Category Archives: Bees

More droning on …

Synopsis : Drones are now being evicted from colonies. How and why does a honey bee colony regulate drone numbers?

Introduction

Over the course of the last eight years posts on The Apiarist have got longer. This year, posts are now five times the length of the 2014 average. I’ve written – and hopefully, you’ll have already read – more words this year than are in The Hobbit.

If this continues until the end of the year we’ll have exceeded the word count in Tolkien’s The Two Towers.

This is probably unsustainable ¹.

The increase is explained in part by the complexity of some topics. It’s compounded by the need to provide some contextual information … and by my prolixity ². The latter is unavoidable, the former is probably necessary, not least because of the significant churn in new beekeepers.

A topic needs to be introduced, explained, justified and concluded.

Without this contextual information a post on oxalic acid trickling could be just:

5 ml of 3.2% w/v per seam when they’re broodless.

And where’s the fun in reading that?

Or writing it?

Furthermore, it’s probably of little use to a beginner who might not know what w/v means. Or what a seam is … or for that matter why being broodless is critical.

Keeping it topical

To maximise the income from site advertising I need to keep readers returning. This means the choice of topics should be important.

However, although some topics are chosen because they’re key concepts in the art and science of beekeeping, the majority are picked simply because I find them interesting.

And this week is one of the latter as I’m going to be droning on about … drones.

Specifically about drone numbers in the colony.

This was prompted by seeing the first drones of the season turfed out of the hive.

Another one bites the dust

Seeing this coincided with me discovering an interesting paper on how the queen’s laying history influences whether she produces drone or worker brood. This, inevitably, led me to other papers on drone production and discussions of how the colony controls drone numbers ³.

Drones are topical now because their days in your colonies are limited.

Already the colony will be producing significantly less drone brood than three months ago. The drones the colony has already produced will still ⁴ be flying strongly on good days.

However, when in the hive they will be being increasingly harassed by the workers.

Herding drones

If you open a colony very gently in the next few weeks ⁵ you might find the corners of the box contain high numbers of drones. The photo above was taken in late August and I’ve seen it several times late in the season. My interpretation is that it’s the only location in the hive in which drones can escape harassment by the workers.

Drone eviction

Drones ‘cost’ the colony a lot to maintain. A drone consumes about four times the amount of food than a worker (Winston, 1987). Therefore, once the fitness benefit of keeping drones falls below the expected costs needed to keep them they become ’surplus to requirements’. At this point the workers turf them out of the hive.

Evicted drones cannot feed themselves, so they perish.

It’s a tough life.

Interestingly, workers preferentially evict old drones. Presumably younger drones are more likely to fly strongly and mate with a virgin queen. Additionally, sperm viability in older drones is reduced, so their genes (and therefore those of the colony) are less likely to be passed on.

This ‘cost’ of maintaining drones is influenced by both the colony and the environment. For example, queenright colonies (which, by definition, have less need for drones) evict more drones than queenless colonies in the autumn, as nectar becomes limiting.

Although most beekeepers associate drone eviction with late summer/early autumn it also occurs when nectar is in short supply e.g. during the ‘June gap’.

It has also been suggested that drone eviction rates are related to colony size. Small queenright nucs, which have less need for drones, are more likely to evict than a full colony.

There’s still a lot we don’t know about drone eviction. For example, since drones tend to accumulate in queenless colonies, do these preferentially evict related drones to maintain potential genetic diversity in the population? ⁶

Hannibal the cannibal

Allowing an unfertilised egg to hatch, feeding the larva, incubating the pupa to emergence and then maintaining the resulting drone is a waste of resources if conditions are not appropriate. For example, doing this during a nectar dearth – particularly when drones are unlikely to be required for mating – makes no sense ⁷.

Therefore, in early spring and late autumn, workers cannibalise developing drone larvae. Effectively they are recycling colony resources. They preferentially cannibalise young larvae rather than older larvae. This makes sense as young larvae are going to need more food to reach maturity.

As above, queenless colonies cannibalise less queen-laid ⁸ drone larvae than queenright colonies.

In addition, some studies have shown that colonies with abundant adult drones cannibalise a greater proportion of developing drones. Again, this makes reasonable sense. Why rear more if you’ve got enough already?

However, to me it makes ‘reasonable’, but not ‘complete’ sense. Drones being reared as larvae are genetically related to the colony, adult drones may well not be. Drones that have drifted in from adjacent hives may therefore reduce the likelihood of the colony passing on its genes under environmental conditions which favour larval destruction but not eviction of adult drones.

Someone needs to look into this in a bit more detail 😉 .

There are lots of other aspects of larval cannibalism that are not understood. For example, how do workers discriminate between drone and worker larvae? Can they – as the queen can – measure the cell dimensions? Drone and worker brood pheromones differ from day 3 or 4. This seems a bit late to account for the cannibalisation of young larvae?

The influence of the queen

Since workers may cannibalise developing larvae ⁹ at different rates (drone vs. worker) it’s necessary to measure the colony’s egg sex allocation to see how the queen may influence drone numbers.

Only a few studies have done this …

There are experiments that suggest (they’re not definitive) that queens in continuously fed colonies lay more drone eggs in spring and summer than in autumn. This implies that day length or temperature may influence the queen, but it could also be a response to colony strength i.e. the queen lays more unfertilised eggs in a colony increasing in size, than in one decreasing.

In addition – and this is where I started down this rabbit hole in the first place – the egg laying history of the queen influences her current egg laying activity.

This easy-to-understand study was conducted by Katie Wharton and colleagues (Wharton, 2007). They confined queens for a period on either drone (DC) or worker comb (WC) – ensuring the queen could only lay drone or worker eggs for 4 days. They then transferred the queens to frames containing a 50:50 mix of drone and worker comb and recorded the amount of drone or worker eggs laid over 24 hours.

WC queens laid more drone eggs but the same amount of worker eggs as DC queens

There was a marked difference in the egg laying activity of the DC or WC queens when given the choice of laying drone or worker eggs. Although both the DC and WC queens laid similar amounts of worker eggs, the WC queens produced significantly more drone eggs as well.

Egg laying history or drone brood quantification?

This is a good study. The authors controlled for a variety of factors including season, colony size and food availability.

They additionally excluded the possibility that the egg laying activity of the queen was influenced by preferential cleaning of particular cells types by the workers, or by the workers backfilling certain cell types with nectar.

Finally, Wharton and colleagues allowed the colony to rear the eggs laid to pupation. The bias already observed was retained i.e. colonies headed by WC queens reared significantly more drone pupae than those headed by DC queens. The workers did not ‘correct’ the negative feedback exhibited by the WC queen, for example by preferentially cannibalising drone brood.

Although I termed this the ‘egg laying history’ of the queen a few paragraphs ago there is another interpretation.

The worker or drone comb already laid up by the queen – during the 4 day confinement period – remained in the colony. It’s therefore possible that the egg laying activity of the queen was influenced by the amount of drone brood already present in the colony.

Either explanation is intriguing.

How does the queen ‘count’ the number of drone or worker eggs she’s laid in the recent past? Alternatively, how does she quantify the amount of drone brood in the colony?

But what about the workers?

The Wharton study largely excluded the possibility that there was preferential cleaning of drone or worker cells by the workers in the hive. In fact, earlier studies have indicated that cell cleaning continues almost constantly and workers were equally likely to clean worker or drone cells.

The Wharton study also addressed – and excluded – the possibility that differences in the backfilling of drone or worker cells might influence egg laying.

However, it’s not understood what determines whether drone cells get backfilled with nectar by workers. My colonies are starting to do this now. Do the workers fill drone cells with nectar because the queen hasn’t laid in these cells or because they only backfill drone cells late in the season?

The former suggests that there is some sort of competition between the egg laying activity by the queen and nectar storage by workers. In contrast, the latter suggests that there are environmental triggers that influence this worker activity.

Or both … of course 😉 .

Comb building

In contrast to some of the studies outlined above, comb building is easy to monitor and – perhaps consequently ¹⁰ – has been well studied.

I’ve already discussed comb building in a recent post about queenless colonies. These preferentially build drone comb (Smith, 2018).

What else influences drone comb production?

Probably the two strongest determinants are the amount of drone comb already in the nest and the season.

Drone comb production is reduced in colonies that already contain lots of drone comb. Many beekeepers never observe this as they only use frames containing worker foundation. The workers squeeze in little patches of drone comb – often in the corners of the frame – but it never exceeds 5% of the total.

Colonies often prefer to build drone comb when given the choice

In contrast, natural nests contain 15-20% drone comb. That’s equivalent to two full frames in a National-sized hive. Once drone comb approaches this level a negative feedback loop operates and the workers build less drone comb. The negative influence of this drone comb (on building more drone comb) is enhanced if the comb contains drone brood.

Colonies drawing comb now (and certainly in the next month or two) will build worker comb. Some beekeepers exploit this to get lovely new worker frames drawn – nucs are particularly good at this ¹¹. In contrast, drone comb is drawn in spring and early summer. The season – presumably day length and temperature – therefore influences drone comb production, and hence drone production.

A thousand words

Well, nearer 2000.

As we near the end of the season we start to see drones evicted from our colonies. It’s interesting to think about the interplay of events that resulted in the colony producing those drones in the first place … and how and why the colony regulates drone production throughout the season.

Wharton (2007) neatly summarised the five stages from comb building to adult drone eviction.

Drone production and maintenance in a honeybee colony

I’ve dealt with these in reverse order because that was the best fit with the photo of the dead drone on the landing board that I started with.

There’s a lot we still don’t understand about the regulation of drone numbers. In particular, I think the majority of studies have ignored the influence of adult drone numbers on any of five stages illustrated above.

This is an important omission as drones move more or less freely between hives. That means that adult drones may well be genetically unrelated to the colony.

Perhaps this means that adult drones do not influence drone production? After all, if they did negatively influence drone production – as suggested above – it would potentially limit the ability of a colony to reproduce its genes. Evolutionarily this doesn’t make sense (at least, to me).

There are a couple of studies that have tried to determine the influence of adult drones, but they have produced conflicting results. Rinderer (1985) added drones to a colony which consequently reduced drone brood production. However, Henderson (1994) did the opposite and showed that removal of adult drones had no effect on drone brood production.

There’s clearly lots more to do …

Notes

I wrote this late on Thursday night. While doing so I watched the page views of my four year old post on Mad honey go ‘off the scale’ (which for a beekeeping site means hundreds of views per hour). The interest wasn’t sparked by my erudite description of grayanotoxin intoxication. Instead it was related to a video of a ‘stoned’ Turkish brown bear cub rescued after eating honey produced from rhododendron nectar.

It’s now abundantly clear that if I want to maintain my outrageous advertising income I should probably write more about hallucinogenic honey and less about the evolutionary subtleties of honey bee sex ratio determination.

That’ll teach me 😉

References

Boes, K.E. (2010) ‘Honeybee colony drone production and maintenance in accordance with environmental factors: an interplay of queen and worker decisions’, Insectes Sociaux, 57(1), pp. 1–9. Available at: https://doi.org/10.1007/s00040-009-0046-9.
Henderson, C.E. (1994) ‘Influence of the presence of adult drones on the further production of drones in honey bee (Apis mellifera L) colonies’, Apidologie, 25(1), pp. 31–37. Available at: https://doi.org/10.1051/apido:19940104.
Rinderer, T.E. et al. (1985) ‘Male Reproductive Parasitism: A Factor in the Africanization of European Honey-Bee Populations’, Science, 228(4703), pp. 1119–1121. Available at: https://doi.org/10.1126/science.228.4703.1119.
Smith, M.L. (2018) ‘Queenless honey bees build infrastructure for direct reproduction until their new queen proves her worth’, Evolution, 72(12), pp. 2810–2817. Available at: https://doi.org/10.1111/evo.13628.
Wharton, K.E. et al. (2007) ‘The honeybee queen influences the regulation of colony drone production’, Behavioral Ecology, 18(6), pp. 1092–1099. Available at: https://doi.org/10.1093/beheco/arm086.
Winston, M.L. (1987) The Biology of the Honey Bee. Cambridge, Massachusetts: Harvard University Press.

Battlefield bees

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Synopsis: For millennia bees were used as weapons of war. They are now being developed as ‘weapons of peace’ to help clear the millions of anti-personnel mines left after active conflict ends. Their legendary scent detection abilities combined with high-tech ‘bee detection’ methods show promise and may help reduce the thousands of civilian casualties that occur decades after the war ends.

Introduction

The weekly posts on this website are about bees and beekeeping.

In the same way that I deliberately shun sponsorship and avoid advertising ¹ I also try and avoid politics, law and other divisive subjects. These cause enough problems without adding my opinions into the mix … and worse, having to moderate the opinions of others in the subsequent comments.

So, although I might write about the detrimental effects of thiamethoxam on bees, I would focus on the science of neurotoxins rather than politics of lifting the ban of the use of this neonicotinoid in the UK .

It is harmful to bees, but the aphid-transmitted yellows viruses may otherwise decimate our sugar beet crop, and might not the alternative pesticides be more harmful to bees?

Do we even need that much sugar beet?

And what about the livelihoods involved?

You can see how quickly it gets very messy.

But, at the same time, I strive to make posts relevant and even topical. When viewed retrospectively – even if just by me – the posts represent a snapshot of my ~~jumbled~~ thoughts of what’s current at the time.

There are ageing posts on oxalic acid treatment that don’t reference ApiBioxal (the good old days as I like to call them), and others that make passing mention of maximising the oil seed rape (OSR) crop, or processing OSR honey ².

All of which means I cannot really avoid mentioning the recent Russian invasion of Ukraine ³.

Six-legged soldiers and one-legged civilians

Two years ago I wrote a post about the use of bees in warfare. For many hundreds of years they were an effective tactical weapon to be dropped on or fired at the enemy.

Not individually – that would be just silly – but a hive at a time.

I’ve been present when someone has dropped a full brood box. The whirling cloud of angry bees was reminiscent in shape, though nothing else, to the mushroom-shaped cloud over Hiroshima. Lots of people got stung in the training apiary during that session.

Unfortunately, the ingenuity of man knows no bounds when it comes to killing and maiming others.

The thermobaric weapons employed today are very different from the torsion-powered ballista siege engines used by the Ancient Greeks over two millennia ago ⁴.

Soldiers now wear carbon and kevlar rather than Corinthian helmets and short-sleeved tunics.

Although you can buy a camouflaged bee suit, it’s not designed for the battlefield, and is likely to be about as much use as the hoodies, jeans and T-shirts that the innocent civilians inevitably caught up – or deliberately targeted – in today’s wars are wearing.

The legacy of war

And long after the battle has ended – years or even decades later – those surviving civilians continue to be maimed and killed by unexploded ordinance and, particularly, by anti-personnel mines.

Minefield sign, Cyprus

So, rather than dwell on the horrors of the present – which is about as far removed from beekeeping as it’s possible to get – I’m going to discuss some more hopeful stories of how bees might help reduce the deaths and injuries caused by anti-personnel mines.

Those readers expecting the (un)usual humour may be disappointed this week … this is not a topic that lends itself well to jokes.

The solutions that scientists are developing to detect anti-personnel mines use a clever combination of the truly awesome scent detection capabilities of honey bees coupled with some very clever technology.

The problem

In 2021 there were 61 countries which were ‘contaminated’ with anti-personnel mines. These mines are typically produced for a few dollars, are perhaps 30 cm in diameter and are buried just sub-surface … and often forgotten.

One definition of the word minefield is ‘a situation or subject presenting unseen hazards’. The plural hazards, and use of ‘field’, indicate that lots of anti-personnel mines are usually buried across an area, thereby rendering it too dangerous to enter.

Farming, trade and communication is inhibited as people have to avoid the minefield(s) … and, if they don’t then the consequences can be devastating.

The International Campaign to Ban Landmines recorded over 7,000 casualties in 2020, 2,500 of whom were killed.

80% of these casualties were civilians and – where the age was known – over 50% of the casualties were children.

Aside from banning the use of anti-personnel mines in the first place, a priority must therefore be to find and removes mines from these areas.

Typically this involves using metal detectors or sniffer dogs for detection, followed by manual clearance. Inevitably, there are considerable risks involved in both the detection and – to a lesser extent – the clearance. These risks make the process time-consuming and expensive.

Which is where honey bees come in …

”I love the smell of Napalm in the morning”

So said Lieutenant Colonel Kilgore (Robert Duvall) in the film Apocalypse Now.

Landmines don’t contain Napalm, but about 90% if them contain TNT (trinitrotoluene).

And not only does TNT have an odour, but landmines continue to emit this odour for years after they have been buried. On a calm day there are vapour plumes of TNT above each of the buried mines ⁵.

The vapour concentration of TNT is measured in parts per trillion (pptr) and is usually in the range 0.01 – 100 pptr.

Just as the usual way to express areas is by comparison to Wales ⁶, the ‘volume comparison’ is typically made to an Olympic swimming pool.

1 part per trillion is about the same as half a teaspoon added to an Olympic swimming pool. Not very concentrated …

… but well within the detection capabilities of honey bees. For comparison, this is similar to that of sniffer dogs ⁷.

To cut a long story short, scientists ⁸ trained bees to feed on syrup laced with trace quantities of TNT. They then tested the ability of these bees to detect targets emitting field-realistic amounts of TNT.

The results were very encouraging. 97-99% of targets were detected with 1-2.5% of false-positives.

More importantly, the false-negatives (targets that were missed) were less than 1%. It’s much more important to not miss any than to ‘find’ some that aren’t there.

Lidar

The authors neatly sum up the benefits and principles of the study:

Bees do not cause mines to explode, do not require a handler, and can be trained more rapidly than dogs. This technique makes use of the natural foraging behavior of bees, which frequently cover ranges up to several km around a hive. The bees identify the sample location by their increased dwell time while flying in its vicinity.

And it’s that last sentence that should give you pause for thought.

How do you detect the “increased dwell time” – a fancy term meaning spending more time flying in one small area than the remainder of the study area – if you’re trying to find mines in an area about the size of a couple of football pitches ⁹.

Remember, as if you’d forgotten, you cannot enter the minefield because of those unseen hazards that are lurking just under the surface waiting to blow your legs off.

Bees are pretty small. I can see them against the clear blue sky at 20-30 metres range, but I can’t see them against mixed foliage at anything like that distance.

One potential solution is light detection and ranging (lidar) technology. Essentially this involves shining a scanning laser across an area and detecting the light scattered when it ‘hits’ objects – such as flying bees ¹⁰. For additional discrimination, changes in the polarisation of the scattered light has been used to distinguish between bees and what the scientists termed ‘clutter’, which I take to mean foliage.

And it works.

Lidar detection of bees; a) bee heatmap, b) chemical detection (5 is a false positive), and c) visual mapping of bees.

Not only in theory but also in practice … lidar has been used to detect bees detecting mines in an active ‘minefield’. Mines were buried in known locations, trained bees were released and their ‘dwell times’ were recorded using lidar (with the detector about 80 metres away).

Football fields and minefields

But there’s a problem.

Lidar involves a laser scanning horizontally 30-60 cm above the ground. Anything lower than this and the foliage prevents accurate (or any) detection of the bees.

And, even though a minefield might be the size of a football field, it doesn’t look like a football field … either when mined in the first place and definitely not after a few years.

Minefield in the Golan Heights

Unfortunately, there’s also an additional problem.

Tragically the sign above was probably erected after someone inadvertently stepped on a mine. Until that fateful day it might have just been ‘that scrubby bit of field bordering the river’.

Detecting the location of mines in a minefield is one problem.

Detecting whether a field is a minefield is a different – albeit related – problem.

And it turns out that bees might be able to help us discriminate between minefields and football fields, or any other sort of equally harmless fields.

I’ll discuss this before returning to the detection of individual mines in a minefield.

REST (and be thankful)

REST is an acronym for Remote Explosive Accent Tracing ¹¹ .

Since those buried anti-personnel and landmines give off a vapour plume of TNT there are methods of sampling the air and testing it for the presence of trace amounts of explosives.

The ‘sampling’ is highly technical and outside the scope of this post ¹².

The ‘testing’ involves sniffer dogs and lots of doggy treat-type rewards. Consequently it is a time-consuming and therefore costly procedure.

However, honey bees are covered in tiny hairs. Through electrostatic interactions, these pick up molecules while they are out foraging. Graham Turnbull ¹³ and colleagues have shown that flying bees can pick up molecules of TNT (from buried mines) which can subsequently be detected.

The bees are therefore used for wide area sampling, but how is the TNT detected? Graham is a physicist whose speciality is organic semiconductor sensing films … essentially thin films that change fluorescence when certain chemicals are deposited on their surface.

The hive entrances were modified so that the bees passed through a tube. This was made of a special material to pick up – and since lots of bees were making the trips, to also concentrate – the molecules that adhered to their hairs whilst out foraging.

Quenched photoluminescence (red bars) compared to negative control areas (black line)

To cut another long story short, the concentrated molecules were then transferred to the semiconductor sensing film and the photoluminescence quantified. A reduction in the photoluminescence (quenching) was indicative of TNT detection ¹⁴.

So honey bees can be used to discriminate between minefields and, er, other fields.

Detecting mines not minefields

I should add that the bees used in the study above were not trained ¹⁵ in any way. They simply placed feeders on the opposite side of the uncontaminated test or mined field to encourage them to sample in a reasonably defined area. The bees were not searching for mines, or the TNT vapour plumes, they were just flying back and forth ‘doing their stuff’ and foraging.

Bees are fast learners and can – as described briefly above – readily be trained to associated particular scents (such as TNT) with rewards (i.e. syrup). When you release these trained bees into an area with TNT vapour plumes they home in on these looking for the rewards … and hence exhibit those previously mentioned ’increased dwell times’ near buried mines.

But there’s still the problem of how the bees can be detected.

Huge advances have been made in unmanned aerial vehicles (UAV’s or drones), GPS and video technology in the 15+ years since the original use of lidar to detect bees detecting landmines.

The combination of these technologies now provides a way to detect individual bees, and consequently anti-personnel mines, within an area.

Using drones to monitor bees

A drone flying 10 m above the minefield ¹⁶ was used to record high resolution video from which the locations of individual bees could be (computationally) determined.

By detecting bees on individual frames (from video taken at 45 fps), rather than tracking each bee, it was (again computationally) relatively straightforward ¹⁷ to generate spatial density maps showing where the bees preferentially concentrated.

Video stills (left) and bee location heat maps (right). The blue circles show mine locations. Bee numbers on scale.

The accurate spatial location was ensured by using a modification (RTK) to GPS which involved an additional ground base station. This increased the standard GPS resolution to provide a horizontal accuracy of 5 cm. The bright coloured foci of ‘increased dwell times’ were less than 0.5 x 0.5 m.

Conclusions and problems

These studies are encouraging. They suggest that a biohybrid system ¹⁸, combining advanced physics and area-wide sampling of bees, with the exquisite scent detection of trained foragers coupled with highly accurate video monitoring, might help reduce the number of victims of landmines that occur long after the original conflict ended.

However, there are a few problems that remain to be resolved.

Bees learn fast, but they also forget fast. The authors developed some reinforcement training exercises to ‘remind’ them that they were searching for things scented with TNT.

Bees are not ideal chemical biosensors. They are potentially easily distracted by a strong nearby nectar flow, they don’t forage ¹⁹ in poor weather and their availability might be seasonal, depending upon the latitude.

Drones have limited flight times and long vegetation still impedes accurate video detection. Either the bees fly at lower altitudes or the foliage is disturbed by rotor downwash and so increases background noise.

Nevertheless, the expected costs and time involved in both the wide-area sampling and mine detection are lower, and the mine detection per se is much safer.

The mines still have to be destroyed, but knowing precisely where they are – and where they are not – is much more than half the battle … in solving the lethal legacy of a possibly long-forgotten battle.

Radar love

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The average beefarmer in the UK is probably somewhere in their mid-60’s ¹. This means that in 1973, when the Dutch rock band Golden Earring had their only notable chart success Radar love, they were about 18.

Bear with me …

As 18 year olds they probably wore denim flares and loud shirts with spearpoint collars. They would go to the local disco to meet similarly-attired members of the opposite sex (whose shorter hair may have been their only distinguishing feature).

They knew when and where to meet … the weekly Saturday night (obviously ² ) disco.

There was no point in turning up at 10 in the morning … the disco was closed 🙁

Similarly, despite their ‘cool threads’, wearing them to the launderette would have resulted in almost certain disappointment … the dance partners they were seeking weren’t likely to be found doing the laundry 🙁

No, the disco was the place to go.

Radar love would have been on the playlist. It reached the top 10 in the charts in many countries.

Hold that thought … we’ll return to Radar love in a few minutes … ³

The birds and the bees

Of course, these young beefarmers didn’t just go to the disco to dance.

Oh no.

They had an ulterior motive 😉

They knew that they had a good chance of meeting a like-minded (and similarly attired) member of the opposite sex who was also ‘looking for love’.

These meetings were effectively ritualised … a particular time and place.

Let’s forget the bell bottoms and hippie shirts now … I only added that detail so that any readers who know an ageing beefarmers can have a little giggle imagining them dressed for the disco 😉

OK, back to the disco … metaphorically.

The disco is not fundamentally dissimilar to the lek used by male grouse ⁴.

Greater sage-grouse at a lek, with multiple males displaying for the less conspicuous females

A lek is defined as a location where males congregate to compete and mate with females. Importantly, there are no direct benefits – such as food or territory – that the females gain from attending the lek ⁵.

How do the males know where to congregate?

Grouse tend to live for several years ⁶. Older grouse know where the lek is because they attended last season. Juveniles probably tag along and learn from their elders despite the fact they are too immature to mate, or lack the social dominance (or plumage ⁷ ) to compete.

As a consequence of this male hierarchy the location of the lek is invariant.

The birds congregate at the same place each year.

One of the features of leks is that males show high levels of fidelity to a single lekking site.

So now we know something about the birds … what about the bees?

Drones congregate in particular – rather ill defined – landscape features called drone congregation areas (DCA’s).

These, like a black grouse lek, are stable from day to day and year to year.

The drones compete (for the queen, though not directly with each other by displaying) and offer the queen no territorial or food benefits … meaning that DCA’s are effectively insect leks ⁸.

Drone congregation areas

There are studies going back well over 50 years on DCA’s. There are no hard and fast rules that define their location (at least to humans … thankfully virgin queens have no problems finding them). However, you can sometimes hear them; they sound like a small swarm, the noise caused by thousands of drones circling 5-40 metres above the ground in a swirling, traffic cone-shaped, perhaps a 100 metres or more in diameter.

How do drones know where to congregate? There is no male hierarchy ⁹. An individual drone lives for just a few weeks and perishes before winter.

The location must be somehow ‘hard-coded’ in the environment. Effectively a set of features that – once located – attract the drones back repeatedly until they either mate with a queen, or die trying ¹⁰.

Many studies have attempted to identify DCA’s – geographic features on the ground, sheltered from strong winds, a dip in the horizon etc. These have tended to produce rather mixed results.

I don’t think we’re anywhere close to being able to point to an intersection of two hedges and say “Over there … that’s where drones will congregate”.

An alternative approach is to go fishing for DCA’s.

Literally.

Having identified a number of potential DCA’s from landscape analysis, you can dangle a virgin queen from a helium balloon and sample the drone density in each of the areas.

It sounds a lot simpler than it is … there’s a nice account by Aude Sorel in Bee Culture if you’re interested.

By definition, the drone congregation areas are the ones you trap the most drones in.

Right?

Well, possibly not.

Perhaps the very method used to sample the drones attracted them there in the first place?

It’s been known since the 1960’s that high concentrations of queen mandibular pheromone can attract drones to almost any location – in one notable example, even 800 metres out to sea ¹¹.

If you use bait, how can you be certain that the areas you define are ‘real’.

A better way to define a DCA would be to observe individual drones accumulating in a particular area … to watch them leaving the hive, fly the tens or hundreds of metres to the same place they flew to yesterday, and record them ‘strutting their funky stuff’.

Have you ever tried to follow a drone in flight?

They’re strong and fast. They need to be to outcompete other drones when chasing the queen.

It’s almost impossible to track them across the apiary, let alone over the hedge, across two fields and into the lee of a copse.

But scientists can now do exactly that … using a technique called harmonic radar tracking.

The title of this post should now make a bit more sense … it’s the use of radar to find where drones go ‘looking for love’ 😉

Harmonic radar tracking

A harmonic radar system emits a stimulus signal. This signal is picked up by a harmonic tag (the transponder) which uses the low frequency stimulus energy to generate a second harmonic which is then re-radiated back out to a receiving system.

The harmonic signal emitter/receiver is portable … if you’ve got a lorry.

Harmonic radar emitter and detector – with Rothamsted Manor in the background.

Fortunately, the transponder is tiny … small and light enough to be glued to the back of a bee.

Drone with harmonic radar transponder attached.

Harmonic radar has been used to study orientation flights in honey bees ¹², to track Asian hornets, and to follow butterfly flight paths ¹³ (amongst other things).

And now it’s been used to map drone congregation areas by tracking the flights of individual drones from the hive.

Harmonic radar is a relatively short range system. You can’t track transponder-tagged insects flying miles away. The effective range is just a few hundred metres for most systems.

However, for drone congregation areas this shouldn’t be a major limitation. Drones generally fly shorter distances to mate than queens (an evolutionary mechanism to avoid inbreeding) and DCA’s have often been found near to apiaries ¹⁴.

Tracking drones by harmonic radar

The study, by Woodgate et al., was published a couple of weeks ago in iScience. The full reference is:

Woodgate et al., Harmonic radar tracking reveals that honeybee drones navigate between multiple aerial leks, iScience (2021), https://doi.org/10.1016/j.isci.2021.102499

It’s available under open access (i.e. free, for anyone) and I recommend you read it if you’re interested.

I’m just going to pick out a few highlights.

During two sequential seasons the authors tracked over 600 flights by at least 78 drones. These included 19 first flights (orientation flights) and – for four drones – 6-8 consecutive flights, including their first ever orientation flight.

Orientation flights were typically observed as multiple loops in different directions, centred on the hive from which the drone originated.

Drone orientation flights

The average duration of these orientation flights was ~13 minutes and the drones observed only took one or two before changing their flight pattern (see below) and seeking drone congregation areas.

Worker bees typically take more (~6) orientation flights than drones. Presumably foragers need to ‘map’ the hive location better because they may end up returning to it (and they’ve failed if they don’t) from any location.

As we’ll see in a minute, drones tend to use particular ‘flyways’ which are probably determined by landscape features. Drones also may return to a different hive to the one they set out from.

Identifying drone congregation areas by harmonic radar tracking

Scientists love ‘heat maps’.

These are a graphical way of depicting levels of activity of one kind or another.

If you overlay the flights by every transponder-tagged drone in each of the two years of this study you generate a map (like C and E shown below). In this study they used a ‘white to red’ scale where the paler the colouration, the more drones were detected in that particular point on the map.

You can easily see the hive location (points 1, 2 and 3) as all flights originated there.

Heat map of the landscape used by drones.

Actually, C and E are a bit confusing because they include the orientation flights which are centred on the hives. If you exclude these you end up with the heat maps D and F on the right.

From these the authors could detect particular areas where the drones tended to concentrate … these are proposed to be the drone congregation areas. There were four within range of the harmonic radar system – A-D above (confusingly labelled on images D and F).

There are a few obvious features of these proposed DCAs:

They are in approximately (but not exactly) the same position in the two study years.
The frequency with which they were visited changes. A is visited less frequently in the second year (panel F) than in the first (panel D).
The most distant DCA (at least that could be mapped in this study) was ~600 metres from the hive.
Each DCA had a roughly symmetrical ‘core’ of 30-50 metres, significantly smaller than many drone trapping studies suggest..

One thing that was noticeable by comparison of the orientation flights and the proposed DCAs was that they did not overlap.

So how do the drones ‘find’ the DCA if they don’t discover them on an orientation flight?

Flyways, straight and convoluted flights

Heat maps are cumulative data.

It was also possible to look at the individual flight paths of drones on their way to and from a DCA (in exactly the same way as they mapped orientation flights).

Analysis of these showed that drones adopted two distinct types of flight – an approximately straight, direct flight interspersed with periods of convoluted, looping flight. There are lots of pictures of these in the paper but, rather than showing another published image, here’s my “no expense ~~made~~ spared” diagram of these two patterns of flight.

Drone flight paths showing distinct direct and convoluted elements.

The convoluted flight defines the drone congregation areas. In these the drones showed very distinctive behaviour – the further they were from the centre of the DCA the more strongly they accelerated back towards the centre.

Drone flight paths (inevitably) overlapped in DCAs.

However, they also overlapped in the straight line flight. Drones tended to use particular flyways from the hives to, and between, the DCAs.

Scientists have previously identified (or at least suggested the existence of) these flyways that drones use to travel to and from the hive and the DCAs ¹⁵.

However, what they had previously not identified was that drones often visit more than one DCA in a single (potential) mating flight.

In 20% of the flights analysed drones visited more than one DCA.

Finally, drones tended to only spend about 2 minutes flying around very fast (at ~5 m/s rather than the sedate ~3 m/s they fly around the hive at ¹⁶ ) within the proposed DCA.

This suggests that drones might routinely patrol several DCAs in a single flight, moving on unless a queen is present.

Harmonic radar mapping the flights of virgin queens

I’ve often preceded the term ‘drone congregation area’ in the text above with the word ‘proposed’. A DCA has a very specific meaning that describes the places where drones congregate to attempt to mate with a virgin queen.

None of the studies above showed queen mating, or even the presence of a queen.

But, of course, the authors tried that as well.

They transponder-tagged queens (94 in total) and tracked their orientation flights and mating flights (26 in total). The orientation flights were remarkably similar to those of the drones; the average number of these flights was 3 and no queen went on more than 6 orientation flights.

Unfortunately the tracking of queen mating flights was less successful ¹⁷.

Queens flew out of range (I’ll return to this shortly), the transponder fell off, or parts of the flight were not picked up by radar. Some of the queens ‘followed’ (or for which tracking was attempted) did get mated, but not apparently in the DCAs identified during the flight tracking of drones.

This type of study clearly needs further work …

Conclusions

Drone congregation areas could be detected using harmonic radar tracking of transponder-tagged drones. Unlike other well-studied lekking areas, males (drones) did not display lek fidelity, but instead visited several in rotation ¹⁸.

The DCAs are a consequence of drones exhibiting a convoluted flight pattern in particular locations. The conservation of the flyways – the routes taken by the drones – between DCAs suggest they might contribute to the location of the DCAs.

Understanding what defines these flyways might allow better prediction of DCA locations.

Previous studies have shown that queens tend to fly further to DCAs than drones, presumably to avoid inbreeding. One possibility is that tagged queens in this study might have been more likely to visit the four DCAs identified if they were placed in mating nucs situated further away from this study site.

But, of course, they could have then flown off in a different direction altogether 🙁

Finally, it’s worth noting that a different pattern of queen mating activity had been described for dark, native (Apis mellifera mellifera) and near-native bees. This is apiary vicinity mating (AVM), and is nicely described by Jon Getty on his website.

I now have some native black bees. I’m also experiencing the worst spring of my entire beekeeping career for queen mating. I am increasingly interested in AVM as a mechanism for saving the queen from drowning or freezing to death while attempting to reach a DCA 🙁

Winter losses

5 Replies

I lost 10% of my colonies this winter.

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

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

April showers frosts

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

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

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

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

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

First inspections and winter losses

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

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

Fresh nectar glistening in a brood frame

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

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

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

Winter losses

Winter losses generally occur for one of four reasons:

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

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

BBKA winter survival survey

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

Lies, damn lies and statistics

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

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

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

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

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

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

Bee Informed Partnership loss and management survey

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

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

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

Perhaps the US climate is less suited to honey bees?

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

I doubt it ⁹.

Running on empty

My two colony losses were due to queen failures.

Old winter bees and no brood

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

Unused winter stores

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

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

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

Strong colony ready for uniting

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

Every little bit helps 🙂

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

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

Drone laying queen

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

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

Drone laying queen …

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

However, many of the bees could be saved …

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

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

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

Under these circumstances I decided to shake the colony out.

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

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

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

I moved the hive away and shook the bees out.

Sure enough … they returned to their original location.

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

This way sisters!

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

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

Drone brood is a Varroa magnet

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

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

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

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

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

Drone-worker-drone …

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

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

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

Boxes of bees

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

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

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

Emptying a box of bees

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

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

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

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

The beekeeping season has definitely started 🙂

Notes

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

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

Going the distance

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

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

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

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

How do bees measure distance?

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

Does that seem logical?

Foragers forage randomly, but usually return directly

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

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

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

Going up!

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

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

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

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

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

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

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

Ouch!

Splash landing

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

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

Bees flying over calm water underestimate distances

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

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

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

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

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

Such as their height above the surface?

Or perhaps the distance travelled?

Going with the flow

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

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

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

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

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

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

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

Esch & Burns experiment to support the optic flow hypothesis

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

Tunnel vision

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

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

The tunnel experiment.

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

‘Tunneling’ bees misreport distances

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

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

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

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

Interpreting the waggle run

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

Determining distances from waggle dance observation

How could the bees get it so wrong?

Increased optic flow

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

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

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

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

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

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

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

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

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

It’s all relative

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

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

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

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

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

How do we know this? ¹⁷

Changing the environment

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

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

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

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

Where do they search for the feeder?

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

Context dependent dance calibration

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

Odometer calibration depends upon the environment

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

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

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

Phew!

Optic flow, waggle dancing and implications for practical beekeeping

None 😉

At least, none that I can think of.

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

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

Notes

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