, appear in and disappear from your field of view much faster … because you’re much closer to them.

Because the tunnel dimensions were known it was possible to calculate the calibration of the bee’s odometer. Classically this would be defined in terms of metres of distance flown generating a particular waggle run length or duration.

These tunnel studies demonstrate that distance flown is not what calibrates the odometer. Instead it’s quantified indirectly in terms of the image motion experienced by the eye. Since environments vary the way to express this is the amount of angular image motion that generates a given duration of waggle.

And, using some mathematical trickery we don’t need to bother with 15, it turns out that this angular motion is only dependent upon distance flown, not the speed of flight.

This is important. Headwinds or tailwinds could change the speed of flight, but not the distance flown 16.

It’s all relative

It’s worth emphasising that the dance followers in experiments 2 (above) should still find the feeder.

The waggle dance would ‘instruct’ them to fly 230 m at the bearing indicated and they’d experience the same visual clues en route.

This means that they should still enter the narrow tunnel and experience increased optic flow because of the encroaching walls. But they’d be experiencing the same optic flow the initial dancing bee had experienced, so would not attempt to fly further down the tunnel.

This means that the optic flow experienced is context dependent. It is related to the environment the bees are foraging in.

This makes sense as the dancing bees and dance followers all occupy the same environment.

How do we know this? 17

Changing the environment

If we change the environment the dance followers search at the wrong distance.

I qualified the statement above when I said that the dance followers should still enter the tunnel and find the feeder.

Actually, most recruits will miss the tunnel entrance – remember it’s smaller that a sheet of A5 paper. At 35 m distance a bee would have to get the bearing correct to about 0.16° to enter the tunnel 18.

So the bees that do not enter the tunnel experience a different environment.

Where do they search for the feeder?

They search at the distance indicated by the waggle duration … so bees that missed the tunnel entrance in experiment 2 (above) would have searched for the feeder 230 m from the hive. Similarly, the dance followers in experiment 4 would have searched 184 m away 19

Context dependent dance calibration

And, finally, the calibration of the odometer depends upon the environment.

Odometer calibration depends upon the environment

If the environment experienced by the dancing bee en route to the feeder in experiments 2 and 4 is different, then it generates a different relationship between waggle run duration and distance.

For example, if one feeder was across a closely mown lawn and the other was across dense shrubby woodland, they would each generate a unique optic flow, so changing the image motion experienced, and hence the waggle run generated.

In the diagram above, you shouldn’t use dance calibration for bees trained to direction A to determine the distance bees going in direction B would forage.

Phew!

Optic flow, waggle dancing and implications for practical beekeeping

None 😉

At least, none that I can think of.

A Caterham 7 isn’t an ideal car for a beekeeper but would be a lot of fun to help you understand optic flow 😉

Most of us keep our bees in mixed environments. Your apiary isn’t situated with a cliff edge on one side and an unbroken prairie on the other. Since the environment is mixed, the waggle dance calibration is not going to be wildly different, whichever way the bees fly off in. You can therefore use an approximate figure of 1 second per kilometre to estimate the the distance at which your bees are foraging, irrespective of the direction they go.


Notes

Most of the referenced studies are at least two decades old. Honey bees have remained a fertile research tool for neurobiologists. Our understanding of honey bee vision continues to improve. However, I cannot discuss any of these more recent studies with reference to optic flow. Anyway, just because they’re old doesn’t make the experiments any less elegant or interesting 🙂

 

The waggle dance

Ask a non-beekeeper what they know about bees and you’ll probably get answers that involve honey or stings.

Press them a little bit more about what they know about other than honey and stings and some will mention the ‘waggle dance’. 

Karl von Frisch

That the waggle dance is such a well-known feature of honey bee biology is probably explained by two (related) things; it involves a relatively complex form of communication in a non-human animal, and because Karl von Frisch – the scientist who decoded the waggle dance – received the Nobel Prize 1 for his studies in 1973.

Von Frisch did not discover the waggle dance. Nicholas Unhoch described the dance at least a century before Von Frisch decoded the movement, and Ernst Spitzner – 35 years earlier still – observed dancing bees and suggested they were communicating odours of food resources available in the environment.

Inevitably, Aristotle also made a contribution. He described flower constancy 2 and suggested that foragers could communicate this to other bees.

Language and communication are important. The development of language in early humans almost certainly contributed to the evolution of our culture, society and technology. Communication in non-human animals, from the chirping of grasshoppers to the singing of whales, is of interest to scientists and non-scientists alike.

It is therefore unsurprising that the ‘dance language’ of honey bees is also of great interest. Although not a ‘language’ in the true sense of the word, Von Frisch described the symbolic language of bees as “the most astounding example of non-primate communication that we know” over 50 years ago. This still applies.

The waggle dance

The waggle dance usually takes place in the dark on the vertical face of a comb in the brood nest, usually close to the nest entrance. The dance is performed by a successful forager i.e. one that has located a good source of pollen, nectar or water, and provides information on the presence, the quality, identity, direction and distance of the source, so enabling nest-mates to find and exploit it.

The dance consists of two phases:

  1. The figure of eight-shaped ‘return phase’ in which the bee circles back, alternately clockwise and anticlockwise, to the start of …
  2. The ‘waggle phase’, which is a short linear run in which the dancer vigorously waggles her abdomen from side to side.

The direction of the food source is indicated by the angle of the waggle phase from gravity i.e. a vertical line down the face of the comb. This angle (α in the figure below) indicates the bearing from the direction of the sun that needs to be followed to reach the food source. 

For example, if the dancer performs a waggle phase vertically down the face of the comb, the food source must be opposite the current position of the sun.

The waggle dance

The distance information is conveyed by the duration of the waggle phase. The longer this run is, the more distant the source. A run of 1 second duration indicates the food source is about 1 kilometre away.

The quality of the food source is indicated by the vigour of the waggling during the waggle phase and the speed with which the return phase is conducted. 

Surely it can’t be that simple?

Yes, it can.

What I’ve described above allows you to interpret the waggle dance sufficiently well to know where your bees are foraging.

Next time you lift a frame from a hive and see a dancing bee, circling around in a little cleared ‘dance floor’ surrounded by a group of attentive workers, try and decode the dance.

Remember that the dance is performed with relation to gravity in the darkened hive. You’re looking to identify the angle from a vertical line up the face of the brood comb to determine the direction from the sun.

Time a few waggle phases (one elephant, two elephants etc.) and you’ll know how far away the food source is.

Really, it’s that simple?

Of course not 😉

The waggle dance was decoded more than half a century ago and remains an active subject for researchers interested in animal communication.

What you’ll miss in your observations is an indication of the type of nectar or pollen resource that the dancing bee is communicating. The dancing worker carries the odour of the food source and may also regurgitate nectar, presumably helping those ‘watching’ (remember, it’s dark … nothing to see here!) determine the type of resource to look for when they leave the hive.

You will also be unable to detect the pulsed thoracic vibrations that the dancing bee produces. These are also indicators of the quality of the food source; better (e.g. higher sucrose content) resources elicit increased pulse duration, velocity amplitude and duty cycle, though the number of pulses is related to the duration of the waggle phase, and so is another potential indicator of distance.

Inevitably, there are also pheromones involved.

There always are 😉

The dancing bee produces two alkanes, tricosane and pentacosane, and two alkenes, Z-(9)-tricosene and Z-(9)-pentacosene. These appear to stimulate foraging activity 3.

But it’s cloudy … or rain stops play … or nighttime

What happens to dancing bees if foraging is interrupted, for example by poor weather or night? 

The dancing bee continues to change the angle of the waggle phase as the sun moves across the sky. This means that a dancing bee will correctly signal the direction to the food source, even if they have not left the hive for several hours.

During their initial orientation flights they learn the sun’s azimuth as a function of the time of day, and use this to compensate for the sun’s time-dependent movement.

Some bees even dance during the night, in which case the watching workers must presumably make their own compensations for the time that has elapsed since the dance 4.

And what happens if the sun is obscured … by clouds, or buildings or dense woodland? How can those directions be followed?

Under these circumstances the foraging bee detects the position of the sun by the pattern of polarised light in the sky. 

Scout bees

The waggle dance is also performed by scout bees on the surface of a bivouacked swarm. In this instance it is used to communicate the quality, direction and distance of a new potential nest site. 

Swarm of bees

Swarm of bees

The intended audience in this instance are other scout bees, rather than the general forager population 5. These scouts use a quorum decision making process to determine the ‘best’ nest site in the area to which the bivouacked swarm eventually relocates.

The shape of the bivouac often lacks a true vertical surface. However, since it’s in the open the dancing bees can orientate the waggle run directly with relation to the sun’s direction, rather than to gravity.

Under experimental conditions the dancing bee can communicate the presence and quality of a food source on a horizontal comb, but – with no reference to gravity – all directional information is lost 6.

The round dance

The duration of the waggle phase is related to the distance from the nest to the food source. Therefore the recognisable waggle dance tends to get difficult to interpret for sources very close to the nest.

It used to be thought that there was a distinct directionless dance (the ’round dance’) for these nearby i.e. 10-40 metres, food sources. However, more recent study 7 suggests that dancers were able to convey both distance and direction information irrespective of the separation of nest and food source. This indicates that bees have just one type of dance for forager recruitment, the waggle dance.

Do all bees communicate using a waggle dance?

There are a very large number of bee species. In the UK alone there are 270 species, 250 of which are solitary.

There’s a clue.

Solitary bees are like me at a disco … they have no one to dance with 🙁

I’ll cut to the chase to help you erase that vision.

The only bees that use the waggle dance are honey bees. These all belong to the genus Apis.

They include our honey bee, the western honey bee (Apis mellifera), together with a further seven species:

  1. Black dwarf honey bee (Apis andreniformis)
  2. Red dwarf honey bee (Apis florea)
  3. Giant honey bee (Apis dorsata)
  4. Himalayan giant honey bee (Apis laboriosa
  5. Eastern honey bee (Apis cerana)
  6. Koschevnikov’s honey bee (Apis koschevnikovi)
  7. Philippine honey bee (Apis nigrocincta)

Dancing and evolution

Dwarf honey bees nest in the open on a branch and dance on the horizontal surface of the nest. The waggle run is orientated ‘towards’ the food source. Apis dorsata is also an open-nesting bee, but forms large vertically-hanging combs. It dances relative to gravity, and indicates the direction by the angle of the waggle run in the same way that A. mellifera does.

The cavity nesting bees, A. cerana, A. mellifera, A. koschevnikovi, and A. nigrocinta produce the most developed form of the dance.

The dances of A. mellifera and A. cerana are sufficiently similar that they can follow and decode the dance of the other.

The complexity of the nest site and the waggle dance reflects the evolution of these bee species. The earliest to evolve (i.e. the most primitive), A. andreniformis and florea, have the simplest nests and the most basic waggle dance. In contrast, the cavity nesting species evolved most recently, form the most complex brood nests and have the most derived waggle dance.

When and why did the waggle dance evolve?

Assuming that the waggle dance did not independently evolve (there’s no evidence it did, and ample evidence due to its similarity between species that it evolved only once) it must have first appeared at least 20 million years ago, when extant honey bee species diverged during the early Miocene.

The ‘why’ it evolved is a bit more difficult to address.

Behavioural changes often arise in response to the environment in which a species evolves.

Bipedalism in non-human primates (like the australopithecines) is hypothesised to have evolved in part due to a reduction in forest cover and the increase in savannah. Apes had to walk further between clumps of trees and bipedalism offered greater travel efficiency.

Perhaps the waggle dance evolved to exploit a particular type or distribution of food reserves?

In this regard it is interesting that the ‘benefit’ of waggle dance communication varies through the season.

If you turn a hive on its side the combs are horizontal 8. Under these conditions the dancing bees can communicate the presence and quality of a food source. However, they cannot communicate its location (either direction or distance).

No directional or distance information is now available

In landmark studies Sherman and Visscher 9 showed that, at certain periods during the season, the absence of this positional information did not affect the weight gain by the hive i.e. the foraging efficiency of the colony.

They concluded that during these periods forage must be sufficiently abundant that simply stimulating foraging was sufficient. Remember those alkanes and alkenes produced by dancing bees that do exactly that?

Tropical habitats

This observation, and some elegant experimental and modelling studies, suggest that dancing is beneficial when food resources are: 

  • sparsely distributed – therefore difficult (and energetically unfavourable) to find by individual scouting
  • clustered or short-lived resources – when it’s gone, it’s gone
  • distributed with high species richness – if there’s a huge range of flowers, which are the most energetically rewarding (sugar-rich) to collect nectar from?

One of the experimental studies that contributed to these conclusions (though there’s still controversy in this area) was the demonstration that waggle dancing was beneficial in a tropical habitat, but not in two temperate habitats. This makes sense, as food resources have different spatiotemporal distribution in these habitats. Tropical habitats are characterised by clustered and short-lived resources.

Therefore the suggestion is that the waggle dance of Apis species evolved, presumable early in the speciation of the genus, in a tropical region where food resources were patchily distributed, available for only limited period and present alongside a wide variety of other (less good) choices.

For example, like individual trees flowering in a forest …

Finally, it’s worth noting that there is evidence that bees that dance are able to successfully exploit food resources further away than would otherwise be expected from their body size.

This also makes sense.

It’s much less risky flying off over the horizon if you know there’s something to collect once you get there 10.


Notes

If you arrived here from my Twitter feed (@The_Apiarist) you’ll have seen the tweet started with the words “Dance like nobody’s watching”, words that are often attributed to Mark Twain. 

The full quote is something like “Dance like nobody’s watching; love like you’ve never been hurt. Sing like nobody’s listening; live like it’s heaven on earth”.

Pretty sound advice.

But it’s not by Mark Twain. It’s actually from a country music song by Susanna Clark and Richard Leigh. This was first released on the Don Williams album Traces in 1987. So only about 90 years out 😉 

Queens and amitraz residues in wax

A question following a recent evening talk to a beekeeping association prompted me to look back at the literature on amitraz and wax residues.

The question was about reuse of honey supers that were present on a colony during miticide treatment.

With the exception of MAQS, there are no approved miticides that should be used if there are honey supers on the hives. The primary reason for this is that there is a risk that the miticide will taint the honey. Since the latter is for human consumption this is very undesirable.

However, it’s not unusual at the end of the season to have a half empty super, or a super containing just uncapped stores. Typically this would be ‘nadired’ i.e. placed below the brood box, with the expectation that the bees will move the stores up into the brood chamber 1.

Two colonies overwintering with nadired supers

And sometimes this super remains in place during the annual early autumn Varroa slaughter. 

The question was something like “Can I reuse the honey super next season?”

My answer

As anyone who has heard me speak will know, my answer was probably rambling, repetitive and slightly incoherent 🙁

However, the gist of it was “Yes, but I don’t”.

With Zoom talks and written questions from the audience you often don’t get all the details. The answer must be sufficiently generic to cover most eventualities 2 including, for example, the range of possible miticides that were used for treatment.

Assuming the nadired super is emptied by the bees during the winter, what are the chances that the wax comb will be contaminated with miticides?

This depends upon the miticide used.

I explained that the organic acids (formic or oxalic) are not wax soluble and so the super can be reused without a problem. 

In contrast, Apistan (a pyrethroid) is known to be wax soluble, so it should probably not be used again to avoid any risk of tainting honey subsequently extracted from it 3.

But (I probably digressed) you really shouldn’t be using Apistan as resistance in the mite population is already widespread.

But what about Apivar (the active ingredient of which is amitraz)?

Since Apivar isn’t wax soluble it would probably be OK to reuse the super … but I qualified this by saying that I don’t reuse them “just to be on the safe side”.

What they don’t tell you about Apivar

This wasn’t really an application of the precautionary principle.

Instead, it reflected a dim memory of some posts I’d read earlier in the year on the Bee-L discussion forum. This is a low volume/high quality forum frequented by scientifically-inclined beekeepers.

It turns out that, although amitraz (the active ingredient in Apivar) is not wax soluble, it’s broken down (hydrolysed) to a formamide and a formamidine

Read that again … I didn’t write the same word twice 😉

The formamide has no residual activity against mites. In contrast, the formamidine retains miticidal activity and is wax soluble

Is this a problem?

Well, possibly. One of the things discussed by Richard Cryberg on Bee-L was that there appears to be no toxicology data on these two products. It’s probably been done, just not published.

Perhaps we can assume that they’re not hideously toxic to humans (or bees)? If it was, amitraz (which is the active ingredient in all sorts of mite and tick treatments, not solely for bees) would carry sterner warnings.

Or should 🙁

The residual miticide activity is potentially more of a problem. A well understood route to developing miticide resistance involves long-term exposure to sub-lethal doses. There are several reports of amitraz resistance in the scientific literature, and bee farmers are increasingly providing anecdotal accounts of resistance becoming a problem.

This, and the possibility of tainting honey, are reason enough in my opinion to not reuse drawn supers that have been on the hive (e.g. nadired) during Apivar treatment.

But it turns out that there are additional potential issues with amitraz residues in comb.

Miticide residues in wax

Commercial wax foundation – like the stuff you buy from Thorne’s or Maisemores or Kemble Bee Supplies – is often contaminated with miticide residues. A large US survey of drawn comb from hives and foundation demonstrated that:

Almost all comb and foundation wax samples (98%) were contaminated with … fluvalinate 4 and coumaphos 5, and lower amounts of amitraz degradates and chlorothalonil 6, with an average of 6 pesticide detections per sample and a high of 39.

I’m not aware of an equivalent published analysis of UK foundation. I’m know one has been done and I’d be astounded if it produced dramatically different results. There’s a global trade in beeswax, some of which will be turned into foundation. The only exception might be certified organic foundations.

Freshly drawn comb

A freshly drawn foundationless frame

I always purchase premium quality foundation but am under no misapprehension that it doesn’t also contain a cocktail of contaminants, including miticides and their ‘degredates’. 

I’d be delighted to be proved wrong but, since I think that’s unlikely, it’s one reason I use an increasing number of foundationless frames … which also saves quite a bit of cash 🙂

Drones and queens and miticides in wax

Numerous studies have looked at the influence of miticide residues on worker, drone and queen development. These include:

  • Sublethal doses of miticides can delay larval development and adult emergence, and reduce longevity 7
  • Tau-fluvalinate- or coumaphos-exposed queens are smaller and have shorter lifespans 8
  • Queens reared in wax-coated cups contaminated with tau-fluvalinate, coumaphos or amitraz attracted smaller worker retinues and had lower egg-laying rates 9.
  • Drones exposed to tau-fluvalinate, coumaphos or amitraz during development had reduced sperm viability 10.

All of which is a bit depressing 🙁

These studies used what are termed ‘field-realistic’ concentrations of the contaminating miticide. They didn’t use wax saturated in miticide, but instead contaminated it with parts per million (ppm), or parts per billion (ppb).

These are the highest concentrations reported in surveys of comb tested in commercial beekeeping operations in the US, so hopefully represent a ‘worst case scenario’.

It’s also worth noting that some commercial beekeepers in the US use significantly more – both in amount and frequency – miticides than are used by amateurs. If you read American Bee Journal or the Beesource forums it’s not unusual to find accounts of spring, mid-season, late-summer and mid-winter treatments, often of the same colonies.

Queen mating

To add to the literature above, a new paper was published in November 2020 which suggested that amitraz residues in wax increased the mating frequency of queens.

The paper is by Walsh et al., (2020) Elevated Mating Frequency in Honey Bee (Hymenoptera: Apidae) Queens Exposed to the Miticide Amitraz During Development. Annals of the Entomological Society of America doi: 10.1093/aesa/saaa041

This piqued my interest. Queen mating frequency is an important determinant of colony fitness.

If a queen mates with more drones there’s inevitably increased genetic diversity in the colony and, in landmark studies by Thomas Seeley, an increase in colony fitness 11

Colony fitness includes all sorts of important characteristics – disease resistance, foraging ability, overwintering success etc.

So, perhaps this is a benefit of amitraz residues in your wax foundation … the reduced egg-laying rate being compensated by increased patrilines 12 and a fitter colony?

The study

Walsh and colleagues grafted queens into JzBz queen cups containing wax laced with one or more miticides. They reared the queens in ‘cell builders’ that had not been miticide treated, shifted mature queen cells to mating nucs and then – after successful mating – quantified two things:

  • the viability of spermatozoa in the queen’s spermatheca
  • the mating frequency of the queen

Irrespective of the miticides incorporated into the wax lining the queen cup, sperm viability was very high (98.8 – 99.5% viable), and no different from queens not exposed to miticides during development. 

Queen cells after emergence in mating nucs

This suggests that miticide contamination of queen cells is unlikely to have a deleterious effect on sperm viability in mated queens.

However, rather oddly, this contradicts a not dissimilar study 5 years ago from some of the same authors where the presence of tau-fluvalinate and coumaphos did reduce sperm viability 13, as did an earlier study looking at the effect of amitraz 14.

This contradiction is pretty-much ignored in the paper … clearly something that “needs further investigation”.

It might be due to experimental differences (for example, they used different methods to determine sperm viability). Alternatively, since the queens were open-mated, it might reflect differences in the miticide-exposure of the donor drones.

Mating frequency

The authors used microsatellite analysis to determine the mating frequency of the queens reared during the study. They compared queens reared in the presence of amitraz or tasty cocktails of tau-fluvalinate & coumaphos, or clorothalonil & chlorpyrifos 15, with those reared in the absence of chemicals contaminating the waxed queen cup.

They measured the observed mating frequency and then calculated the effective mating frequency (me). Conveniently they describe the difference between these parameters:

The observed mating frequency refers to the total number of drone fathers represented in a queen’s worker progeny. The effective mating frequency uses the proportion of each subfamily within a colony and compensates for calculating potentially skewed estimates of paternity (i.e., unequal subfamily proportions in sampled pupae) and intracolony genetic relatedness.

‘Convenient’ because it saves me having to explain it 😉

The observed mating frequencies of the control queens (untreated wax), or those reared in the presence of amitraz or tau-fluvalinate & coumaphos cocktails were not statistically different. However, queens reared in clorothalonil & chlorpyrifos-laced wax had a lower observed mating frequency.

Strikingly though, when calculated, the effective mating frequency of amitraz- or tau-fluvalinate & coumaphos-exposed developing queens was significantly higher (~12.9-13.4) than either the untreated controls or clorothalonil & chlorpyrifos (~8.2-8.8) 16.

And … ?

The amitraz result is new.

The influence of tau-fluvalinate & coumaphos on effective mating frequencies has been reported previously (by some of the same authors 17) which, since this was a new study in a different region, is at least encouraging because it supports the earlier work.

Taken together, these results suggest that miticide residues (of at least two chemically different types) increase the number of drones that a queen mates with.

The discussion of the paper speculates about why this difference is observed. 

The number of drones a queen mates with is influenced by several things. These include the number and duration of the mating flights. Perhaps the amitraz-exposed queen can’t count properly, or loses her ability to judge time … or just flies more slowly?

All of these would result in exposure to more drones.

Before returning to the hive, a queen must be able to determine whether she has mated with sufficient drones. It is suggested that stretch receptors in the oviducts are involved with this, forming a negative feedback stimulus once the oviducts are full. Perhaps amitraz impairs stretch receptor function or signalling?

Clearly there’s a lot left to learn.

Hyperpolyandry

The effective mating frequencies determined in the presence of amitraz (and tau-fluvalinate & coumaphos) were higher than the controls. However, they still appear rather low when compared with previous reports of hyperpolyandrous 18 colonies with up to 77 distinct patrilines (I’ve written about this previously, including descriptions of how it was determined).

Don’t mix the two observations up. In the studies of hyperpolyandry they analysed queens to determine their patriline.

A queen from a very rare patriline is still a queen, so can be screened.

In contrast, if you only screen a handful of workers (from the thousands present in the colony), you are very unlikely to find extremely rare patrilines. Those you do find will be the ones that are most common. 

A logical extension of the studies reported by Walsh et al., would be to determine whether hyperpolyandry also increased in amitraz-exposed colonies. If the effective mating number is increased you should observe a larger number of patrilines.

Alternatively, perhaps Withrow and Tarpy (who published the hyperpolyandry paper 19) should look again at whether the colonies they screened had a long history of amitraz exposure.

And what about that nadired super?

It’s probably fortunate I’d not fully read the literature before answering the question after my talk. 

If I had, I’d have tried to paraphrase the ~2000 words I’ve just written … so making my answer interminably long.

Of course, it’s unlikely that an amitraz (Apivar) contaminated super will ever be visited by a queen (but these things do happen 🙁 ).

Or be a location for developing queen cells. 

So, in this regard, I think it’s irrelevant whether the super is reused.

In contrast, the wax solubility and residual miticide activity of one of the hydrolysis products of amitraz is more of a concern. I don’t want this near honey I’m going to extract, and I’d rather not have it in the hive at all.

All of which explains the “Yes, but I don’t” answer to the original question about whether the super can be reused.

Fondant feeding on a colony with a nadired super

The super in the picture above will be removed early next season, before the queen starts laying in it. The super will be empty and I’ll melt the wax out in my steam wax extractor. 

In a good nectar flow the bees will draw a full super of comb very quickly. Yes, they’ll use some nectar that would otherwise be used make honey, but that’s a small penalty.

And what will I do with the extracted wax? 

I’ll probably trade it in for new foundation 20.

And since this is what many beekeepers do it explains why I’m certain that most commercial foundation is contaminated with miticides 🙁

But don’t forget …

Mite management is important. Miticides are chemicals and, like other medicines, have both beneficial and detrimental effects. The beneficial effects far outweigh the detrimental ones. If you do not treat, the likelihood is that mites and viruses will kill the colony … if not immediately, then eventually.


 

The winter cluster

We had our first snow of the year last night and the temperature hasn’t climbed above 3°C all day. The hills look lovely and, unsurprisingly, I’ve not seen a single bee venturing out of the hives.

Winter wonderland

If you crouch down close to the hive entrance and listen very carefully you’ll be able to hear …

… absolutely nothing.

Oh no! Are they still alive? Maybe the cold has killed them already?

If you rap your knuckles against the sidewall of the brood chamber you’ll hear a brief agitated buzz that will quickly die back down to silence.

Don’t do that 😯

Don’t disturb them unless you absolutely have to. They’re very busy in there, huddling together, clustering to maintain a very carefully regulated temperature.

Bees and degrees

Any bee that did venture forth at 3°C would get chilled very rapidly. Although the wing muscles generate a lot of heat (see below), this uses a large amount of energy.

If the body temperature of an individual bee dips below ~5.5°C they become semi-comatose. They lose the ability to move, or warm themselves up again. Below -2°C the tissues and haemolymph starts to freeze.

However, as long as they’re not exposed to prolonged chilling (more than 1 hour) they can recover if the environmental temperature increases 1.

An individual bee has a large surface area to volume ratio, so rapidly loses heat. Their hairy little bodies help, but it’s no match for prolonged exposure to a cold environment.

But the bees in your hives are not individuals. Now, perhaps more than any other time in the season, they function as a colony. Survival, even for a few minutes at these temperatures, is dependent upon the insulation and thermoregulation provided by the cluster.

All for one, one for all

The temperature in the clustered colony is always above the coma-inducing 5.5°C threshold, even for the bees that form the outer surface layer, which is termed the mantle.

And the temperature in the core of the cluster is much warmer still, and if they’re rearing brood (as they soon will be 2) is maintained very accurately.

The mantle

The temperature inside the hive entrance, some distance from the cluster, is the same as the external ambient temperature. On a cold winter night that might be -5°C (in Fife), or -35°C (in Manitoba).

Studies have shown that clustered colonies can survive -80°C for 12 hours, so just a few degrees below freezing is almost balmy.

The winter cluster

Due to thermal radiation from the clustered colony, the temperature of the airspace around the colony increases as you get nearer the cluster. Draught free hives – and beekeepers that refrain from rapping on the brood box sidewall – will reduce movement of this air, so reducing thermal losses from convection.

The clustered colony is not a uniform ‘ball’ of bees. It has two distinct layers. The outer layer is termed the mantle and is very tightly packed with bees facing inwards. These bees are packed in so tightly that their hairy bodies trap air between them, effectively forming an insulating quilt.

To reduce heat loss further these mantle bees have a countercurrent heat exchanger (between the abdomen and the thorax) that reduces heat loss from the haemolymph circulating through their projecting abdomens.

The mantle temperature is maintained no lower than about 8°C, safely above coma-inducing lower temperatures.

Penguins and flight muscles

I’ve seen it suggested that the mantle bees circulate back into the centre of the cluster to warm up again, but have been unable to find published evidence supporting this. It’s an attractive idea, and it’s exactly what penguins do on the Antarctic ice sheet … but that doesn’t mean it’s what bees do.

Penguins, not bees

Although bees can cope with temperatures of 8°C, they cannot survive this temperature for extended periods. If bees are chilled to below 10°C for 48 hours they usually die. This would support periodically recirculating into the centre of the cluster to warm up.

Bees do have the ability to warm themselves by isometric flexing of their flight muscles. Essentially they flex the opposing muscles that raise and lower the wings, without actually moving the wings at all.

This generates a substantial amount of heat. On a cool day, bees warm their flight muscles by this isometric flexing before leaving on foraging flights. They have to do this as the flight muscles must reach 27°C to generate the wing frequency to actually achieve flight. Since bees will happily forage above ~10°C this demonstrates that the isometric wing flexing can raise the thoracic flight muscle temperature by at least 15-17°C.

But, briefly back to the penguin-like behaviour of bees, neuronal activity is reduced at lower temperatures. In fact, at temperatures below 18°C bees don’t have sufficient neuronal activity to activate the flight muscles for heat generation. This again suggests there is a periodic recycling of bees from the mantle to the centre of the cluster.

How can bees fly on cool days if it’s below this 18°C threshold? The day might be cooler, but the bee isn’t. The colony temperatures are high enough to allow sufficient neuronal activity for the foragers to pre-warm their flight muscles to forage on cool days.

Anyway, enough of a digression about flight muscles, onward and inward.

The core

Inside the mantle is the core. This is less densely occupied by bees, meaning that they have space to move around for essential activities such as brood rearing or feeding.

The temperature of the core varies according to whether the colony is rearing brood or not. If the colony is broodless the core temperature is maintained around 18°C.

The tightly packed mantle bees reduce airflow to the core. As a consequence of this the CO2 levels rise and the O2 levels fall, to about 5% and 15% respectively (from 0.04% CO2 and 21% O2 in air). A consequence of this is that the metabolic rate of bees in the core is decreased, so reducing food consumption and minimising the heat losses from respiration.

Brood rearing

My clustered winter colonies are probably just thinking about starting to rear brood 3.

Bees cannot rear brood at 18°C. Brood rearing is very temperature sensitive and occurs optimally at 34.5-35.5°C.

Outside that narrow temperature band things start to go a bit haywire.

Pupae reared at 32°C emerge looking normal (albeit a day or so later than the expected 21 days for a worker bee), but show aberrant behaviour. For example, they perform the waggle dance less enthusiastically and less accurately 4. In comparison to bees reared at 35°C, the ‘cool’ bees performed only 20% of the circuits and the ‘waggle run’ component was a less accurate predictor of distance to the food source.

Neurological examination of bees reared at 35°C showed they had increased neuronal connections to the mushroom bodies in the brain, when compared with those reared as little as 1°C warmer or cooler. This, and the behavioural consequences, shows how critical the brood nest temperature is.

The cluster position

The cartoon above shows the cluster located centrally in the hive. This isn’t unusual, though the cluster does tend to move about within the volume available as they utilise the stores.

You can readily determine the location of the cluster. Either insert a Varroa tray underneath an open mesh floor for a few days …

All is well ...

Tell tale signs of a brood-rearing cluster …

… or by using a perspex crownboard. I have these on many of my colonies and it’s a convenient way of determining the size and location of the cluster with minimal disturbance to the colony.

Perspex crownboard

Perspex crownboard …

Though you don’t need to check on them like this at all.

The photograph above was from late November (6 years ago). The brood box is cedar and therefore provides relatively poor insulation.

While checking the post-treatment Varroa drop in my colonies this winter it was obvious that cluster position varied significantly between cedar and poly hive types.

In poly hives (all my poly hives are either Abelo or Swienty) it wasn’t unusual to find the cluster tight up against one of the exterior side walls. In contrast, colonies hived in cedar brood boxes tended to be much more central.

This must be due to the better insulation of polystyrene compared with cedar.

Insulation

Although I don’t think I’ve noticed this previously in the winter, it’s not uncommon in summer to find a colony in a poly hive rearing brood on the outer side of the frames adjacent to the hive wall. This is relatively rare in cedar boxes, other than perhaps at the peak of the summer.

If you’re interested in hive insulation, colony clustering and humidity I can recommend trying to read this paper by Derek Mitchell.

I don’t provide additional insulation to my colonies in the winter. It’s worth noting that all my hives have open mesh floors. In addition, the crownboard is topped by a 5 cm thick block of insulation throughout the year, either integrated into the crownboard or just stacked on top.

Perspex crownboard with integrated insulation

If you use perspex crownboards you must have insulation immediately above them. If you don’t you get significant amounts of condensation forming on the underside which then drips down onto the cluster.

The winter cluster and miticide treatment

The only time you’re likely to see the winter cluster is when treating with an oxalic acid-containing miticide. And only then when trickle treating.

With the choice between vaporising or trickle treating, I tend to be influenced by the ambient temperature.

If the cluster is very tightly clustered (because it’s cold) I tend to trickle treat.

If it is more loosely clustered I’m more likely to vaporise.

The threshold temperature is probably about 8°C, but I’m not precious about this. The logic – what little is applied – is that the oxalic acid crystals permeate the open cluster better than they would a closed cluster.

I’ve got zero evidence that this actually happens 😉

However, it’s worth reiterating the point I made earlier about airflow through the mantle. Since this is restricted in a tightly clustered colony – evidenced by the reduced O2 and elevated CO2 levels – then it seems reasonable to think that OA crystals are less likely to penetrate it either.

Of course, there’s an assumption that the trickled treatment can penetrate the cluster, and doesn’t just coat the mantle bees with a sticky OA solution.

Which neatly brings us back to penguins … if these mantle bees do recirculate through the cluster core they’ll take some of the OA with them, even if it didn’t get there directly.

Finally, it’s worth noting that cluster formation starts at about 14°C. As the temperature drops the cluster packs together more tightly. Between 14°C and -10°C the volume of the cluster reduces by five-fold.

By my calculations 5, at 2°C and 8°C the cluster is three and four times it’s minimal volume respectively, so perhaps both OA vapour and trickled solution could permeate perfectly well.


 

Smell the fear

With Halloween just around the corner it seemed appropriate to have a fear-themed post.

How do frightened – or even apprehensive – people respond to bees?

And how do bees respond to them?

Melissophobia is the fear of bees. Like the synonym apiphobia, the word is not in the dictionary 1 but is a straightforward compounding of the Greek μέλισσα or Latin apis (both meaning honey bee) and phobos for fear.

Melissophobia is a real psychiatric diagnosis. Although people who start beekeeping are probably not melissophobic, they are often very apprehensive when they first open a colony.

If things go well this apprehension disappears, immediately or over time as their experience increases.

If things go badly they might develop melissophobia and stop beekeeping altogether.

Even relatively experienced beekeepers may be apprehensive when inspecting a very defensive colony. As I have discussed elsewhere, there are certain times during the season when colonies can become defensive. These include when queenless, during lousy weather or when a strong nectar flow ends.

In addition, some colonies are naturally more defensive than others.

Some could even be considered aggressive, making unprovoked attacks as you approach the hive.

A defensive response is understandable if the colony is being threatened. Evolution over eons will have led to acquisition of appropriate responses to dissuade natural predators such as bears and honey badgers.

I’m always careful (and possibly a little bit apprehensive) when looking closely at a completely unknown colony – such as these hives discovered when walking in the Andalucian hills.

If Carlsberg did apiaries ...

Apiary in Andalucia

How do bees detect things – like beekeepers or bears – that they might need to mount a defensive response against?

Ignore the bear

Bees have four senses; sight, smell, touch and taste. Of these, I’ve briefly discussed sight previously and they clearly don’t touch or taste an approaching bear 2 … so I’ll focus on smell.

Could they use smell to detect the scent of an approaching human or bear that is apprehensive of being stung badly?

Let’s forget the grizzly bear 3 for now. At over 200 kg and standing 2+ metres tall I doubt they’re afraid of anything.

Let’s instead consider the apprehensive beekeeper.

Do bees respond to the smell of a frightened human (beekeeper or civilian)?

This might seem a simple question, but it raises some interesting additional questions.

  • Is there a scent of fear in humans?
  • Can bees detect this smell?
  • Have bees evolved to generate defensive responses to this or similar smells?

If two beekeepers inspect the same colony and one considers them aggressive and the other does not, is that due to the beekeepers ‘smelling’ different?

I don’t know the answers to some of these questions, but it’s an interesting topic to think about the stimuli that bees have evolved to respond to.

The scent of fear

This is the easy bit.

Is there a distinctive scent associated with fear in humans?

The Scream by Edvard Munch (1895 pastel version)

Using some rather unpleasant psychological testing researchers have determined that there is a smell produced in sweat secretions that is associated with fear. Interestingly, the smell alone appears not to be detectable. The female subjects tested 4 were unable to consciously discriminate the smell from a control neutral odour.

However, the ‘fear pheromone’ alone caused changes in facial expression associated with fright and markedly reinforced responses to visual stimuli that induced fear.

Females could respond to the fear pheromone produced by males (and vice versa) and earlier MRI studies (involving significantly less unpleasant experiments) had shown that this smell was alone able to induce changes in the amygdala, the region in the brain associated with emotional processing.

So, there is a scent of fear in humans. We can’t consciously detect it, but that doesn’t make it any less real.

Can bees detect it?

Can bees smell the scent of fear?

This is where things get a lot less certain.

I’m not aware that there have been any studies on whether bees can definitively identify the fear pheromone produced by humans.

To conduct this study in a scientifically-controlled manner you would need to know precisely what the pheromone was. It would then be tested in parallel with one or several irrelevant, neutral or related (but different) compounds. In each instance you would have to identify a response in the bee that indicated the fear pheromone had been detected.

All of which is not possible as we don’t definitely know what the fear pheromone is chemically.

We do know it’s present in the sweat of frightened humans … but that’s about it. This makes the experiment tricky. Comparisons would also have to be made with sweat secretions present in the same 5 human when not frightened.

And what response would you look for? Usually bees are trained to respond in a proboscis extension test. In this a bee extends its proboscis in response to a recognised smell or taste.

But, as none of this has been done, there’s little point in speculating further.

So let’s ask the question the other way round.

Would bees be expected to smell the scent of fear?

Smell is very significant to bees.

They have an extremely sensitive sense of smell, reflected in their ability to detect certain molecules as dilute as one or two parts per trillion. Since many people struggle with visualising what that means it’s like detecting a grain of salt in an Olympic swimming pool 6.

Part of the reason we know that smell is so important to bees is because evolution has provided them with a very large number of odorant receptors.

Odorant receptors are the proteins that detect smells. They bind to chemical molecules from the ‘smell’ and these trigger a cellular response of some kind 7. Different odorant receptors have different specificities, binding and responding to the molecules that are present in one or more odours.

Odorant receptor diversity and sensitivity

Bees have 170 odorant receptors, more than three times the number in fruit flies, and double that in mosquitoes. Smell is clearly very important to bees 8.

This is perhaps not surprising when you consider the role of odours within the hive. These include the queen and brood pheromones and the chemicals used for kin recognition 9.

In addition, bees are able to find and use a very wide range of plants as sources of pollen and nectar and smell is likely to contribute to this in many ways.

Finally, we know that bees can detect and respond to a wide range of other smells. Even those present at very low levels which they may not have been exposed to previously. For example Graham Turnbull and his research team in St Andrews, in collaborative studies with Croatian beekeepers, are training bees to detect landmines 10 from the faintest ‘whiff’ of TNT they produce. This deserves a post of its own.

So, while we don’t know that bees could detect a fear pheromone, there’s a good chance that they should be able to.

Evolution of defensive responses

We’re back to some rather vague arm waving here I’m afraid.

In a rather self-fulfilling manner we don’t know if bees have evolved a defensive response to the fear pheromone of humans as – for reasons elaborated above – we don’t actually know whether they do respond to the fear pheromone.

We could again ask this question in a slightly different way.

Might bees be expected to have evolved a defensive response to the fear pheromone?

Long before we developed the poly nuc or the fiendishly clever Flow Hive, humans have been attracted by honey and have exploited bees to harvest it.

The ancient Egyptians kept bees in managed hives over 5000 years ago.

However, we can be reasonably certain that humans provided suitable nesting sites (which we’d now call bait hives) to attract swarms from wild colonies well before that.

But we’ve exploited bees for tens or hundreds of thousands of years more than that.

The ‘Woman(Man) of Bicorp” honey gathering (c. 8000 BC)

There are examples of Late Stone Age (or Upper Paleolithic c. 50,000 to 10,000 years ago) rock art depicting bees and honey from across the globe, with some of the most famous being in the Altamira (Spain) cave drawings from c. 25,000 years ago.

Survival of the fittest

And the key thing about many of these interactions with honey bees is that they are likely to have been rather one-sided. Honey hunting tends to be destructive and results in the demise of the colony – the tree is felled, the brood nest is ripped apart, the stores (and often the brood) are consumed.

None of this involves carefully caging the queen in advance 🙁

This is a strong selective pressure.

Colonies that responded earlier or more strongly to the smell of an apprehensive approaching hunter gatherer might be spared. These would survive to reproduce (swarm). Literally, the survival of the fittest.

All of this would argue that it might be expected that bees would evolve odorant receptors capable of detecting the fear pheromone of humans.

There’s no fire without smoke

There are (at least) two problems with this reasoning.

The first problem is that humans acquired the ability to use fire. And, as the idiom almost says, there’s no fire without smoke. Humans were regularly using fire 150-200,000 years ago, with further evidence stretching back at least one million years that pre-humans (Homo erectus) used fire.

And, if they were using fire you can be sure they would be using smoke to ‘calm’ the bees millenia before being depicted doing so in Egyptian hieroglyphs ~5,000 years ago.

It seems reasonable to expect that the use of smoke would mask the detection of fear pheromones, in much the same way that it masks the alarm pheromone when you give them a puff from your trusty Dadant.

The other problem is that it might be expected that the Mesolithic honey hunters had probably ‘got the job’ precisely because they weren’t afraid of bees. In extant hunter gatherer communities it’s known that there are specialists that have a particular aptitude for the role. Perhaps these beekeepersrobbers produce little of no fear pheromone in the first place?

What about other primates?

It’s well know that non-human primates (NHP’s), like chimpanzees and bonobo, love honey. They love it so much that they are responsible for an entire research area studying tool use by chimps.

Bonobo ‘fishing’ for termites using a tool (I couldn’t find a suitable one robbing honey)

Perhaps NHP’s produce a fear pheromone similar to that of humans? Since they haven’t learned to use fire (and they are very closely related to humans) bees may have evolved to respond to primate fear pheromone(s), and – by extension – to those of humans.

However, chimpanzees and related primates prefer to steal honey from stingless bees like Meliponula bocandei. The only information I could find suggested they avoided Apis mellifera, or “used longer sticks as tools“.

Perhaps not such a strong selective pressure after all …

More arm waving

A lot of the above is half-baked speculation interspersed with a smattering of evolutionary theory.

Bees clearly respond in different ways to different beekeepers. I’ve watched beekeepers retreat from a defensive colony which – later on the same training day – were beautifully calm when inspected by a different beekeeper.

Trainee beekeepers

Trainee beekeepers

Although this might have been due to differences in the production of fear pheromones, it’s clear that the bees are also using other senses to detect potential threats to the colony.

Look carefully at how outright beginners, intermediate and expert beekeepers move their hands when inspecting a colony.

The tyro goes slow and steady. Everything ‘by the book’. Not calm, but definitely very controlled.

The expert goes a lot faster. However, there’s no banging frames down, there are no sudden movements, the hands move beside the brood box rather than over it. Calm, controlled and confident.

In contrast, although the “knowing just enough to be dangerous” intermediate beekeeper is confident, they are also rushed and a bit clumsy. Hands move back and forwards over the box, movements are rapid, frames are jarred … or dropped. A bee sneaks inside the cuff and stings the unprotected wrist. Ouch!

“That’s an aggressive colony. Better treat it with care.”

You see what I mean about arm waving?

I strongly suspect movement and vibration trigger defensive responses to a much greater extent than the detection of fear pheromones in humans (if they’re detected at all).

Closing thoughts

You’ll sometimes read that bees respond badly to aftershave or perfumes. This makes sense to me only if the scent resembles one that the bees have evolved a defensive response against.

Don’t go dabbing Parfum de honey badger behind your ears before starting the weekly inspection.

Mellivora capensis – the honey badger. Believe me, you’re not worth it.

But why would they react aggressively to an otherwise unknown smell?

After all, they experience millions of different – and largely harmless – smells every day. Bees inhabit an environment that is constantly changing. One more unknown new scent does not immediately indicate danger. There would be an evolutionary cost to generating a defensive response to something that posed no danger.

And a final closing thought for you to dwell on …

Humans have probably been using fire to suppress honey bee colony aggression for hundreds of thousands of years.

Why haven’t bees evolved defensive responses to the smell of smoke? 11

Happy Halloween 🙂


 

Does DWV infect bumble bees?

Covid (the disease) is caused by a virus called SARS-Cov-2. SARS is an abbreviation of severe acute respiratory syndrome and the suffix ‘Cov’ indicates that it’s a coronavirus. The final digit (2) shows that it’s the second of this type of virus that has caused a pandemic. The first was in 2003, and was caused by a virus we now call SARS-Cov-1. That virus had a case fatality rate of 11%, but only infected ~8500 people worldwide. 

SARS-Cov-2 is not a human virus, by which I mean it’s not a virus that has been present in the human population for a long time. It’s actually a virus that most probably originated in bats.

We’re still not sure how SARS-Cov-2 jumped species from bats to humans.

SARS-Cov-1 made the same transition from bats and we do have a pretty good idea how this happened. Before the virus was found in bats it was detected in palm civets and raccoon dogs, both of which are farmed for food and sold in live game markets. Neither animal shows any symptoms when infected with SARS-Cov-1.

Bats were also sold in the same live game markets in Guangdong province in China and it seems likely that the virus either crossed directly from bats to humans, or went via a third species such as the palm civet.

Pathogen spillover

It’s likely that SARS-Cov-2 followed the same route. We are entering a second – likely extended – period of geographic lockdown due to Covid. Since SARS-Cov-2 made its cross-species jump from bats to humans it has infected at least 41 million people and killed over 1.13 million.

Pathogens that jump from one species to another can have catastrophic consequences for the recipient population 1 or, as in the case of palm civets, might cause no harm whatsoever.

The term ‘pathogen spillover’ is often used to describe the event when a pathogen – whether viral, bacterial or a parasite – escapes or spills over from its natural host to another species. 

CSI in the apiary … motive, opportunity, means

To make the species jump a number of criteria must occur. The pathogen has to be present at a high enough level to be infectious in the ‘donor’ species. For convenience, let’s consider this as the motive to jump species 2.

Secondly, the pathogen needs to have an opportunity to jump species. For example, because the donor and ‘recipient’ species share the same habitat or regularly come into contact.

Finally, it has to have the means to replicate in the recipient. If it cannot replicate it can never get established in the recipient population or cause disease.

In fact, this is an oversimplification. It also needs to be transmissible between individuals of the recipient species (or it will never spread in the recipient species).

So what has all this to do with the bumble bees in the title of this post?

Honey bees get a bad press from some scientists and environmentalists. They compete with native solitary and other bees and pollinators for environmental resources – like pollen and nectar. Increasingly, particularly in agricultural areas, these can be in limiting supply at certain times of the season. For examples, because all the hedgerows have been grubbed up and the wildflower meadows obliterated.

In addition, there are a number of studies that have suggested that honey bee viruses have spilled over into other pollinators, in particular bumble bees, and that this pathogen spillover has contributed to the decline in free-living bee populations.

Do honey bee viruses have the motive, opportunity and means to cause disease in bumble bees?

Are honey bee viruses responsible for the decline in bumble bee populations?

Correlation and causation

There are numerous studies showing that the most widespread honey bee virus, deformed wing virus (DWV), can be detected in wild-caught bumble bees.

Let me pose a quick question … does detection mean ‘replication’?

Deformed wing virus “does what it says on the tin” in honey bees. When transmitted by Varroa it causes developmental defects in pupae that appear as wing deformities in newly emerged workers.

DWV symptoms

DWV symptoms

There’s also one study that implicates DWV in directly causing disease in bumble bees.

This influential paper, published 15 years ago, demonstrated that some bumble bees had the characteristic crippled wings seen in symptomatic emerging honey bee workers. The ‘smoking gun’ was that DWV was also detected in these bumble bees.

Exhibit A : Evidence for DWV infection in bumble bees – click image for full legend.

This is the only figure in the paper and there have been no substantive follow-up papers. For some reason they showed ‘symptomatic’ Bombus terrestris (the buff-tailed bumble bee; panel A, left) and PCR detection data for B. pascorum (the common carder bee).

Nevertheless, this association between presence and symptoms was sufficient for the authors to conclude “we demonstrated that DWV is pathogenic to at least two bumble bee species … causing wing deformity similar to clinically DWV-infected honey bees”.

Here’s a second important question … does the detection of DWV in bumble bees demonstrate it is responsible for the symptoms observed? 3

As a virologist interested in the evolution, replication and transmission of viruses – and a beekeeper – this seemed like a worthwhile topic to explore in a bit more detail.

There might be a correlation between the presence of DWV in bumble bees, but does this account for causation of the DWV-like symptoms seen in the bumble bees?

Motive and opportunity

Let’s get these two out of the way quickly (though we’ll return to opportunity in the notes at the end).

Remember, viruses don’t want to do anything. I’m using motive as a hideously contrived reference to whether the pathogen, DWV, is present at high and infectious levels in honey bees.

It is.

Healthy, mite-naive (but reared in a hive with Varroa) workers can carry as little as ~1000 DWV viruses. Similar levels of DWV are also present in hives from Varroa-free regions – at least of the UK 4. In contrast, after parasitisation by Varroa, symptomatic or asymptomatic adult worker honey bees regularly have more that 109 (one trillion) viruses coursing through their little bodies. 

This virus is highly infectious. When injected into honey bees, as few as 10 viruses are sufficient to start a new infection and are replicated several million-fold within 24-48 hours.

Let’s agree that they have the ‘motive’ … to spread to other hosts.

They also have the opportunity, at least in the broadest sense of the word. Honey bees and bumble bees share the same environment. They collect nectar and pollen from the same flower species. They can even regularly be seen visiting the same flower simultaneously. In addition, it’s not unusual to see bumble bees trying to access honey bee hives to steal nectar.

I’d argue that the virus appears to have ample opportunity to move from one species to the other.

Does DWV replicate in bumble bees?

This is an important question. The data figure (‘Exhibit A’) shown above does not answer this question. Their assay simply detects the presence of DWV, with no indication of whether the virus is replicating.

And the reason this is important is really explained in the section on motive and opportunity above. DWV is ubiquitous and present at extraordinarily high levels in many honey bees. It’s present in honey bee faeces. It can be detected on flowers that honey bees have visited, or in pollen collected from those flowers.

DWV is absolutely everywhere.

So, if it is everywhere, there’s a good chance it might simply contaminate things … like bumble bees that look a bit sick for other reasons.

However, if DWV is involved in causing disease in bumble bees then the virus must replicate in bumble bees.

Virus replication – select for full size and legend.

I’ve used this figure before. To be certain that the virus is replicating you need to identify the intermediate replication products (the negative strand RNA shown as red arrows above).

Almost none of the large number of papers that have reported “DWV infected bumble bees” have identified these intermediate replication products.

Many studies didn’t even bother looking for these critical intermediate products.

So we did

There are two ways we could have investigated this. We could have collected large numbers of bumble bees from the fields and screened them in the lab for these replication intermediates. The problem with this approach is that DWV might be very rare in bumble bees, or might be common, but only replicate rarely. The additional problem is that it involves going out and collecting bees from the environment – that’s hard work 😉 5

An alternative approach is to buy a nest of bumble bees 6 and to inject a few bumble bee pupae and adults with DWV. This is what we did. In parallel, to ‘prove’ the virus used can replicate in honey bees, we injected a few honey bee pupae.

We injected the bees as it’s the definitive way we have of being sure that the virus was present.

‘Gene jockeys’

I’ve got some gifted molecular virologists in my lab. These are scientists who use genetic engineering or biotechnology techniques to address tricky questions (gene jockeys). They are particularly skilled at manipulating nucleic acids, such as the genomes of viruses.

If DWV is so common (it is – see above) how would we know that the replication intermediates were from the virus we injected into the bumble bees, rather than from a virus that was possibly already present?

To solve this puzzle Alex (the lead author on our recent paper) did two things. She engineered a unique genetic marker into the virus which we could look for (and that knew was absent from other similar viruses that might already be present in bumble bees). In addition, she ensured that the virus she injected into the bumble bees contained none of the negative strand RNA that is produced as the intermediate when the virus replicates. 

And … to cut a long story short, we could detect the negative strand RNA replication intermediate in injected bumble bees. In addition, the virus contained the unique genetic marker Alex had engineered into the virus genome, so we were absolutely certain it was the injected virus that was replicating. All of the controls worked exactly as expected.

DWV does replicate in bumble bees … at least under the conditions used in our experiment.

But it does not replicate very fast

The bumble bees we used for these experiments are commercially produced under very clean conditions. When we tested control bumble bees they contained no DWV at all, even using our most sensitive assays.

In contrast to injected honey bee pupae, DWV replicates rather slowly in bumble bees. In honey bees it will amplify a million-fold in 48 hours. However, in bumble bees, in 48 hours we only observed a 100-10,000 fold increase in DWV levels. It was only when we injected large amounts of DWV to bumble bees that we could recapitulate the virus levels seen in symptomatic honey bees.

This was a little puzzling as we’ve assumed that the very rapid replication of DWV in honey bees contributes to pathogenesis. The virus needs to replicate fast to spread through the pupa and infect the particular tissues and organs that, when damaged, result in the characteristic symptoms seen.

If it only replicates slowly in bumble bees how can it cause symptoms?

But hold on … does it cause symptoms?

Not as far as we can tell. 

We never found any injected bumble bees with deformed wings, or any that looked anything like the symptoms seen with DWV in honey bees. The actual quote from the paper is:

Strikingly, none of the eclosed bumble bees showed any signs of the wing deformities that are characteristic of DWV infection of honey bees”. 

Morbidity of DWV in bumble bees

Injected pupae developed until eclosion and were either non-viable, discoloured or apparently normal in appearance (see (c) above). We observed similar numbers of these three types whether the pupae were injected with DWV or mock-injected with buffer alone (see (b) above). What’s more, of those injected with the virus, the level of the virus was the same irrespective of the appearance (or viability) of the bumble bee (see (a) above).

We know that these bumble bees contain replicating DWV but see no evidence for overt disease. I acknowledge that they may have invisible symptoms. However, we see no evidence for the wing deformities reported in the 2005 paper from Genersch and colleagues (‘Exhibit A’, above).

Heavy going? But I’ve only just started … 

So, let’s briefly return to our “motive, opportunity, means” crime analogy and summarise where we’ve got to so far. 

DWV is present at very high levels in at least some honey bees. In addition, bumble bees are likely to regularly come into contact with DWV in the environment. This could happen through contaminated pollen, when attempting to rob hives, or by direct interaction with honey bees when both visit flowers. Finally, and most tellingly, DWV replicates in bumble bees.

So, if I was Peter Falk as Lieutenant Columbo, I’d argue that DWV has the motive, opportunity and means to potentially cause disease in bumble bees.

But, as is apparent from the figure above, it appears not to actually cause disease.

Which is puzzling. 

Just one more thing 7

There’s a major problem with the experiments I’ve discussed so far.

Like all experiments 8 they were tightly controlled, with single variables and lots of statistical analysis to demonstrate our confidence in their reproducibility.

The problem wasn’t technical, it was how well they recapitulated potential transmission in the environment.

We’re not aware of anything that goes around “injecting” bumble bee pupae or adults 9. They are not parasitised by Varroa and, although there are bumble bee mites (such as Parasitellus), they don’t feed on bumble bees in the same way that Varroa feeds on honey bees.

How else might bumble bees acquire DWV?

The obvious route is orally, while feeding. 

There are a few issues with feeding bees DWV. The method is simple enough … you add the virus to sugar syrup and they slurp it down. However, you have little control over how much individual bees consume. Do some bees feed directly and other acquire food when fed by other bees (trophallaxis)? It gets rather difficult control.

In addition, we knew from our studies of honey bees that they are far less susceptible to infection per os (by mouth) than by injection. And by far less I mean tens of thousands of fold less susceptible, at least as adults.

Feeding bumble bees DWV

We chose to investigate two routes of feeding.

The first was to directly feed individual bees in the laboratory. Using this approach we failed to detect any evidence for infection or DWV replication in fed adult bumble bees, even when fed 100 million DWV viruses.

Not an encouraging start. However, larvae generally have increased susceptibility to pathogens, so we also investigated feeding larvae in the laboratory. In these studies we did manage to establish infection. However, to do so we had to add 100 million DWV viruses to he food and only achieved a 50% infection rate. 

The second approach we used was direct feeding of complete bumble bee colonies maintained in the lab.

Bumble bees are easier to keep in the lab than honey bees as they don’t need to be free-flying. Each colony occupies a 30cm3 perforated plastic box, supplied with pollen and syrup. We fed three colonies 100 million DWV per bee per day for 4-6 weeks 10.

That’s a huge amount of virus for a protracted period. Our reasoning here was straightforward. Perhaps there was a particular developmental stage that had increased susceptibility? Perhaps adult bees feeding larvae would – for whatever reason – reduce their resistance to infection via the oral route?

We screened every egg, larva, pupa and adult bee for replicating DWV at the end of the experiment.

There was none present 🙁 ( or perhaps 🙂 , depending upon your viewpoint )

Summary and conclusions

We generated unequivocal evidence that DWV replicates in bumble bees. Specifically in Bombus terrestris, the buff tailed bumble bee. I’d be surprised if it did not replicate in other Bombus species, but that will need to be investigated. 

Infection of adult bees was only possible by injection, with no evidence for infection during feeding.

Bumble bee larvae can be infected with DWV while feeding, but only when fed very large amounts of virus directly. When larvae were reared by a colony supplied with DWV-laced food for several weeks the larvae did not become infected.

These results were recently published in Scientific Reports (Gusachenko et al., 2020) and are freely available. The title of the paper neatly sums up the study “Evidence for and against deformed wing virus spillover from honey bees to bumble bees: a reverse genetic analysis”

What does this mean in terms of our understanding of pathogen spillover from managed honey bee colonies to free living bees? 

If ecologists and environmental scientists are going to make the case that honey bees are threatening the survival of free-living solitary or bumble bees (due to pathogen spillover) they need to:

  1. formally demonstrate that the honey bee virus replicates in the free-living bee
  2. show that this replication is detrimental to the free-living bee
  3. provide evidence for a natural route of transmission by which infection can occur

In my view, and using legal terms again, it’s case proven for the first point in Bombus terrestris 11. In contrast, if I was the judge I’d throw out the other cases due to lack of evidence.

Monkey puzzles

There are viruses everywhere. Every living species has one or more viruses that infect it. Inevitably, because viruses replicate to very high levels, species other than the natural host may become exposed.

But almost always this has no consequences at all …

And to illustrate that I’ll briefly describe a study of monkey viruses infecting humans in Cameroon from several years ago. HIV is one of a very large group of viruses called immunodeficiency viruses. The ‘H’ stands for human, though the virus originated in chimpanzees and is very closely related to the simian immunodeficiency viruses (SIV).

A study of hunters living in the forests of Cameroon showed evidence for exposure to multiple different SIV isolates in tribespeople who hunted non-human primates or who were involved in butchering them or preparing them for market or cooking. There was no evidence these viruses were spreading in the human population, or that there was any sickness or disease associated with prior exposure. 

Environmental exposure happens all the time, and is relatively easy to detect. That’s exactly what had happened with these monkey viruses.

But evidence for environmental exposure, whilst easy to get, is not sufficient to support a claim that the virus causes disease at the level of the individual, or that it threatens an entire population.

As a virologist I think it’s interesting that DWV replicates in Bombus terrestris. However, I’ve yet to see any convincing evidence that DWV spillover from honey bees is responsible for the decline in wild bee populations.

Circumstantial evidence is not the same as convincing evidence.


Notes

What are the missing experiments that we didn’t do?

A key one is to determine whether long-term infection of bumble bees with DWV results in disease. We didn’t see overt deformed wing disease, but it’s possible that infection for weeks could have caused this or other symptoms. 

Although we fed bumble bee nests for weeks with DWV we saw no evidence of larval infection. This was despite previously demonstrating that larvae could be infected with high levels of DWV orally. One possibility is that DWV is inactivated by adult bees. I think it would be interesting to look at the level of infectious DWV in larval food.

Are there natural routes of exposure to DWV that result in bumble bee infection?

Does DWV ever cause environmental contamination at a high enough level to naturally infect bumble bees? For example, is the level of DWV in honey bee faeces high enough and does it ever contaminate pollen?

We are not going to do these studies so it will be interesting to see if others do … or whether simply the presence of the virus (whether replicating or not) will be proof’ that honey bee viruses are responsible for the decline in free-living bee populations.

Diutinus bees

Diutinus is Latin for long-lasting.

Diutinus bees are therefore long-lasting bees. These are the bees that, in temperate regions, maintain the colony through the winter to the warmer days of spring.

I’ve discussed the importance of these bees recently., and I’ve regularly made the case that protecting these ‘long-lived’ bees from the ravages of Varroa-vectored viruses is critical to reduce overwintering colony losses.

Winter is coming …

In most cases the adjective diutinus is replaced with ‘winter’, as in winter bees; it’s a more familiar term and emphasises the time of year these bees are present in the hive. I’ll generally use the terms interchangeably in this post.

Diutinus does not mean winter

From a scientific standpoint, the key feature of these bees is that they can live for up to 8 months, in contrast to the ~30 days a worker bee lives in spring or summer. If you are interested in what induces the production of long-lived bees and the fate of these bees, then the important feature is their longevity … not the season.

Furthermore, a proper understanding of the environmental triggers that induce the production of long-lived bees might mean they could be produced at other times of the season … a point with no obvious practical beekeeping relevance, but one we’ll return to in passing.

It’s worth emphasising that diutinus bees are genetically similar to the spring/summer bees (which for convenience I’ll refer to as ‘summer bees’ for the rest of the post). Despite this similarity, they have unique physiological features that contribute to their ability to thermoregulate the winter cluster for months and to facilitate spring build-up as the season transitions to spring.

What induces the production of winter bees? Is it a single environmental trigger, or a combination of factors? Does summer bee production stop and winter bee production start? What happens at the end of the winter to the winter bees?

Segueing into winter bee production 

The graph below shows the numbers of bees of a particular age present in the hive between the end of August and early December.

Colony age structure from August to December – see text for details

Each distinct colour represents bees reared in a particular 12 day ‘window’. All bees present before the 31st of August are blue. The next 12 day cohort of bees are yellow etc. The area occupied by each colour indicates the number of bees of a particular age cohort.

Note that egg laying (black) is negligible between early October and late November when it restarts.

The graph shows that that there is no abrupt change from production of summer bees to production of winter bees.

For example, about 95% of the blue bees have disappeared by December 1. Of the yellow bees, which first appeared in mid-September, about 33% are present in December. Finally, the majority of the lime coloured bees, that first put in an appearance in early October, are present at the end of December.

The colony does not abruptly stop producing short-lived summer bees on a particular date and switch to generating long-lived ‘diutinus’ winter bees. Instead, as late summer segues into early autumn, fewer short lived bees and more long lived bees are produced. 

Note that each cohort emerge from eggs laid 24 days earlier. The orange cohort emerging from 24/09 to 05/10 were laid within the first two weeks of September. This emphasises the need to treat early to reduce mite levels sufficiently to protect the winter bees.

Winter bees are like nurse bees but different

Before we consider what triggers the production of diutinus bees we need to discuss how they differ from summer bees, both nurses and foragers.

Other than being long-lived what are their characteristics?

Interaction of key physiological factors in nurse (green), forager (red) and winter bees (blue). Colored disks indicate the relative abundance of each factor.

The four key physiological factors to be considered are the levels of juvenile hormone (JH), vitellogenin (Vg) and hemolymph proteins and the size of the hypopharyngeal gland (HPG).

As summer nurse bees transition to foragers the levels of JH increases and Vg decreases. This forms a negative feedback loop; as Vg levels decrease, JH levels increase. Nurse bees have high levels of hemolymph proteins and large HPG, the latter is involved in the production of brood food fed to larvae.

So if that describes the summer nurse bees and foragers, what about the winter bees?

Winter bees resemble nurse bees in having low JH levels, high levels of VG and hemolymph proteins and large HPG’s. 

Winter bees differ from nurse bees in being long lived. A nurse bee will mature into a forager after ~3 weeks. A winter bee will stay in a physiologically similar state for months.

There have also been time course studies of JH and Vg levels through the winter. In these, JH levels decrease rapidly through October and November and are at a minimum in mid-January, before rising steeply in February and March.

As JH levels rise, levels of Vg and hemolymph proteins decrease and the size of the HPG decreases i.e. as winter changes to early spring winter bees transition to foragers.

Now we know what to look for (JH, Vg levels etc) we can return to think about the environmental triggers that cause these changes.

No single trigger

In temperate regions what distinguishes winter from autumn or spring? 

Temperatures are lower in winter.

Daylength (photoperiod) is shorter in winter.

There is less pollen and nectar (forage) available in winter.

Under experimental conditions it’s quite difficult to change one of these variables without altering others. For example, shifting a colony to a cold room (i.e. lowering the ambient temperature to <10°C) leads to a rapid decrease in JH levels (more winter bee-like). However, the cold room was dark, so perhaps it was daylength that induced the change? Alternatively, a secondary consequence of moving the colony is that external forage was no longer available, which could account for the changes observed.

And forage availability will, inevitably, influence brood rearing.

Tricky.

Reducing photoperiod alone does induce some winter bee-like characteristics, such as increases in the protein and lipid content of the fat bodies. It also increases resistance to cold and starvation. It can even cause clustering at elevated (~19°C) temperatures. However, critically, a reduced photoperiod alone does not appear to make the bees long lived. 

Remember also that a reduced photoperiod will limit foraging, so reducing the nutritional status of the colony. This is not insignificant; pollen trapping 2 in the autumn accelerates the production of winter bees.

But again, this may be an indirect effect. Reduced pollen input will lead to a reduction in brood rearing. Feeding pollen to broodless winter colonies induces egg-laying by the queen.

Brood, brood pheromones and ethyl oleate

One of the strongest clues about what factor(s) induces winter bee production comes from studies of free-flying summer colonies from which the brood is removed. In these, the workers rapidly change to physiologically resemble winter bees 3.

How does the presence of brood prevent the generation of diutinus bees?

There are some studies which demonstrate that the micro-climate generated in the colony by the presence of brood – elevated temperature (35°C) and 1.5% CO2 – can influence JH levels. 

However, brood also produces pheromones – imaginately termed brood pheromone – which does all sorts of things in the colony. I’ve discussed brood pheromone previously in the context of laying workers. The brood pheromone inhibits egg laying by worker bees.

Brood pheromone also contributes to a enhancement loop; it induces foraging which results in increased brood rearing and, consequently, the production of more brood pheromone.

One way brood pheromone induces foraging is by speeding the maturation of middle-aged hive bees into foragers. Conversely, when raised in the absence of brood, bees have higher Vg levels, start foraging later and live longer.

But it’s not only brood that produces pheromones.

Workers also produce ethyl oleate, a pheromone that slows the maturation of nurse bees, so reducing their transition to foragers.

A picture is worth a thousand words

All of the above is quite complicated.

Individual factors, both environmental and in the hive, have direct and indirect effects. Experimentally it is difficult to disentangle these. However, Christina Grozinger and colleagues have produced a model which encapsulates much of the above and suggests how the production of winter bees is regulated. 

Proposed model for regulation of production of winter bees.

During autumn there is a reduction in forage available coupled with a reduced daylength and lower environmental temperatures. Consequently, there is less foraging by the colony. 

Since more foragers are present within the hive, the nurse bees are exposed to higher levels of ethyl oleate, so slowing their maturation.

There’s less pollen being brought into the colony (reduced nutrition), so brood production decreases and so does the level of brood pheromone. The reduced levels of brood pheromone also reduces nurse bee maturation.

As shown in the diagram, all of these events are in a feedback loop. The reduction in levels of brood pheromone further reduces the level of foraging … meaning more foragers are ‘at home’, so increasing the effects of ethyl oleate.

All of these events have the effect of retarding worker bee maturation. The workers remain as ‘nurse-like’ long-lived winter bees.

Is that all?

The difference between nurse bees and winter bees is their longevity … or is it?

In the description above, and in most of the experiments conducted to date, the key markers of the levels of JH, Vg and hemolymph proteins, and the size of the HPG, are what has been studied. 

I’d be astounded if there are not many additional changes. 

Comparison of workers and queen bees have shown a large range of epigenetic changes induced by the differences in the diet of young larvae 4. Epigenetic changes are modifications to the genetic material that change gene expression.

I would not be surprised if there were epigenetic changes in winter bees, perhaps induced by alteration of the protein content of their diet as larvae, that influence gene expression and subsequent longevity. Two recent papers suggest that this may indeed happen; the expression of the DNA methyltransferases (the enzymes that cause the epigenetic modifications) differs depending upon the demography of the colony 5 and there are epigenetic changes between the HPG in winter bees and bees in spring 6.

Clearly there is a lot more work required to fully understand the characteristics of winter bees and how they are determined.

Don’t forget …

It’s worth emphasising that the local environment (forage and weather in particular) and the strain of the bees 7 will have an influence on the timing of winter bee production.

Last week I discussed a colony in my bee shed that had very little brood on the 2nd of October (less than one side of one frame). When I checked the colonies last weekend (11th) there were almost no bees flying and no pollen coming in. A colleague checked an adjacent colony on Monday (13th) and reported it was completely broodless. These bees are ‘local mongrels’, selected over several years to suit my beekeeping.

Early autumn colonies

In contrast, my colonies on the west coast are still busy. These are native black bees. On the 14th they were still collecting pollen and were still rearing brood. 

The calendar dates in the second figure (above) are therefore largely irrelevant.

The transition from summer bees to the diutinus winter bees will be happening in your colonies, sooner or later. I suspect it’s already completed in my Fife bees.

Whether genetics or environment has a greater influence on winter bee production remains to be determined. However, I have previously described the good evidence that local bees are better adapted to overwintering colony survival.

To me, this suggests that the two are inextricably linked; locally selected bees are better able to exploit the environment in a timely manner to ensure the colony has the winter bees needed to get the colony through to spring.