Category Archives: Science

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.


 

Winter bee production

There are big changes going on in your colonies at the moment.

The summer foragers that have been working tirelessly over the last few weeks are slowly but surely being replaced. As they die off – whether from old age or by being eaten by the last of the migrating swallows – they are being replaced by the winter bees.

Between August and late November almost the entire population of bees will have changed. The strong colonies you have now (or should have) will contain a totally different workforce by the end of the year.

Forever young

The winter bees are the ones responsible for getting the colony from mid/late autumn through to the following spring. They are sometimes termed diutinus bees from the Latin for “long lived”.

These are the bees that thermoregulate the winter cluster, protecting the queen, and rearing the small amounts of brood during the cold, dark winter to keep the colony ticking over.

Midwinter cluster

A midwinter colony

Physiologically they share some striking similarities with so-called hive or nurse bees 1 early in the summer.

Both hive bees and winter bees have low levels of juvenile hormone (JH) and active hypopharyngeal glands. Both types of bee also have high levels of vitellogenin, high oxidative stress resistance and corpulent little bodies.

But early summer nurse bees mature over a 2-3 week period. Their JH levels increase and vitellogenin levels decrease. This induces additional physiological changes which results in the nurse bee changing into a forager. They sally forth, collecting nectar, pollen and water …

And about three weeks later they’re worn out and die.

Live fast, die young

And this is where winter bees differ. They don’t age.

Or, more accurately, they age   v  e  r  y    s  l  o  w  l  y.

In the hive, winter bees can live for 6 months if needed. Under laboratory conditions they have been recorded as living for up to 9 months.

They effectively stay, as Bob Dylan mumbled, forever young.

Why are winter bees important?

Although not quite eternal youth … staying forever young is useful as their longevity ensures that the colony does not dwindle and perish in the middle of winter.

With little or no nectar or pollen available in the environment the colony reduces brood rearing, and often stops altogether for a period.

But what about the kilograms of stores and cells filled with pollen in the hive? Why can’t they use that?

Whilst both are present, there’s nothing like enough to maintain the usual rate of brood rearing. If they tried the colony would very quickly starve.

Evolution has a very effective way of selecting against such rash behaviour 🙁

If you doubt this, think how quickly hives get dangerously light during the June gap. With no nectar coming in and thousands of hungry (larval) mouths to feed the colony can easily starve to death during a fortnight of poor weather in June.

The winter bees ‘hold the fort’, protecting the queen and rearing small amounts of brood until the days lengthen and the early season pollen and nectars become available again.

And, just as the winter bees look after the viability of the colony, the beekeeper in turn needs to look after the winter bees … we rely on them to get the colony through to spring.

Lots of bees

Can you identify the winter bees?

But before we discuss that, how do you identify and count the winter bees? How can you tell they are present? After all, as the picture above shows 2, all bees look rather similar …

Counting the long lived winter bees

The physiological changes in winter bees, such as the JH and vitellogenin levels, are only identifiable once you’ve done some rather devastating things to the bee. These have the unfortunate side effect of preventing it completing any further bee-type activities 🙁

Even before you subject them to that, their fat little bodies aren’t really sufficiently different to identify them visually.

But what is different is their longevity.

By definition, the diutinus or winter bees are long lived.

Therefore, if you record the date when the bee emerged you can effectively count back and determine how old it is. If it is more than ~6 weeks old then it’s a winter bee.

Or the queen 😉

And, it should be obvious, if you extrapolate back to the time the first long lived bees appear in the hive you will have determined when the colony starts rearing winter bees.

The obvious way to determine the age of a bee is to mark it upon emergence and keep a record of which marks were used when. Some scientists use numbered dots stuck to the thorax, some use combinations of Humbrol-type paint colours.

I’m not aware that anyone has yet used the barcoding system I discussed recently, though it could be used. The winter bee studies I’m aware of pre-date this type of technology.

Actually, some of these studies date back almost 50 years, though the resulting papers were published much more recently.

This is painstaking and mind-numbingly repetitive work and science owes a debt of gratitude to Floyd Harris who conducted many of the studies.

Colony age structure – autumn to winter

Here is some data showing the age structure of a colony transitioning from late summer into autumn and winter. There’s a lot in this graph so bear with me …

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

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

The graph shows the numbers and ages of bees in the colony.

The ages of the bees is indicated on the vertical axis – with eggs and brood (the youngest) at the bottom, coloured black and brown respectively. The adult bees can be aged between 1 and ~100 days old 3. The number of bees is indicated by the width of the coloured bar at each of the nine 12 day intervals shown.

All of the adult bees present in the hive at the end of August are coloured blue, irrespective of their age. There are a lot of these bees at the end of August and almost all of them have disappeared (died) by mid-November 4.

The remaining colours indicate all the bee that emerge within a particular 12 day interval. For example, all the bees that emerge between the 31st of August and the 12th of September are coloured yellow.  Going by the width (i.e. the numbers of bees of that age) of the yellow bars it’s clear that half to two-thirds of these bees die by mid October, with the rest just getting older gracefully.

But look at the cohort that emerge between the end of September and early October, coloured like this 5. The number of these bees barely changes between emergence and early December. By this time they are 72 days old i.e. an age that most summer bees never achieve.

Brood breaks and climate

In the colony shown above the queen continued laying reduced numbers of eggs – the black bars – until mid-October and then didn’t start again until the end of November. During this period the average age of the bees in the colony increased from ~36 days to ~72 days and the strength of the colony barely changed.

The figure above comes from a BeeCulture article by Floyd Harris. The original data isn’t directly referenced, but I suspect it comes from studies Harris conducted in the late 70’s in Manitoba, some of which was subsequently published in the Journal of Apicultural Research. In addition, Harris co-authored a paper presenting similar data in a different format in Insectes Sociaux which describes the Manitoban climate as having moderate/hot summers and long, cold winters.

My hives in Scotland, or your hives in Devon, or Denmark or wherever, will experience a different climate 6.

However, if you live in a temperate region the overall pattern will be similar. The summer bees will be replaced during the early autumn by a completely new population of winter bees. These maintain the colony through to the following spring.

The dates will be different and the speed of the transition from one population to the other may differ. The timing of the onset of a brood break is likely to also differ.

However, the population changes will be broadly similar.

And, it should be noted, the dates may differ slightly in Manitoba (and everywhere else) from year to year, depending upon temperature and forage availability.

Colony size and overwintering survival

Regular readers might be thinking back to a couple of posts on colony size and overwintering survival from last year.

One measured colony weight, showing that heavier colonies overwintered better 7. A second discussed the better performance of local bees in a Europe-wide study of overwintering survival. In this, I quoted a key sentence from the discussion:

“colonies of local origin had significantly higher numbers of bees than colonies placed outside their area of origin”

I can’t remember when during the season those studies recorded colony size, but I’m well aware that large colonies in the winter survive better.

The colonies that perish first in the winter are the pathetic grapefruit-sized 8 colonies with ageing queens or high pathogen loads.

In contrast, the medicine ball-sized ‘boomers’ go on and on, emerging from the winter strongly and building up rapidly to exploit the early season nectar.

But what the graph above shows is that the bees in a strong colony in late summer are a completely different population from the bees in the colony in midwinter.

The strength of the midwinter colony is determined entirely by when winter bee rearing starts and the laying rate of the queen, although of course both may be indirectly influenced by summer colony strength.

The influence of the queen

Other than this potential indirect influence, it’s possibly irrelevant how large the summer colony is in terms of winter colony size (and hence survival).

After all, even if the summer bees were three times as numerous, their fate is sealed. They are all going to perish six weeks or so after emergence.

Are there ways that beekeepers can influence the size of the overwinter colony to increase its chances of survival?

I wouldn’t pose the question if the answer wasn’t a resounding yes.

It has been known for a long time 9 that older queens stop laying earlier in the autumn than younger queens. As explained above, the longer the queen lays into the autumn the more winter bees are going to be produced.

Mattila et al., 10 looked at the consequences of late season (post summer honey harvest) requeening of colonies. In these they removed the old queen and replaced her with either a new mated or virgin queen, or allowed the colony to requeen naturally.

Using the ’12 day cohort’ populations explained above, the authors looked at when the majority of the winter bees were produced in the colony, and estimated the overall size of the winter colony.

The influence of new queens on winter bee production.

The influence of new queens on winter bee production. Note shift to the right in B, C and D, with new queens.

With the original old queen, 53% of winter bees were produced in the first two cohorts of winter bees. With the requeened colonies 54-64% of the winter bees were produced on average 36 days later, in the third and fourth cohorts of winter bees.

This indicates that young queens produce winter bees later into the autumn.

This is a good thing™.

In addition, though the results were not statistically significant, there was a trend for colonies headed by new queens to have a larger population of bees overwinter.

Perhaps one reason the requeened colonies weren’t significantly larger was that the new queens delay the onset of winter bee rearing. I’ll return to this at the end.

The influence of deformed wing virus (DWV)

Regular readers will know that this topic has been covered extensively, and possibly exhaustively, elsewhere on this site … so I’ll cut to the chase.

DWV is the most important virus of honey bees. When transmitted by Varroa destructor there is unequivocal evidence that it is associated with overwintering colony losses. The reason DWV causes overwintering losses is that it reduces the longevity of the winter bees.

The virus might also reduce the longevity of summer bees but,

  1. there’s so many of them to start with
  2. there’s loads more emerging every day, and
  3. they only survive a few weeks anyway,

that this is probably irrelevant in terms of colony survival.

Dainat et al., (2012) produced compelling evidence showing that DWV reduces the longevity of winter bees 11. The lifespan was reduced by ~20%.

A consequence of this is that the winter bees die off a little faster and the colony shrinks a little more. At some point it crosses a threshold below which it cannot thermoregulate the cluster properly, further limiting the ability of the colony to rear replacement bees (assuming the queen is able to lay at a low rate).

This colony is doomed.

Even if they stagger through to the longer days of spring they contain too few bees to build up fast. They’re not dead … but they’re hardly flourishing.

Winter bees and practical beekeeping

I think there are three ways in which our understanding of the timing of winter bee production should influence practical beekeeping:

Firstly … The obvious take-home message is that winter bees must be protected from the ravages of DWV. The only way to do this is to minimise the mite population in the colony before the winter bee rearing starts.

The logical way to do this is to treat using an approved miticide as soon as practical after the summer honey is removed 12.

I discuss the importance of the timing of this treatment in When to treat?, which remains one of the most-read posts on this site.

Secondly … Avoid use of miticides (or other colony manipulations) that reduce the laying rate of the queen in early autumn.

When I used to live at lower latitudes I would sometimes use Apiguard. This thymol-containing miticide is very effective if used when the temperature is high enough. However, in my experience a significant proportion of queens stop laying when it is being used. Not all, but certainly more than 50%.

I don’t know why some stop and others don’t. Is it genetic? Temperature-dependent?

Whatever the reason, they stop at exactly the time of the season you want them to be laying strongly.

Thirdly … consider requeening colonies with young queens after the summer honey is removed. This delays the onset of winter bee production and results in the new queen laying later into the year. The later start to winter bee production gives more time for miticides to work.

A win-win situation.


 

The new normal

For many, beekeeping associations provide the bookends that bracket the practical beekeeping season. In meetings during the dark, wet, cold winter months we can at least discuss bees, reminisce about the season just gone or plan for the season ahead.

Usually with tea and biscuits 🙂

Or in the more civilised associations (and a quick plug here for the Fortingall & District BKA) with fantastic homemade cakes 😉

Elderflower lemon drizzle cake

Beekeeping associations, through the training and social events that they organise and the contacts that they enable, provide an important support framework for beekeepers, both new and old.

Training new beekeepers is one important function associations provide, but more experienced beekeepers also benefit from co-operative purchasing schemes for foundation or fondant 1 and – of course – from the winter seminar programmes.

Double whammy

The Covid-19 pandemic has dealt a double whammy to many associations.

Training events, necessitating flagrant breaches of social distancing during hands-on practical beekeeping demonstrations, are a problem. Many associations delivered the theoretical coursework before lockdown was imposed, but were subsequently unable to provide the practical component of the training for beginners.

It’s difficult to spot the queen from 50 centimetres sometimes, let alone 2 metres.

Returning a marked and clipped queen – tricky to do at arm’s length

The independent first inspections for 2020 beginners are likely to have been a pretty tough challenge for many. Congratulations to those who got through them and the rest of the season with little support.

I’m hearing that some associations have cancelled or postponed all training events for the ’20/’21 winter season.

The imposition of lockdown 2 in March probably had little impact on the ’19/’20 winter seminar programmes, but they’re likely to have a significant impact going forwards.

I give quite a few talks on science and practical beekeeping in most winters. Audiences and venues vary, depending on the association. I’ve talked in drafty church halls to groups of 15, or swanky conference centres to ten times that number.

There is always a good turnout by new beekeepers, or even by those who have yet to start keeping bees.

Not your typical beekeeping audience … or church hall

However, there is generally a gender imbalance, with more men than women attending. And – and I’m afraid there’s no gentler way to write this – there’s an age imbalance as well, with the enthusiastic young ‘uns outnumbered by older, and in some cases old, beekeepers.

Statistics

This age and gender imbalance inevitably make the ’20/’21 winter seminar programmes an endangered species, at least in the format we’ve grown used to over past seasons.

If you look at the statistics for serious Covid-19 cases it is clear that there is a strong bias towards elderly males. There are other biases as well … underlying medical complications and ethnicity also have a major influence, though whether the latter is socio-economic, genetic or due to the presence of comorbidities remains unclear.

All of which means that spending an hour in a drafty church hall listening to a talk on bait hives is probably unwise … not least because the social distancing needed precludes any chance to huddle together for warmth when the one bar electric heater blows a fuse.

Zoom …

In the brave new, socially distanced, world we’re currently inhabiting, drafty church halls and excellent homemade cakes are now just a distant memory. 

Instead we have Teams talks, Demio demonstrations and WebEx webinars. 

And Zoom, but I can’t think of a suitable alliteration to go with Zoom 🙁

For many office-based workers, lockdown resulted in the substitution of boardroom meetings with spare bedroom virtual meetings. 

Hastily repurposed guest bedrooms have become home offices. The combination of IKEA furniture, a reasonably recent laptop and a fast internet connection has enabled ‘business as usual’.

Almost.

All of my meetings – with administrators, colleagues, my research team and students – have been online since late March (and in certain cases since early March).

Academics are used to collaborating globally and so were already familiar with Zoom, Teams or Skype for conference calls and job interviews. These have just continued 3, and been extended to now include all the in-person meetings that used to happen.

One or two colleagues have embraced this expanded use of the technology to have their own ‘green screens’. This allows their head and upper torso to be projected in front of a selected image – of a tropical beach, their favourite golf course or local boozer.

The Maldives … the perfect backdrop for a dull committee meeting

The really professional ones even change out of their pyjamas before calls … 😉

But many beekeepers will be largely unfamiliar with the technology and the advantages it offers … and disadvantages it imposes.

Online beekeeping talks

I’ve both attended and delivered online beekeeping talks. Not a huge number, but enough to have a fair idea of what works and what doesn’t. In addition, I’ve taken part – as audience or presenter – in hundreds of non-beekeeping online events.

For readers who have yet to take part here’s a general guide of what to expect.

The speaker and topic are advertised in advance and those interested in listening/watching register to attend. The talk is hosted by the beekeeping association who provide a ‘chairperson’ or ‘master of ceremonies’. This person has the unenviable task of dealing with the speaker, the technology and the audience. 

And two of these three might do something incomprehensibly stupid … and the internet can break.

On the evening of the talk 4 you login via a website (using a username/password provided on registration) and launch the necessary software to take part in the event.

Eventbrite beekeeping talk

Sometimes this can be through the web browser, but – more usually – it involves downloading and installing software onto your computer. Which might be an issue for some people wanting to take part. Do this in advance of the start time of the talk, not in the last 2 minutes before kickoff.

After an introduction by the chairperson, control of the graphics is usually handed to the speaker who delivers the talk. To avoid awkward ‘noises off’ 5 the chairperson usually mutes all other microphones

Why unenviable?

I previously described the chairpersons role as unenviable.

While the speaker blathers away the chair is probably:

  • dealing with email enquiries about how to launch the software
  • justifying why there isn’t a video of the speaker actually speaking (it’s turned off to save bandwidth), and
  • telling someone that they are the only person unable to hear the presenter. Therefore, it must be their audio output settings that are wrong.

And if that isn’t enough, the chair will be collecting and collating questions during and after the talk, for reasons I’ll discuss shortly.

Finally, it’s not unusual for the chair to also ensure that the talk is recorded so that those who couldn’t download the software or hear the presenter can attempt to listen to it in the future.

That’s a lot to deal with.

Questions and answers

Good talks generate questions.

As a speaker, there’s nothing worse than a talk being met by an echoing wall of silence.

Hello? Is there anybody [out] there? Just nod if you can hear me 6.

Some are points of clarification, others are after elaboration or explanation of a contrary view.

Some questions are nothing whatsoever to do with the talk 😉

They might not even be about beekeeping.

All require an answer of some kind.

And this is where the technology gets in the way of communication. 

Questions from the floor, in which the audience member switches on their microphone, clearly enunciates the question, and turns off the mike returning ‘control’ to the host and speaker cause delays.

Often significant delays. However, even short breaks interrupt good communication – think back to the lag on transatlantic satellite phone calls. 

The speaker asks the question clearly … but omits to turn on the mike.

Or they fail to the turn off the mike, so the entire audience hears the follow up “and I hope he answers quickly as Strictly’s on in a few minutes”.

For a couple of questions this is just about acceptable. For twenty or thirty it is not.

So, the beleaguered chair takes written questions from the audience, collates them, removes duplicates … and then asks the presenter on behalf of the audience.

I refer you back to the word ‘unenviable’. If you take part, cut the chair some slack …

Disadvantages of online talks

Unfortunately a subset of beekeepers who would have attended a gathering on the second Tuesday of the month in the church hall will never attend an online beekeeping talk.

For a start, they might not even own a computer. 

They might – and I have considerable sympathy for this view – mainly attend talks for the craic, the opportunity to catch up with friends and the chance of some homemade lemon drizzle cake.

All of those are good reasons to attend a talk in person … and in the case of lemon drizzle cake I’d say a compelling reason to attend 😉

As a regular speaker at associations I’d add here that the craic and the homemade cake are the parts of the evening I enjoy the most. After all, I’ve heard the talk before. I might even have heard the questions before 😉

None of these more social things are achievable online. Everyone listens in their own little bubble, isolated from the shared experience.

If they can’t bake it’s going to be a long evening 🙁

For others, the technology will continue to be a problem. They can see the pictures but can’t hear the words. Or vice versa. Or worse …

No Zoom for you …

The fact that 190 others don’t have the same problems just makes it a more frustrating and unrewarding experience. Being live, there’s no real chance of resolving these ‘local’ problems without delaying the talk and irritating the rest of the audience.

Over time the numbers unable to handle the technology will reduce.

In some cases it’ll be because they have learned to master it – either by perseverance, or by the beekeeping association providing some sort of training sessions.

In other cases it’ll be because they simply gave up 🙁

Like those who don’t have a computer in the first place, this means online talks are serving a different audience and some association members are likely to be excluded by the switch to online talks.

Advantages of online talks

But it’s not all bad news. I can see some benefits for both the speaker and audience from online presentations.

Associations can invite speakers from anywhere.

They don’t have to be from the same county.

Or the same country.

This broadens the topics that can be covered and provides the opportunity to discover different beekeeping practices from other areas (always remembering that this might simply confuse beginners).

There are some good speakers out there – just look through past programmes for the BBKA, SBA or WBKA Annual Conventions or the National Honey Show.

Associations can ‘share’ speakers by running joint events or inviting neighbouring association members to register.

As a speaker, this means that audiences tend to be larger. Bigger audiences are almost always better 7. Since everyone is logging on, rather than driving across the county to the venue, there’s less chance a spot of bad weather will put people off.

This is a huge advantage for the speaker as well … I’ve regularly talked, answered questions, drunk tea, chatted, eaten lemon drizzle cake, drunk more tea, said my goodbyes and then driven for three hours to get home 8.

The Beast from the East ...

The Beast from the East …

I’ve also had to cancel talks at relatively short notice due to ‘adverse driving conditions’ – which in Scotland means a bit more than a dusting of snow. 

None of that happens in our brave new digital world.

Is this the new normal?

For the foreseeable future I think it is. The national lockdown is being replaced by local restrictions where virus transmission is increasing. However, school and university students have yet to return and this will likely lead to increased transmission in some areas (in Scotland, we’re already seeing this, though transmission is usually ascribed to “unregulated house parties” rather than within the school 9 ).

A vaccine remains some way off. It’ll be even longer until we have vaccinated a large enough proportion of the population to interrupt transmission.

Or to know how long immunity lasts.

All of which means that indoor social events, like talks about bait hives or swarm control, are likely to be undesirable, unattractive or simply not allowed.

What can we do to improve things?

Delivering a talk online is a very much less rewarding experience than doing so in person.

There’s no ability to properly engage with the audience – no banter, no eye contact, no jokey comments.

You can’t tell whether the old boy in the back row has switched off or just nodded off.

Or perhaps he’s simply cheesed off because he disagrees with everything I’m saying.

You don’t necessarily know who is in the audience – you might know overall numbers, but not whether the local bee inspector or beefarmer is logged on. Knowledge of the audience can influence the way you pitch a talk.

It’s an oddly sterile undertaking. This makes judging the pacing and content of the talk very much more difficult. With a live audience it’s usually possible to tell whether they’re ‘keeping up’ or ‘tuning out’. You can’t do this online.

If the audience is present you can ask questions and get immediate answers … anyone who has heard me talk will know I ask about drifting and ‘how many of your bees are your bees?’. There are ways of doing this online, but it requires familiarity with more software (or additional features of the current software).

Starting materials ...

Practical demonstrations and online presentations – tricky

In the meantime …

  1. Associations can help their members embrace the technology by providing limited training where it is needed. 
  2. Think creatively about topics that can be covered within the limitations of the technology. For example, some of the talks I’ve been to in person have had a practical component. I’ve attended excellent candle dipping and skep making workshops 10. Likewise, I talk about DIY for beekeeping which involves handing round examples of my hastily cobbled together beautifully crafted floors or roofs. These sorts of things might be achievable entirely online, perhaps with more videos. However, preparing these will certainly require a lot more work by the presenter to be effective.
  3. Provide feedback to speakers – what worked and what didn’t work?

Welcome to the new normal … I hope to “see” you online sometime 🙂


Notes

The new normal means “a previously atypical or unfamiliar situation, behaviour, etc., which has become standard, usual, or expected” (OED). Although now associated with the Covid-19 pandemic, it’s usage can be traced back over 200 years. The big increase in usage was in reference to the 2008 financial crisis, and – historically – it is often used in reference to economic events. 

The new normal – Google ngram results

Orientation flights

Part of the reason for the success of honey bees is the division of labour between workers of different ages. Young workers (hive bees) clean the cells, nurse larvae and look after the queen. Older workers are foragers, collecting pollen and nectar (and water) from across the landscape.

To be successful, foragers need to know where to look and how to return.

The ‘where to look’ is partly accounted for by the well-known waggle dance 1.

In this post I’m going to discuss the second component of successful foraging – the homing ability of foragers.

More specifically, I’m going to discuss how the bee first learns about the location of the hive. 

Orientation flights

Bees do not instinctively know where the hive (or the tree they are nesting in for a wild colony) is located. They have to learn this before embarking on foraging trips to collect nectar or pollen.

This learning takes the form of one (or usually several – as we shall see) orientation flights. These enable the bee to memorise the precise location of the hive with relation to geographic landmarks. On subsequent foraging flights the bees use these landmarks to return to the hive.

Orientation flight have a characteristic appearance …

… and are very nicely described in the introduction to a paper by Capaldi and Dyer 2:

An orientation flight at the nest entrance begins as a departing bee turns and hovers back and forth, turning in short arcs, apparently looking at the hive entrance. Then, the bee increases the size of the arcs until, after a few seconds, she flies in circles while ascending to heights of 5–10 metres above the ground. This spiraling flight takes the bee out of sight of human observers. She returns a few minutes later, always without nectar or pollen.

Which I couldn’t have written any better, so have reproduced verbatim.

There are a number of features of the orientation flight that are immediately obvious from this description (which all beekeepers will recognize). These include:

  • A ‘local’ component, in the immediate vicinity of the hive
  • Wider ranging flight at a greater altitude and a longer distance
  • Direct observation does not allow the location, duration or track of these distant flights to be monitored
  • The bee returning from the orientation flight does not bring pollen or nectar with her

Do orientation flights allow orientation?

How do we know that these flights enable the bee to learn where the hive is located?

Early studies conducted by Becker (1958) showed that bees captured after a single orientation flight and then re-released up to 700 metres away from the hive could find their way ‘home’. In contrast, bees that had not gone on an orientation flight before were, by definition, disoriented and did not return to the hive.

However, the percentage that returned after undertaking a single orientation flight was related to the distance of the release point, and was never more than ~60% (at 200 metres). 

In contrast, reorienting older foragers (for example, as happens after moving a hive to a new location) were much better (~90%) at returning to the hive after a single reorientation flight. 

Capaldi and Dyer extended these early studies by Becker to investigate the impact of the visibility of local landmarks on orientation and reorientation, and also measured the speed with which bees returned after being displaced.

Landmarks

These studies showed that a single orientation flight allowed bees to identify the landmarks in the immediate vicinity (100 – 200 m) of the hive. When released from more distant locations, returning flights were faster and more successful (i.e. fewer lost bees) when the bees had sight of the landmarks in the vicinity of the hive.

The hive itself was effectively invisible except at very short ranges. This makes sense for a tree-nesting animal. One tree looks much the same as another 3, but if you learn that the nest is in the tree between the very tall conifer and the long straight hedgerow – two features visible from hundreds of metres distant – then orientation is straightforward.

This suggests that apiaries located near distinctive landscape features may be preferable in terms of increased returning forager rates.

“Distinctive” as far as an orienting worker bee is concerned, which may not be the same as distinctive to the beekeeper of course 😉

Reorienting bees (compared to first flight bees) took longer to explore the environment and were better at returning. Either these bees learn differently (a distinct possibility) or their prior experience in the wider landscape gives them an advantage when the hive is relocated.

Where do you go to my lovely?

The early studies by Becker and those by Dyer and colleagues defined many of the parameters that characterise orientation flights. What they did not do is show where the bees actually go during the orientation flights?

Do they just zoom around randomly?

Do they fly ever-increasing spirals?

Perhaps they perform some sort of grid search, exploring individual landscape features carefully for future reference?

Recent developments with harmonic radar have allowed tracking of individual bees during orientation flights over hundreds of metres. These have provided further insights into the process.

Because harmonic radar is also relevant to other studies of honey bee flight – for example, the impact of neonicotinoids on foraging ability – I’ll digress slightly from orientation flights to describe the technology.

Harmonic radar

Harmonic radar has revolutionised tracking studies of insects in much the same way as GPS tags have provided unique insights into bird migration (or, for that matter, shark migration).

The radar system has two components. The insect is tagged with a tiny antenna attached to a Schottky diode (together termed the transponder). The transmitter/detector is a ground-based scanner that transmits the radar signal. This is used as the energy source by the diode which re-emits a harmonic of the original signal which can then be detected.

Tagged bumble bee (left) and harmonic radar detector (right)

The transponder weighs less than a normal pollen load, though presumably there is some wind resistance from the antenna. In studies of bees with and without transponders fitted the orientation flights were of a similar duration, suggesting any wind resistance didn’t appreciably impact the flying ability of the bee.

Orientation (a-c) and foraging (d) flights monitored by harmonic radar.

 

Orientation flights 4 were taken by bees between 3 and 14 days post-emergence, with the mean onset of foraging being 14 days post-emergence.

Bees took between 1 and 18 orientation flights, though there wasn’t a direct relationship between the number of flights and the age of the bee, suggesting they may learn at different rates.

Initial orientation flights were generally in the immediate vicinity of the hive. Older workers – pre-foraging – ventured further afield. More recent studies have addressed this in greater detail (see below).

Orientation flights were distinctly different from foraging flights. The former were slower and less direct. The ground speed of orienting bees was ~3.6 m/s in contrast to foragers who flew at ~5.6 m/s and, as shown in D above, foraging flights were very much ‘there and back’ straight lines.

Venturing forth …

A more recent study 5 has used harmonic radar to investigate multiple orientation flights by individual bees, effectively analysing how the bee explores the landscape as it ages towards a forager.

This was a remarkable study. It involved addition and subsequent removal of the transponder from 115 individual bees during 184 orientation flights. When the orienting bee returned they recaptured it, removed the transponder and allowed it to reenter the hive. When it reappeared for another orientation flight they reattached the transponder.

Anyone complaining about their inability to mark queens 6 should do this as a training exercise 😉

The scientists also recorded several foraging flights of a smaller number of the same bees, to allow comparison with their behaviour during orientation flights.

Orientation and foraging flights of five individual bees.

Flights were defined as short or long range, but long range orientation flights were still significantly shorter than foraging flights.

  • Short range flights were made in poor weather and familiarised the bees with the immediate vicinity of the hive.
  • Consecutive long range flights reduced in duration as the bees learnt about the immediate hive vicinity i.e. the long range flights included some local exploring at first as well.
  • Orientation flights explored different areas of the landscape, rather than focusing on one sector.
  • Subsequent foraging flights involved areas that the bees may have never visited during orientation flights.
  • Some very long duration foraging flights may involve a degree of exploration, though it’s not clear whether this is truly orientation, or actual scouting activity.

Not all bees performed short and long range flights though early long range lights did involve local exploration as well. 

Ground clues and conclusions

The final part of this study investigated the influence of visual landscape features on orientation flights. This deserves a post in its own right as the techniques are quite involved.

Essentially they generated heat maps of the flights overlaid onto the geography. Using this approach they determined that some features visible from the air e.g. borders between grassland and a track, influenced the direction of flight and hence the orientation flights. 

There are additional studies of the influence of visible landmarks on bee flight which I’ll return to at some point in the future.

Again, like the comment made above about visible landmarks, it suggests to me that apiaries situated near such distinctive features may aid orientation and subsequent homing flights by honey bees.

When you next stand by your hive entrance on a warm, sunny afternoon and watch young workers flying to and fro across the entrance before spiralling up and away out of sight, you’ll know that it is an essential component of their training to be effective foragers.

They don’t forage for long – perhaps three weeks at most – but they are very effective, partly because they know where to return to.


Notes

Where do you go to my lovely? was the title of a rather syrupy (my blog, my opinion! … Apologies if it’s a favourite of yours 😉 ) song by Peter Sarstedt in 1969. It’s notable for some quite clever rhyming lyrics and a particularly dodgy mustache he sports in the YouTube video. (I’m just linking it, rather than embedding just because of the mustache).

It has nothing to do with bees.

 

A virus that changes bee behaviour

Particle physicists might not agree, but I think that evolution is the most powerful force in the universe. It is responsible for the fabulous diversity of life, for everything from the 6,000,000 kg Pando clonal colony of quaking aspen covering 43 hectares of the Fishlake National Forest in Utah, to the teeniest of tiniest of viruses.

As a microbiologist I’m acutely aware of the role evolution has played in the genetic arms race between hosts and pathogens. This is what is responsible for the multi-faceted immune system higher organisms carry – the antibodies, the lymphocytes, the complement and interferon responses, and everything else.

In turn, the fast replicating bacteria and viruses have evolved countermeasures to subvert these immune mechanisms, to switch them off entirely or to decoy them into targeting the wrong thing. This ‘arms race’ has gone on since well before the evolution of multicellular organisms (~600 million years ago) … and continues unabated.

Evolution is powerful for one simple reason; if a particular genetic combination 1 ‘works’ it will be passed on to the progeny. If a virus evolves a way to resist the immune response of the host, or to spread between hosts more efficiently, then the trait will be inherited.

Molecular mechanisms and behavioural changes

Some of changes work at the molecular level, invisible without exquisitely sensitive in vitro analysis; protein A binds to protein B and, in doing so, stops protein B from doing whatever it should have be doing. These are important but often very subtle.

Rabies: Slaying a mad dog, 1566 illustration from Wellcome Images

Other changes are far more obvious. Take rabies for example (or don’t, it’s not recommended as it has a near-100% case fatality rate) … the primary host of the rabies virus are carnivorous mammals. Infection causes gross behavioural changes that facilitate virus transmission. The animal becomes bolder and much more aggressive, resulting in virus transmission through biting.

Recent studies have elegantly demonstrated that this (also) is an example of protein A binding to protein B at the molecular level, it’s just that the phenotype 2 is very much more marked.

A protein on the surface of the virus resembles a snake venom toxin and has the ability to bind to nicotinic acetylcholine receptors present in the central nervous system of the mammalian host. These receptors ‘do what they say on the tin’ and bind the neurotransmitter acetylcholine. If the virus protein binds the receptor the response to acetylcholine is blunted and this, in turn, leads to hyperactivity, one of the key behavioural responses caused by rabies viruses.

That’s enough about mad dogs.

If virus-induced behavioural changes are so obvious, why haven’t lots of different examples already been identified and characterised?

Sniffles

Part of the problem is the blurring of distinctions between overt behavioural changes and the direct symptoms induced due to the virus replicating.

Human rhinovirus, the aetiological agent of the common cold, causes upper respiratory tract infections. You get a runny nose and you sneeze a lot.

Gesundheit

Your behaviour changes.

However, it’s generally accepted that the sneezing and runny nose are a result of the physiological response to infection, rather than a virus-induced behavioural response to facilitate transmission.

It’s worth noting that all that “stuff” that comes out of your nose contains infectious virus, so it’s perhaps an artificial distinction between the general symptoms of sickness and evolutionarily-selected host behavioural changes caused by the virus.

Which in a roundabout way …

… allows me to finally introduce the topic of bee viruses that cause host behavioural changes involved in their transmission.

Or, rather, one bee virus that does this … though I’m certain that there will be more.

Israeli Acute Paralysis Virus (IAPV) is an RNA virus transmitted horizontally by direct contact between bees, or while feeding on developing pupae, by the parasitic mite Varroa destructor. It was implicated as a causative agent of Colony Collapse Disorder (CCD), though really compelling evidence supporting it as the primary cause never materialised.

It’s a virus UK beekeepers should be aware of, but unworried by, as it is extremely rare in the UK.

A recent paper has shown two behavioural changes in response to IAPV infection in honey bees. One of them – that facilitates horizontal transmission between colonies – is also partially explained at the molecular level.

The paper was published a couple of months ago:

Geffre et al., (2020) Honey bee virus causes context-dependent changes in host social behavior. Proc. Natl. Acad. Sci. USA 117:10406-10413. 3

I’m going to focus on the results, rather than the methods, though the methods are rather cool. They used barcoded bees to allow the automated image analysis of every bee in a colony for some of the studies where they had introduced known IAPV infected individuals.

Responses of nestmates to IAPV infected bees

Imagine watching a few hundred waggle dances and being able to recount the position, distance and response of every bee ‘watching’ 4 the dance, and then being able to summarise the results.

Over five days.

Non-stop.

Including nights (and yes, bees do still waggle dance at night – a subject for the future).

The scientists orally infected groups of 30 bees with a sub-lethal dose of IAPV, marked them and released them into an observation hive. They then recorded their movements around the hive and their interactions with other bees in the colony. In particular, they focussed on trophallaxis interactions where one bee ‘feeds’ another.

Trophallaxis is also considered to be a method of communication in the hive and has been implicated in disease transmission.

The authors love their whisker plots and statistical analysis.

Who doesn’t? 😉

However, they generally make for rather underwhelming images in a bee blog for entertaining reading. Here .. see what I mean …

Number of trophallaxis interactions per hour.

Suffice to say that the results obtained were statistically significant.

They showed that the infected bees in the colony actually moved about the colony more than their nestmates. Conversely, they were engaged in fewer trophallaxis interactions i.e. it appeared as though they were being ‘ignored’ by their nestmates.

Were they really being ignored altogether or did their nestmates approach them, detect something was amiss and move away?

Antennation

Antennation is the mechanism by which bees recognise nestmates. They use the sensitive chemoreceptors on their antenna to detect cuticular hydrocarbons (CHC) which are distinctive between bees from different hives.

Antennation is a precursor to trophallaxis.

After all, bees do not want to feed a foreigner, or exchange chemicals involved in communications, or even potentially risk being exposed to a new pathogen.

Good as the barcoding and camera system is, it’s not good enough to record antennation within the observation hive. To do this they manually 5 recorded antennation events between IAPV-infected bees and nestmates in cages in the laboratory 6.

In these studies IAPV-infected bees were engaged in the same number of antennation events as control bees. This strongly suggests that the nestmate could detect there was something ‘wrong’ with the IAPV-infected individuals. In support of this conclusion, the authors also demonstrated that bees inoculated with a double stranded RNA (dsRNA) stimulator of the honey bee immune response were also also antennated equally, but engaged in less trophallaxis interactions.

Therefore, these studies appear to show that nestmates exhibit a behavioural response to IAPV-infected bees (and bees with elevated immune responses, recapitulating their response to pathogen infection) that is likely to be protective, reducing the transmission of horizontally acquired viruses.

It’s worth noting two things here.

  1. There were no virus transmission studies conducted. It’s assumed that the lack of trophallaxis reduces virus transmission. That still needs to be demonstrated.
  2. This response is not induced by the virus on the host. It’s a response by nestmates of the host to virus infected individuals (or individuals that present as ‘sick’). As such it’s not the same as the rabies example I started this post with.

Virus-induced behavioural responses

But do the IAPV-infected bees behave differently when they come into contact with other bees who are not their nestmates?

After all, IAPV is a pathogenic virus and its continuing presence within a population (not just a single hive) depends upon it being spread from hive to hive.

For a highly pathogenic virus this is very important. If you spread from bee to bee within a hive and kill the lot you also go extinct … this partly explains the mechanism by which highly virulent viruses become less virulent over time.

But back to IAPV. What happens when IAPV-inoculated bees interact with bees from a different hive?

For example, what would happen if they drifted from one hive to another in a densely populated apiary? Drifting is a significant contributor to the spread of bees between adjacent colonies – studies show that 1% of marked bees drift to adjacent hives over a 3 day window. This partially accounts for the genetic mix of workers (up to 40% are unrelated to the queen that heads the colony) in a hive, a fact generally unappreciated by beekeepers.

The authors first showed that IAPV-infected bees could apparently leave and return to the hive with a similar frequency as uninfected foragers. Their flying was not compromised.

They then resorted again to recording interactions in the laboratory between IAPV-infected bees or control dsRNA-inoculated bees and workers from a different hive.

This was where it gets particularly interesting.

Hello stranger

The dsRNA-immunostimulated bees (remember, these induce a generalised immune response characteristic of a ‘sick’ bee) were treated aggressively by unmatched workers from a different hive.

In contrast, the IAPV-infected bees (which were ‘sick’ and would have been undergoing immunestimulation caused by the IAPV infection) experienced significantly less aggression than both uninoculated workers (which induced an intermediate response) and the dsRNA-inoculated.

This strongly suggests that IAPV is somehow able to modulate the appearance or behaviour (and one often determines the other) of the host to make it more acceptable to an unmatched worker.

They extended this study to conduct “field-based assays at the entrances of three normally managed honey bee colonies”, monitoring whether IAPV-infected bees were more likely to be accepted by the guard bees at the entrance of the hive.

They were. The IAPV-infected bees received a less aggressive reception and/or entered the hives much more easily than the controls.

But what about proteins A and B?

Good question.

The behavioural alterations described above must be explainable in terms of the molecular changes that IAPV induces in the bees. By that I mean that the virus must make, or induce the making of, a chemical or protein or other molecule, the presence of which explains their acceptance by the foreign guard bees.

And the obvious candidates are the cuticular hydrocarbons (CHC) that are recognized during antennation, which I introduced earlier.

And here the story leaves us with some tantalising clues, but no definitive answer.

The scientists demonstrate that there were marked differences between the CHC profile of IAPV-infected and control bees. Again, they used their favoured whisker plots to show this, but collated all of the CHC data into an even more difficult to explain scatter plot of linear discriminant analysis.

CHC profiles (relative abundance) shown using linear discriminant analysis.

The key take home message here is that for each of the CHC’s analysed there were differences in both the quality and relative abundance between the control bees, bees immunestimulated with dsRNA and the IAPV-infected bees.

These differences were so marked that you can see distinct clustering of points in the analysis above … these bees ‘look’ 7 different to the guard bees that antennate them.

This is a great story.

It’s as yet incomplete. To complete the understanding we will need to know which of those CHC’s, or which combination, when suppressed (or overrepresented) induce the guard bees to say “Welcome, step this way … “.

We’ll then of course need to find out how IAPV induces the change in CHC profile, which takes us right back to protein A and protein B again.

Ever the pedant

Much as I like this science I’d perhap argue that, again, the virus isn’t directly inducing a behavioural change in the host.

What it’s doing is inducing a behavioural change in the response to the infected host (by the guard bee). So perhaps this again isn’t quite the same as the rabies example we kicked off with.

A behavioural change in the host might include IAPV-infected bees drifting more, or drifting further. Alternatively, perhaps a colony with widespread IAPV infection could more easily indulge in robbing neighbouring colonies as they would experience less aggression from guard bees.

Smaller is better ...

Reduced entrance to prevent robbing …

I can see immediate evolutionary benefits to a virus that induced these types of behavioural changes. It’s not an original idea … the late Ingemar Fries suggested it in a paper two decades ago 8.

I’m also certain that researchers are looking for evidence supporting these types of directly-induced behavioural changes caused by viral pathogens in honey bees.

All religion, my friend, is simply evolved out of fraud, fear, greed, imagination, and poetry

Edgar Allen Poe may or may not have said this.

However, while we’re on the thorny subject of pathogen-induced behavioural changes in the host, it might be worth mentioning a couple of more controversial areas in which it has been proposed.

In the snappily titled paper “Assortative sociality, limited dispersal, infectious disease and the genesis of the global pattern of religion diversity” Fincher and Thornhill argue 9 that the wide diversity of religions in the tropics (compared to temperate regions) is driven by infectious disease selecting for three anti-contagion behaviours; in-group assortative sociality; out-group avoidance; and limited dispersal. It’s an interesting idea and I’m pleased I don’t have to test it experimentally. Their argument is that these three behavioural changes select for fractionation, isolation and diversification of the original culture … and hence the evolution of religions.

Conversely, perhaps microorganisms induce religious behaviours (rather than religion per se) that facilitate their transmission. This is exemplified in the entertainingly titled paper “Midichlorians – the biomeme hypothesis: is there a microbial component to religious rituals?” by Panchin et al., (2014). They argue that microbes – and they are really thinking about the gut microbiota here – might be able to influence their hosts (humans) to gather for religious rituals at which both ideas (memes) and infections are more easily transmitted.

Perhaps something to think about when mindlessly spinning out all that summer honey in the next few weeks?

Party, party

I think it’s fair to say that both the papers in the section above have some way to go until they achieve mainstream acceptance … if they ever do.

Furthermore, the general area in which parasites, bacteria and viruses, induce changes in the behaviour of their hosts’ is really in its infancy. We are aware of a lot of behavioural changes, but few are understood at the molecular level 10. As such, we often don’t know whether the association is correlative or causative.

Evolution is certainly a powerful enough selective force to ensure that even extremely subtle benefits to the pathogen may become a genetically-fixed feature of the complex interaction it has with the host.

Respiratory viruses, such as the common cold, Covid-19 and influenza infect millions of people globally and are readily transmitted by direct or indirect contact.

That’s why most of the readers of this post have a face mask nearby and a bottle of hand sanitizer ‘at the ready’. Or should.

Direct transmission benefits the virus as it does not have to survive on a door handle, milk bottle or petrol filling pump.

But direct transmission requires that people meet and are in close contact.

And a paper 10 years ago demonstrated that infection with influenza virus resulted in increased social interactions in the 48 hours post-exposure, compared with the same period pre-exposure 11.

It’s amazing what viruses can do … or might do … or (just look around you) are doing.