Category Archives: Science

Picking winners, part 2

Synopsis : Some larvae are nutritionally deprived and may produce sub-optimal queens. Grafting may miss the ‘best’ larvae the colony would select for rearing as emergency queens.

Introduction

A fortnight ago I discussed the preference colonies show for heavy eggs – or more accurately for larvae reared from heavy eggs – when producing queens under the emergency response.

Why might this be interesting?

The longevity of the queen and the absolute dependence the colony has on her quality means that the choice of larvae they rear new queens from is of fundamental importance.

These are the larvae that develop to produce queens with the traits that benefit the colony – in fecundity, disease resistance and a range of other characteristics.

It cannot be random.

If they make the right choice the colony will flourish, swarm and their genes will be perpetuated.

That is a significant evolutionary selective pressure. Its application over the 90 million years or so since the evolution of eusociality has resulted in the honey bees we have today.

Now, these traits favoured by the bees might not all benefit our beekeeping, but some of them should. Longevity, fecundity and disease resistance are likely to be evolutionarily favourable traits, and will also be useful for beekeepers.

Defensiveness and swarminess … er, not so much 😉 .

But the bees have little time to select the best larvae. They have 6 days from the day the egg is laid until a larva is too old 1 to reliably develop into a queen.

In practice they select very young larvae (or even 3 day old eggs) so ensuring the resulting queen is fed for the maximum time with royal jelly, thereby producing a larger queen with more ovarioles.

So what do the bees choose?

Given the choice, which larvae are selected by the bees to rear new queens?

Artificial experiments and nepotism

The ‘heavy eggs’ experiment I discussed a fortnight ago was primarily designed to study kin selection and nepotism in honey bees. The study was conducted over a decade ago 2, but wasn’t published until 2021, though the results were known before then 3.

If you remember, nepotism in honey bees is a nice idea; particular patrilines of workers (fathered by the same drone) should favour larvae of the same patriline. However, there have been no convincing studies that actually support this, and there are compelling theoretical arguments why nepotism could actually be detrimental to the colony.

Parts of the study I’m going to discuss this week were also designed to test for nepotism. I’m going to ignore these 4 and instead focus on some more interesting results that I think have practical relevance for beekeeping.

In addition, the study this week uses methods that are more typical of those used by beekeepers and that avoid the artificiality of rearing larvae in vitro before reintroducing them to a queenless colony.

In this regard I’d argue that they more closely resemble what’s happening in a colony rearing emergency queens. Furthermore, they should be easier for beekeepers to understand, and to repeat … not for experimental purposes, but when rearing queens.

Sagili et al., (2018)

The majority of the studies I’m going to discuss are from Sagili et al., (2018). The title ’Honey bees consider larval nutritional status rather than genetic relatedness when selecting larvae for emergency queen rearing’ neatly summaries their conclusions, but some of the detail is worth discussing in a bit more detail.

It’s always interesting to know what goes on in the hive.

During inspections we see frames of brood – capped and open cells. Other than the larvae getting bigger as they get older they all look much of a muchness … but they’re not.

All larvae are equal, but some are more equal than others 😉 .

Hungry mouths

The queen lays an egg in an empty cell. Other than the egg, the cell remains empty for 3 days when the egg hatches to release the larva. Without prompt and regular feeding the larva will starve or suffer setbacks in development.

Unrealised potential … a frame with eggs and young larvae

For this reason the nurse bees make frequent visits to the occupied cells to determine their content and needs.

Is it an egg or a larva?

Is it hungry?

And these visits continue during the 5 days of larval development.

How many visits do they make, how often is a larva fed, and are all larvae treated equally?

Sagili and colleagues used observation hives and video cameras to record nurse bees visiting cells containing larvae of precise ages 5. They recorded visits over 4 hours to 2 day old larvae, and one hour observations of 5 day old larvae.

In four separate hives, 4-8% of the young (2 day old) larvae did not receive a visit from a nurse bee during the 4 hour period they were filmed.

Of the 5 day old larvae, again ~10% didn’t receive a visit during the observation period and, of those fed, the longest interval between feeds was ~36 minutes. However, over one hour, the older larvae that were being fed were visited very regularly; the median interval between feeds was a little under 4 minutes and they were fed for a total of ~7 minutes over one hour.

Clearly some larvae, for whatever reason, get little or no attention for extended periods, whereas those that are visited, are fed very frequently.

Nutritionally deprived and non-deprived larvae

Larvae that are infrequently visited are likely to be nutritionally deprived … or, using the technical jargon beloved of beekeepers and scientists alike, hungry 6.

Do nurse bees respond differently to nutritionally deprived and non-deprived larvae?

Which are visited first and fed first?

The scientists caged the queen on a frame and allowed her to lay eggs for 24 hours. They then removed the queen (caging her elsewhere in the hive) and waited for the eggs to hatch. 24 hours later the larvae were caged under either 13 mm mesh or 3 mm mesh. Workers can access the larvae through the 13 mm mesh, but cannot get through 3 mm mesh. Cages were left in place for four hours to create two populations of larvae on the same frame; nutritionally deprived and non-deprived.

Small and large mesh cages over day-old larvae

They then again used video recording of randomly selected larvae to record and quantify the attention and feed visits they received.

The purpose of this part of the study was to determine whether the nurse bees could discriminate between larvae that were nutritionally deprived and those that were not.

And they could …

Inspections and feeding visits to nutritionally deprived and non-deprived larvae

Deprived larvae were visited (inspected) sooner, the black bars in the graph above, and fed earlier. You can just about determine this from the graph; the stats are more convincing (but less comprehensible 😉 ).

In addition, deprived larvae received more frequent inspections, more frequent feeds and were fed for longer.

Acceptance of larvae for queen rearing

A colony rendered suddenly queenless will attempt to rear a replacement under what is called the emergency response. Suitable young larvae are selected, fed a diet rich in royal jelly and the cell is reshaped to be orientated vertically.

This vertical orientation is a major inducement for the workers to continue to feed the developing larva with royal jelly (She et al., 2011). This is exploited in queen rearing techniques that involve the grafting of young larvae into wax or plastic cups which are then placed, open end down, in a queenless colony.

Sagili et al., investigated whether nutritionally deprived and non-deprived larvae were favoured when queens were reared under the emergency response.

Interestingly, they did so using larvae in natural comb and following grafting into plastic queen cups.

Which were favoured for queen rearing? Grafted or natural, nutritionally deprived or non-deprived?

In both instances the larvae were presented to a queenless and broodless colony, using 6 recipient colonies in each case.

Larval acceptance for queen rearing using two different methods – grafted and in natural comb

In the case of grafting, 12 of each type of larvae were presented on a cell bar frame. When transferring comb (prepared as described before using caged larvae) the entire frame was introduced.

The recipient colony did not discriminate between the nutritionally deprived and non-deprived larvae when they were grafted, but they showed a marked preference for the non-deprived larvae in natural comb.

In addition, they reared significantly more queens from grafted larvae than they did from larvae in comb.

All larvae are equal, but some are more equal than others …

Since I’m enthusiastic about queen rearing, this last set of experiments was by far the most interesting part of the study.

There are two results that are particularly striking.

Firstly, more queens were reared from grafted larvae than were reared following the transfer of a frame of larvae. The difference was significant, with almost twice as many queens being produced following grafting. It’s also worth noting that the bees only had 24 grafted larvae to choose from, compared to a much larger number of larvae on the transferred natural comb.

More is better … right?

Secondly, the workers showed no preference between nutritionally deprived and non-deprived grafted larvae, but showed a strong preference for the well-fed larvae in natural comb.

So, what do these results mean?

Let’s have a quick recap:

  • developing larvae receive different amounts of ‘attention’ from nurse bees
  • about 10% of developing larvae received no (or only a limited number of) visits during an extended observation period. The presumption is that these larvae are likely to be nutritionally deprived (though this was not demonstrated)
  • nurse bees can readily distinguish between nutritionally deprived and non-deprived larvae; the former receive earlier inspections, are fed sooner and more frequently
  • when rearing emergency queens, workers preferentially select larvae in natural comb that are not nutritionally deprived. In contrast, they make no distinction between nutritionally deprived and non-deprived grafted larvae

We know from numerous studies that high quality queens must be well fed during larval development. The best queens are produced from very young larvae (or even 3 day old eggs) that are then fed for an extended period with copious amounts of royal jelly in a strong hive full of nurse bees.

Queens produced under these conditions are larger and have more ovarioles, so should lay more eggs for longer.

It makes sense that nurse bees can distinguish between ‘hungry’ and replete larvae … the former need feeding or they won’t develop properly. The former may already have been held back developmentally … not an ideal start for a new queen.

Since nurse bees can determine the nutritional status of very young larvae, logic would dictate that they would select those that are not nutritionally deprived to rear new queens from.

After all, the future of the colony, and any resulting swarms will depend on it.

So, why don’t they make a similar distinction when presented with grafted larvae?

Selection vs. maintenance of larvae for queen rearing

The key difference between the grafted larvae and those in natural comb was the orientation of the ‘cells’ in which the larvae were presented to the queenless and broodless colony.

Grafted larvae were presented in a vertically orientated plastic queen cup, whereas the cells in natural comb are horizontal 7.

Cell bar frame with vertically orientated plastic Nicot queen cups

The interpretation is that larvae in vertically orientated cells are not selected by the nurse bees, but are instead just maintained as developing queens.

In contrast, larvae in horizontal(ish) natural comb are selected as the starting material for new queens, and the resulting reshaping of the comb to form the queen cell leads to their maintenance as developing queens.

Significance for beekeeping and queen rearing

The increased number of queens reared from grafted larvae probably reflects this ‘maintenance’ response being triggered in the nurse bees. ’Any’ larva presented in a suitably orientated cell must have been preselected as suitable … so the bees feed them up with royal jelly.

The nurse bees don’t know that these grafted larvae were selected by the beekeeper, not on the basis of them being nutritionally replete, but more likely because they were visible, about the right size, and in an accessible part of the comb.

But … think back to the first experiment. This suggested that ~10% of all larvae are nutritionally deprived because they have received, at best, infrequent visits over the last few hours. If the beekeeper hadn’t picked them during grafting, it’s unlikely the bees would have selected them as being suitable for producing queens.

Are 10% of grafted queens sub-optimal? Remember, the differences may be subtle.

The other point I found interesting is that the bees reared fewer queens in natural comb than from grafted larvae.

Why?

Charged queen cell

One possibility is that the reshaping of the comb is a physical limitation and restricts queen production. Perhaps this is why so many are at the edge of combs? 8 

Another is that the bees only rear ’enough’ queens for their needs, but that they can only determine what ‘enough’ is using larvae in natural comb.

Whilst these are certainly possible I think there’s an intriguing alternative … only a small proportion, even of well fed larvae, are considered suitable by the colony for queen rearing and so selected by the nurse bees.

Yes, the bees favour larvae that are not nutritionally deprived, but perhaps there are additional characteristics that are also desirable (and that vary between larvae).

Quantity and quality

Grafting larvae is a well established method of producing large numbers of queens. If the donor colony is good quality there’s every reason to expect that the resulting queens will be good.

Most, if not all, commercially reared queens come from grafted larvae (where quantity is paramount and quality might be a secondary concern) … probably hundreds of thousands of queens a year are produced like this. It’s the method I’ve used for many years.

Queen cells from grafted larvae …

But this paper raises two or three interesting ideas:

  • about 10% of larvae selected at random for grafting are likely to be nutritionally deprived and so would not have been chosen by the nurse bees. The presumption is that these will produce sub-standard queens.
  • nurse bees might be a lot more selective in the larvae they choose for emergency queens, only favouring a subset of even those not nutritionally deprived.
  • queen rearing methods that present larvae in natural comb might produce fewer queens but those queens may have the desirable characteristics selected by the bees (potentially resulting in better quality but a smaller quantity).

If this last point is correct, it’s worth noting that queen rearing methods – like the Hopkins method – that use larvae in frames placed horizontally over the colony 9, may trigger the queen maintenance response rather than allowing the selection of larvae by the nurse bees.

It would be very interesting to determine whether the bees would discriminate between nutritionally deprived and non-deprived larvae presented on a horizontal frame.

Two final thoughts;

  1. Grafting works well, but that doesn’t mean it’s the best way to produce top-quality queens.
  2. The desirable characteristics nurse bees favour (for colony survival and reproduction) may not be beneficial for beekeeping.

But I’d be surprised if they weren’t 🙂


Note

I’ve not had a chance to discuss it, but Free et al., (1989) previously demonstrated that nutritionally deprived larvae received more attention from nurse bees. I’ll deal with how the workers detect the nutritional status of larvae in the future.

References

Free, J.B., Ferguson, A.W., and Simpkins, J.R. (1989) The Effect of Different Periods of Brood Isolation on Subsequent Brood-Cell Visits by Worker Honeybees (Apis Mellifera L.). Journal of Apicultural Research 28: 22–25 https://doi.org/10.1080/00218839.1989.11100815. Accessed November 22, 2022.
Sagili, R.R., Metz, B.N., Lucas, H.M., Chakrabarti, P., and Breece, C.R. (2018) Honey bees consider larval nutritional status rather than genetic relatedness when selecting larvae for emergency queen rearing. Sci Rep 8: 7679 https://www.nature.com/articles/s41598-018-25976-7. Accessed November 21, 2022.
Shi, Y.Y., Huang, Z.Y., Zeng, Z.J., Wang, Z.L., Wu, X.B., and Yan, W.Y. (2011) Diet and Cell Size Both Affect Queen-Worker Differentiation through DNA Methylation in Honey Bees (Apis mellifera, Apidae). PLOS ONE 6: e18808 https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0018808. Accessed November 22, 2022.

Picking winners, part 1

Synopsis : Queenless colonies prefer to rear new queens from heavy eggs. How was this determined and what are the implications for our queen rearing?

Introduction

Arguably the most important decision a colony will ever make is the selection of the eggs or larvae from which a new queen is raised. Other decisions are obviously important, such as the nest site a swarm occupies, but if the choice of ’starting material’ for the new queen is poor then the resulting colony is unlikely to thrive.

Actually, I suspect this isn’t arguable at all; whether it’s a replacement queen to take over after the colony swarms, or a supersedure queen to replace the ageing matriarch as she runs out of sperm or energy, a poorly chosen larva will – sooner or later – result in the demise of the colony.

Let’s hope they’ve chosen a good ‘un (they will have!)

Conversely, a good larva, fed well by nurse bees, that mates with enough drones and evades marauding swallows on the return to the hive – and the clumsily wielded hive tool of the beekeeper – will end up heading a strong colony. This strong colony will collect a surfeit of pollen and nectar, so ensuring good overwintering survival. It will be better able to defend itself against wasps or other robbing bees, and will be less susceptible to disease 1.

Reproduction

A strong, healthy colony will build up well in the spring and produce one or more swarms 2. If these survive – undoubtedly also helped by having good genetics – the colony will have reproduced and can be considered successful.

A small swarm ...

Honey bee reproduction in action

This type of ’success’ is what evolution selects for, so you can be absolutely certain that the choice of eggs/larvae from which new queens are reared is not random.

Cooperation vs. nepotism

Rearing new queens involves cooperation. In fact, as eusocial insects, almost everything that happens in the colony is cooperative. Multiple nurse bees feed the developing queens, hundreds of scout bees survey the environment for new nest sites and thousands of related workers provision the hive with pollen and nectar.

It’s often stated that these workers are ‘half sisters’ … they share the same mother (the queen) but different fathers (drones).

And there are quite a lot of fathers … .

The queen mates with at least a dozen drones during the mating flights she takes. Some calculations suggest it’s significantly more than a dozen drones. Whatever the number, workers fathered by the same drone will be more related to each other than they will be to workers fathered by a different drone.

On average workers within a single patriline (i.e. fathered by the same drone) are supersisters and share 75% of their genes. In contrast, workers in different patrilines (i.e. different drones) only share 25% of their genes.

And this is potentially a problem for cooperation.

It might be expected that nurse bees would select their supersister larvae when rearing new queens. Doing so would help ensure the propagation of their genes in subsequent generations, rather than those of their half sisters.

This would be an example of nepotism; ’showing special favour or unfair preference to a relative’ 3.

Lots of studies have attempted – largely unsuccessfully – to demonstrate nepotism in social insects, but that doesn’t mean it’s not worth looking again.

Do worker honey bees exhibit nepotism when selecting larvae to rear new queens?

Nepotism vs. colony diversity

It’s easy to talk yourself out of an experiment.

You have a good idea, do a bit of reading, discuss it with your friends and collaborators and then – belatedly – consider the underlying theory.

At which point it all sort of falls apart and you find numerous reasons not to do the experiment in the first place.

It was a daft idea because of x, y and z.

Think of all the time and money you’ve saved … back to the drawing board.

And there are good theoretical reasons why nepotism is unlikely to be seen in social insects like honey bees.

The most compelling of these is that genetic diversity within the colony is beneficial.

And nepotism, by definition, reduces diversity.

A quick recap on the diversity story … colonies with limited genetic diversity e.g. those headed by poorly mated queens, are less ‘fit’ than colonies with extensive genetic diversity. Fitter colonies are bigger, stronger, healthier and more likely to reproduce. The seminal study on this was by Mattila and Seeley (2007) which I discussed briefly in Polyandry and colony fitness.

So, theoretically, nepotism is a ‘bad thing’ … don’t bother doing the experiment.

But hold on a second, we also know that different patrilines of workers ‘smell’ very different to each other because they produce distinct cuticular hydrocarbons (CHC).

If nepotism is such a ‘bad thing’ why retain the (evolutionarily ‘expensive’) genetic machinery to generate all these different CHC’s? Why not just make all workers from one queen distinct from those derived from a different queen?

Individual colonies need to have distinct CHC’s to prevent robbing, but why are different patrilines distinct in their CHC profile?

Maybe nepotism occurs after all?

Better do the experiment.

Nepotism and larval selection

The study I’m going to briefly discuss was recently published by AL-Kahtani and Bienefeld (2021). It’s interesting and reasonably definitive in my view. However, whilst it addresses the ”Do bees exhibit nepotism during larval selection?” question 4 I think there are features of the study that are somewhat artificial which might restrict the generality of the conclusions they reach.

More interestingly, and of relevance to practical beekeeping, they show that bees are highly selective in their choice of eggs/larvae.

Can beekeepers exploit this to produce better quality queens?

The experiment was very simple.

Simplified diagram of the experimental method (see text for details)

Unmated queens from diverse areas of Germany were instrumentally inseminated with sperm from 10 drones, each selected from different unrelated geographic areas.

Six colonies were established (only three shown above) which were subsequently split into a queenright egg-producing colony (EPC; presumably a nuc, though it’s not stated) and a queenless larvae-rearing colony (LRC).

Eggs laid within a 6 hour window were incubated for 48 hours in an incubator, weighed and then allowed to hatch. For the first 48 hours after hatching the larvae were artificially reared by feeding them a sugar/protein diet 5.

This artificial rearing was done to avoid any bias from non-genetic colony odours e.g. due to pollen/nectar.

After 48 hours, 30 larvae, 10 from the matched EPC and 10 from each of the unrelated EPC’s were grafted into plastic queen cups and presented to the LRC for rearing as queens.

The larvae selected were obvious as these were fed and wax was deposited to create the surrounding queen cell.

Did LRC’s preferentially select larvae from the matched EPC?

No.

This larval transfer was done several times to get statistically meaningful results, using six colonies, repeated either twice or three times in successive years. In total 450 grafted larvae were presented to the LRC’s.

Larval acceptance rates were ~48-60%, a figure often exceeded when grafting for queen rearing.

Capped queen cells

Capped queen cells produced using the Ben Harden queenright queen rearing system

I suspect this rather mediocre acceptance rate reflects the in vitro rearing of the larvae for the first 2 days, potentially compounded by the age of the larvae which – at 48 hours – are at least 30 hours older than optimal.

But the acceptance rate doesn’t really matter as it was similar whether the larvae were derived from the matched or unmatched EPC. This therefore ’contradicts the hypothesis that kinship plays a central role in the selection of larvae for queen breeding’ (to quote the authors verbatim).

Larval selection is not nepotistic.

But certain larvae were preferentially selected

Despite the fact that the bees didn’t appear to care whether the eggs were from a related colony or not, they did preferentially select larvae produced by certain queens.

And I’ve already given you a clue of the characteristic favoured by the workers … though the characteristic per se wasn’t directly selected by the workers bees.

The six different queens used in this study produced eggs that differed slightly in weight. On average, the heaviest and lightest eggs varied in weight by ~5%.

There was a significant and direct correlation between the average weight of eggs produced by a queen and the likelihood that the resulting larvae would be selected by the larval rearing workers.

Heavier eggs produced larvae that were favoured by the cell raising colony.

Relationship between average egg weight and whether they were accepted for queen rearing

Of course, the cell raising colony never saw the eggs … these were hatched in an incubator and fed for two days before grafting and introduction.

Nevertheless, there was something about heavy eggs that the larval rearing colony favoured.

A total of 248 virgin queens were produced from the 450 larvae grafted (55%). These virgins were weighed and subsequently naturally mated, resulting in 190 egg-laying queens (42% of grafts, or 77% of virgins). Of these, 147 came to a grisly end as they were dissected two months after they started laying to count the number of ovarioles (the sub compartments of the ovaries in which the developing oocytes are produced).

Queen weight and ovariole number have previously been considered as markers of queen quality. Perhaps disappointingly, there were no significant differences in terms of virgin queen weight, ovariole number or the delay in onset of egg laying between queens produced from heavy or light eggs.

Crude criteria of what’s best

I’m not unduly concerned that the crude criteria we use to judge the quality of these queens (weight and ovariole number) failed to demonstrate significant differences. These criteria may not be the same as the ones selected by the bees 6. The fact that we cannot measure differences in the resulting queens does not mean that there were not qualitative differences in queens reared from heavy eggs that would benefit the colony.

Or would have been if they hadn’t been dissected 🙁 .

Where have all my young girls gone?

Bigger AND better … or just bigger?

It just means we were probably not measuring the right things.

However, extending this experiment from the relatively straightforward ‘heavy eggs are favoured’ observation is not trivial. If the scientists cannot see a difference in the queens then they might have to look at colony performance over time, or in adverse years, or when swarming, or in hard winters etc. Each of these may directly or indirectly act as a selective pressure on the queen quality, and hence the choice the bees make in the initial eggs or larvae that are selected for queen rearing.

Caveats

The relationship between egg weight and larval acceptance shown above is based upon the average egg weight produced by each of the 6 queens used in the study.

These average weights varied by about 5% (154.9 to 162.7 micrograms). That’s not a lot, and is narrower than the range of egg weights produced by an individual queen. Unfortunately, this data isn’t presented, but it can be inferred from the standard deviation of the mean egg weight.

For example, the average weight of the heaviest eggs was 162.7 micrograms with a standard deviation of 22.2 micrograms. With some basic assumptions of the distribution of weights, that means that 68% of the eggs were between 140.5 and 184.9 micrograms, but the remaining 32% were heavier or lighter.

Clearly, queens produce eggs that vary considerably in weight … and this has also been shown in previous studies (e.g. AL-Kahtani et al., 2013).

I would have liked to see a graph of the weight of individual eggs and an indication of whether or not the resulting larva was accepted as starting material for a new queen.

Secondly, there are methodological problems – acknowledged by the authors – in the relationship between queen quality and egg weight. So few queens were reared from lightweight eggs that it was difficult to determine if these produced poor quality flyweight queens with low numbers of ovarioles.

You can only work with what’s available.

Emergency response and egg/larval selection

The other two caveats I have are to do with the experimental design. The study involved rearing queens under the emergency response; larvae were presented to queenless and broodless colonies. For survival, they had to rear queens from the material presented (but still exhibited a preference).

However, I’d suggest that the vast majority of queens reared by honey bees – over the millions of years that have shaped the evolutionary choices we are now testing – are produced under either the swarming or supersedure responses.

Is egg/larval choice under the emergency response the same?

We don’t know 7.

The non-random construction of queen cells.

Finally, it has been shown that colonies prefer to rear queens from 3 day old eggs rather than 48 hour old larvae. I understand why the authors reared the eggs in vitro, but it does rather ignore the known preferences of the colony (see The bees know best for more on this topic).

Yes … they had no option other than to choose between the offered 48 hour old larvae … but would they have made the same choice if they had been given the eggs in the first place?

Why are heavier eggs preferred … ?

This is where we get to speculation and I’m going to save the discussion for after a follow-up post 8 in the next fortnight or so.

The bottom line is we don’t really know, but we have some pretty good ideas (though some are extrapolated from other social insects).

However, there’s a related question; ”How are the heavier eggs/larvae selected?” … and I think it’s fair to say this remains unclear 9.

… and is this relevant to our queen rearing?

When I rear queens I select larvae from a colony that shows some or all of the traits that I favour in my bees.

I’m a simple beekeeper and I have very simple needs … I want my bees to be calm, well-tempered, steady on the comb and frugal in winter. The best colonies that exhibit these traits are used as a source for grafting larvae when queen rearing.

Nice bees and a nice queen

In contrast, colonies that exhibit lots of chalkbrood, have poor temper, run about the comb or – worst of all – ‘follow’ are never used for queen rearing. Nor are they allowed to replace their queen during swarm control. Instead, these are requeened (as early as practical) from better stock.

I’ve described before my ‘rule of thirds’. When comparing the sum total of the various traits I care about, the best third are used for queen rearing. These queens are used to requeen the worst third and – if there are spares – the intermediate quality third as well.

However, if I run out of queens I’m reasonably happy to let the middle third requeen themselves (for example, during swarm control).

You’d be surprised how quickly the average quality of your bees improves using a strategy like this.

Grafting larvae vs. letting the bees choose

But the ‘best third’ are defined solely by my criteria.

I ignore any preferences the bees might have by choosing the larvae when grafting.

Assuming the queens that head these top third ‘good’ colonies produce a range of egg sizes (which they will), the bees would preferentially select the larvae from the largest eggs.

I just pick the larvae of the right age that I can see 10 and transfer them to a Nicot plastic queen cup.

Eggs and young larvae

Eggs and young larvae

Not the same thing at all.

Perhaps it doesn’t matter? After all, thousands of apparently satisfactory queens are reared by grafting every season.

Perhaps the characteristics the bees select for – whatever they are – are irrelevant for beekeeping? We don’t know, but I’d bet that some of the criteria that benefit the bees – and are evolutionarily selected – might well benefit beekeeping.

Poor ‘take’

Sometimes I get 100% ‘take’ i.e. all the grafted larvae accepted and reared as queens 11.

Sometimes it’s less, a few times it’s almost none 🙁 .

Cell bar frame with three day old queen cells, The Apiarist.

3 day old queen cells …

In the latter instance I usually assume that the cell raising colony is not sufficiently ‘receptive’ but perhaps I’ve chosen undersized larvae (for their age), or perhaps the donor queen only produces undersized larvae (again, for their age)?

In the best tradition of “If at first you don’t succeed, try, try and try again” I usually just have another go. Almost always, I have another go in exactly the same way.

Perhaps if I used larvae from a different (but still good) colony the take would be improved?

Or perhaps if I presented the larvae in a manner that allowed the bees to select those to be developed into queens I might get improved acceptance? 12

I could end up with more queens and – potentially – better queens.

It’s blowing a hoolie tonight

And, as another autumn storm winds itself up to come barreling in from the Atlantic, dreaming about balmy May afternoons in the apiary and improved ways to produce better queens is about as close as I can get to beekeeping 😉 .


References

AL-Kahtani, S.N. and Bienefeld, K. (2021) ‘Strength surpasses relatedness–queen larva selection in honeybees’, PLOS ONE, 16(8), p. e0255151. Available at: https://doi.org/10.1371/journal.pone.0255151.
Al-Kahtani, S.N., Wegener, J. and Bienefeld, K. (2013) ‘Variability of Prenatal Maternal Investment in the Honey Bee (Apis mellifera)’, Journal of Entomology, 10(1), pp. 35–42. Available at: https://doi.org/10.3923/je.2013.35.42.
Mattila, H.R. and Seeley, T.D. (2007) ‘Genetic Diversity in Honey Bee Colonies Enhances Productivity and Fitness’, Science, 317(5836), pp. 362–364. Available at: https://doi.org/10.1126/science.1143046.

Winter covers and colony survival

Synopsis : A recent study shows increased overwinter colony survival of ‘covered’ hives wrapped in Correx and with insulation under the roof. What provides the most benefit, and are the results as clear cut as they seem?

Introduction

A recent talk by Andrew Abrahams to the Scottish Native Honey Bee Society coincided with me catching up my 1 backlog of scientific papers on honey bees. I’d been reading a paper on the benefits of wrapping hives in the winter and Andrew commented that he did exactly that to fend off the worst of the wet weather. Andrew lives on the island of Colonsay about 75 km south of me and we both ‘benefit’ from the damp Atlantic climate.

The paper extolled the virtues of ‘covered’ hives and the data the researchers present looks, at first glance, compelling.

For example, <5% of covered hives perished overwinter in contrast to >27% of the uncovered control hives.

Wow!

Why doesn’t everyone wrap their hives?

However, a closer look at the paper raises a number of questions about what is actually benefitting (or killing) the colonies.

Nevertheless, the results are interesting. I think the paper poses rather more questions than it answers, but I do think the results show the benefits of hive insulation and these are worth discussing.

Bees don’t hibernate

Hibernation is a physiological state in which the metabolic processes of the body are significantly reduced. The animal becomes torpid, exhibiting a reduced heart rate, low body temperature and reduced breathing. Food reserves e.g. stored fat, are conserved and the animal waits out the winter until environmental conditions improve.

However, bees don’t hibernate.

Winter cluster 3/1/21 3°C (insulation block removed from the crownboard)

If you lift the lift the roof from a hive on a cold midwinter day you’ll find the bees clustered tightly together. But, look closely and you’ll see that the bees are moving. Remove the crownboard and some bees will probably fly.

The cluster conserves warmth and there is a temperature gradient from the outside – termed the mantle – to the middle (the core).

If chilled below ~5.5°C a bee becomes semi-comatose 2 and unable to warm herself up again. The mantle temperature of the cluster never drops below ~8°C, but the core is maintained at 18-20°C when broodless or ~35°C if they are rearing brood. I’ve discussed the winter cluster in lots more detail a couple of years ago.

The metabolic activity of the clustered winter bees is ‘powered’ by their consumption of the stores they laid down in the autumn. It seems logical to assume that it will take more energy (i.e. stores) to maintain a particular cluster temperature if the ambient temperature is lower.

Therefore, logic would also suggest that the greater the insulation properties of the hive – for a particular difference in ambient to cluster temperature – the less stores would be consumed.

Since winter starvation is bad for bees (!) it makes sense to be thinking about this now, before the temperatures plummet in the winter.

Cedar and poly hives

I’m not aware of many comparative studies of the insulation properties of hives made from the two most frequently used materials – wood and polystyrene. However, Alburaki and Corona (2021) have investigated this and shown a small (but statistically significant) difference in the inner temperature of poly Langstroth hives when compared to wooden ones.

Poly hives were ~0.5°C warmer and, perhaps more importantly, exhibited much less variation in temperature over a 24 hour period.

Temperature and humidity in poly and wood hives

In addition to the slight temperature difference, the humidity within the wooden hives was significantly higher than that of poly.

The hives used in this study were occupied by bees and the temperature and humidity were recorded from sensors placed in a modified frame in the ‘centre of the brood box’. The external ambient temperature averaged 0°C, but fluctuated over a wide range (-10°C to 20°C) during the four month study 3.

Temperature anomalies

Whilst I’m not surprised that the poly hives were marginally warmer, I was surprised how low the internal hive temperatures were. The authors don’t comment on whether the ‘central’ frame was covered with bees, or whether the bees were rearing brood.

The longitudinal temperature traces (not reproduced here – check the paper) don’t help much either as they drop in mid-February when I would expect brood rearing to be really gearing up … Illogical, Captain.

The authors avoid any discussion on why the average internal temperature was at least 5-8°C cooler than the expected temperature of the core of a clustered broodless colony, and ~25°C cooler than a clustered colony that was rearing brood.

My guess is that the frame with the sensors was outside the cluster. For example, perhaps it was in the lower brood box 4 with the bees clustered in the upper box?

We’ll never know, but let’s just accept that poly hives – big surprise 😉 – are better insulated. Therefore the bees should need to use less stores to maintain a particular internal temperature.

And, although Alburaki and Corona (2021) didn’t measure this, it did form part of a recent study by Ashley St. Clair and colleagues from the University of Illinois (St. Clair et al., 2022).

Hive covers reduce food consumption and colony mortality

This section heading repeats the two key points in the title of this second paper.

I’ll first outline what was done and describe these headline claims in more detail. After that I’ll discuss the experiments in a bit more detail and some caveats I have of the methodology and the claims.

I’ll also make clear what the authors mean by a ‘hive cover’.

The study was conducted in central Illinois and involved 43 hives in 8 apiaries. Hives were randomly assigned to ‘covered’ or ‘uncovered’ i.e. control – groups (both were present in every apiary) and the study lasted from mid-November to the end of the following March.

Ambient (blue), covered (black) and control (dashed) hive temperatures

There were no significant differences in internal hive temperature between the two groups and – notably – the temperatures were much higher (15°-34°C) than those recorded by Alburaki and Corona (2021).

All colonies, whether covered or uncovered, got lighter through the winter, but the uncovered colonies lost significantly more weight once brood rearing started February. The authors supplemented all colonies with sugar cakes in February and the control colonies used ~15% more of these additional stores before the study concluded.

I don’t think any of these results are particularly surprising – colonies with additional insulation get lighter more slowly and need less supplemental feeding.

The surprising result was colony survival.

Less than 5% (1/22) of the covered hives perished during the winter but over 27% (6/21) of the control hives didn’t make it through to the following spring.

(Un)acceptable losses

To put these last figures into context the authors quote a BeeI Informed Partnership survey where respondents gave a figure of 23.3% as being ’acceptable’ for winter colony losses.

That seems a depressingly high figure to me.

However, look – and weep – at the percentage losses across the USA in the ’20/’21 winter from that same survey 5.

Bee Informed Partnership 2021 winter colony losses (preliminary data)

This was a sizeable survey involving over 3,300 beekeepers managing 192,000 colonies (~7% of the total hives in the USA).

If hive covers reduce losses to just 5% why does Illinois report winter losses of 47%? 6

Are the losses in this manuscript suspiciously low?

Or, does nobody use hive covers?

I don’t know the answers to these questions, but I also wasn’t sure when I started reading the paper what the authors meant by a hive ‘cover’ … which is what I’ll discuss next.

Hive covers

The hives used in this study were wooden Langstroths and the hive covers were 4 mm black corrugated polypropylene sleeves.

This is what I call Correx … one of my favourite materials for beekeeping DIY.

These hive covers are available commercially in the USA (and may be here, I’ve not looked). At $33 each (Yikes) they’re not cheap, but how much is a colony worth?

Significantly more than $33.

I’ve not bothered to make the conversion of Langstroth Deep dimensions (always quoted in inches 🙁 ) to metric and then compared the area of Correx to the current sheet price of ~£13 … but I suspect there are savings to be made by the interested DIYer 7.

However, knowing (and loving) Correx, what strikes me is that it seems unlikely to provide much insulation. At only 4 mm thick and enclosing an even thinner air gap, it’s not the first thing I’d think of to reduce heat loss 8.

4 mm Correx sheet

Thermal resistance is the (or a) measure of the insulating properties of materials. It’s measured in the instantly forgettable units of square metre kelvin per watt m2.K/W.

I couldn’t find a figure for 4 mm Correx, but I did manage to find some numbers for air.

A 5 mm air gap – greater than separates the inner and outer walls of a 4 mm Correx hive cover – has a thermal resistance of 0.11 m2.K/W.

Kingspan

It’s not possible to directly compare this with anything meaningful, but there is data available for larger ‘thicknesses’ of air, and other forms of insulation.

An air gap of 100 mm has a thermal resistance of about 0.17 m2.K/W. For comparison, the same thickness of Kingspan (blown phenolic foam wall insulation, available from almost any building site skip) has a thermal resistance of 5, almost 30 times greater.

And, it turns out, St. Clair and colleagues also added a foam insulation board on top of the hive crownboard (or ‘inner cover’ as they call it in the USA). This board was 3.8 cm thick and has somewhat lower thermal resistance than the Kingspan I discussed above.

It might provide less insulation than Kingspan, but it’s a whole lot better than Correx.

This additional insulation is only briefly mentioned in the Materials and Methods and barely gets another mention in the paper.

A pity, as I suspect it’s very important.

Perspex crownboard with integrated 50 mm Kingspan insulation

I’m very familiar with Kingspan insulation for hives. All my colonies have a 5 cm thick block present all year – either placed over the crownboard, built into the crownboard or integrated into the hive roof.

Two variables … and woodpeckers

Unfortunately, St. Clair and colleagues didn’t compare the weight loss and survival of hives ‘covered’ by either wrapping them in Correx or having an insulated roof.

It’s therefore not possible to determine which of these two forms of protection is most beneficial for the hive.

For reasons described above I think the Correx sleeve is unlikely to provide much direct thermal insulation.

However, that doesn’t mean it’s not beneficial.

At the start of this post I explained that Andrew Abrahams wraps his hives for the winter. He appears to use something like black DPM (damp proof membrane).

Hive wrapped in black DPM (to prevent woodpecker damage)

Andrew uses it to keep the rain off the hives … I’ve used exactly the same stuff to prevent woodpecker damage to hives during the winter.

It’s only green woodpeckers (Picus viridis) that damage hives. It’s a learned activity; not all green woodpeckers appear to know that beehives are full of protein-rich goodies in the depths of winter. If they can’t grip on the side of the hive they can’t chisel their way in.

When I lived in the Midlands the hives always needed winter woodpecker protection, but the Fife Yaffles 9 don’t appear to attack hives.

Here on the west coast, and on Colonsay, there are no green woodpeckers … and I know nothing about the hive-eating woodpeckers of Illinois.

So, let’s forget the woodpeckers and return to other benefits that might arise from wrapping the hive in some form of black sheeting during the winter.

Solar gain and tar paper

Solar gain is the increase in thermal energy (or temperature as people other than physicists with freakishly large foreheads call it) of something – like a bee hive – as it absorbs solar radiation.

On sunny days a black DPM-wrapped hive (or one sleeved in a $33 Correx/Coroplast hive ‘cover’) will benefit from solar gain. The black surface will warm up and some of that heat should transfer to the hive.

And – in the USA at least – there’s a long history of wrapping hives for the winter. If you do an internet search for ‘winterizing hives’ or something similar 10 you’ll find loads of descriptions (and videos) on what this involves.

Rather than use DPM, many of these descriptions use ‘tar paper’ … which, here in the UK, we’d call roofing felt 11.

Roofing felt – at least the stuff I have left over from re-roofing sheds – is pretty beastly stuff to work with. However, perhaps importantly, it has a rough matt finish, so is likely to provide significantly more solar gain than a covering of shiny black DPM.

I haven’t wrapped hives in winter since I moved back to Scotland in 2015. However, the comments by Andrew – who shares the similarly warm and damp Atlantic coastal environment – this recent paper and some reading on solar gain are making me wonder whether I should.

Fortunately, I never throw anything away, so should still have the DPM 😉

Winter losses

Illinois has a temperate climate and the ambient temperature during the study was at or below 0°C for about 11 weeks. However, these sorts of temperatures are readily tolerated by overwintering colonies. It seems unlikely that colonies that perished were killed by the cold.

So what did kill them?

Unfortunately there’s no information on this in the paper by St. Clair and colleagues.

Perhaps the authors are saving this for later … ’slicing and dicing’ the results into MPU’s (minimal publishable units) to eke out the maximum number of papers from their funding 12, but I doubt it.

I suspect they either didn’t check, checked but couldn’t determine the cause, or – most likely – determined the cause(s) but that there was no consistent pattern so making it an inconclusive story.

But … it was probably Varroa and mite-transmitted Deformed wing virus (DWV).

It usually is.

Varroa

There were some oddities in their preparation of the colonies and late-season Varroa treatment.

Prior to ‘winterizing’ colonies they treated them with Apivar (early August) and then equalised the strength of the colonies. This involves shuffling brood frames to ensure all the colonies in the study were of broadly the same strength (remember, strong colonies overwinter better).

A follow-up Varroa check in mid-October showed that mite levels were still at 3.5% (i.e. 10.5 phoretic mites/300 bees) and so all colonies were treated with vaporised oxalic acid (OA).

Sublimox vaporiser

Sublimox vaporiser … phoretic mites don’t stand a chance

In early November, mite levels were down to a more acceptable 0.7%. Colonies received a second OA treatment in early January.

For whatever reason, the Apivar treatment appears to have been ineffective.

When colonies are treated for 6-10 weeks with Apivar (e.g. early August to mid-October) mite levels should be reduced by >90%.

Mite infestation levels of 3.5% suggest to me that the Apivar treatment did not work very well. That being the case, the winter bees being reared through August, September and early October would have been exposed to high mite levels, and so acquired high levels of DWV.

OA treatment in mid-October would kill these remaining mites … but the damage had already been done to thediutinus’ winter bees.

That’s my guess anyway.

An informed guess, but a guess nevertheless, based upon the data in the paper and my understanding of winter bee production, DWV and rational Varroa management.

In support of this conclusion it’s notable that colonies died from about week 8, suggesting they were running out of winter bees due to their reduced longevity.

If I’m right …

It raises the interesting question of why the losses were predominantly (6 vs 1) of the control colonies?

Unfortunately the authors only provide average mite numbers per apiary, and each apiary contained a mix of covered and control hives. However, based upon the error bars on the graph (Supporting Information Fig S1 [PDF] if you’re following this) I’m assuming there wasn’t a marked difference between covered and control hives.

I’ve run out of informed guesses … I don’t know the answer to the question. There’s insufficient data in the paper.

Let’s briefly revisit hive temperatures

Unusually, I’m going to present the same hive temperature graph shown above to save you scrolling back up the page 13.

Ambient (blue), covered (black) and control (dashed) hive temperatures

There was no overall significant difference in hive temperature between the control and covered colonies. However, after the coldest weeks of the winter (7 and 8 i.e. the end of February), hive temperatures started to rise and the covered colonies were consistently marginally warmer. By this time in the season the colonies should be rearing increasing amounts of brood.

I’ve not presented the hive weight changes. These diverged most significantly from week 8. The control colonies used more stores to maintain a similar (actually – as stated above – marginally lower) temperature. As the authors state:

… covered colonies appeared to be able to maintain normal thermoregulatory temperatures, while consuming significantly less stored food, suggesting that hive covers may reduce the energetic cost of nest thermoregulation.

I should add that there was no difference in colony strength (of those that survived) between covered and control colonies; it’s not as though those marginally warmer temperatures from week 9 resulted in greater brood rearing.

Are lower hive temperatures ever beneficial in winter?

Yes.

Varroa management is much easier if colonies experience a broodless period in the winter.

A single oxalic acid treatment during this broodless period should kill 95% of mites – as all are phoretic – leaving the colony in a very good state for the coming season.

If you treat your colonies early enough to protect the winter bees there will inevitably be some residual mite replication in the late season brood, thereby necessitating the midwinter treatment as well.

I’m therefore a big fan of cold winters. The colony is more likely to be broodless at some point.

I was therefore reassured by the similarity in the temperatures of covered and control colonies from weeks 48 until the cold snap at the end of February. Covered hives should still experience a broodless period.

I’m off for a rummage in the back of the shed to find some rolls of DPM for the winter.

I don’t expect it will increase my winter survival rates (which are pretty good) and I’m not going to conduct a controlled experiment to see if it does.

If I can find the DPM I’ll wrap a few hives to protect them from the winter weather. With luck I should be able to rescue an additional frame or two of unused stores in the spring (I often can anyway). I stack this away safely and then use it when I’m making up nucs for queen mating.

I suspect that the insulation over the crownboard provides more benefit than the hive ‘wrap’. As stated before, all my colonies are insulated like this year round as I’m convinced it benefits the colony, reducing condensation over the cluster and keeping valuable warmth from escaping. However, wrapping the hive for solar gain and/or weather protection is also worth considering.


References

Alburaki, M. and Corona, M. (2022) ‘Polyurethane honey bee hives provide better winter insulation than wooden hives’, Journal of Apicultural Research, 61(2), pp. 190–196. Available at: https://doi.org/10.1080/00218839.2021.1999578.

St. Clair, A.L., Beach, N.J. and Dolezal, A.G. (2022) ‘Honey bee hive covers reduce food consumption and colony mortality during overwintering’, PLOS ONE, 17(4), p. e0266219. Available at: https://doi.org/10.1371/journal.pone.0266219.

Wild, feral or escapees?

Synopsis : How far do swarms move? Can estimates of environmental apiary and hive densities help determine whether “isolated, lost or ancient” bees are anything of the sort?

Introduction

A little more on feral colonies this week. It’s an interesting topic to think about as the temperature drops, the wind picks up and the trees change into their autumn finery.

A riot of autumn colour

If there are any colonies in the local woods, how did they get there and what are their chances of survival?

I discussed some answers to this last week, using the specific example of old-growth forests in Germany (Kohl et al., 2022). In those cases the reality was that the majority of colonies perished within a year – the average time they survived was only ~32 weeks.

The most likely explanation for their presence in the forest was ‘spillover’ of lost swarms from managed colonies in neighbouring farmland. My assumption – though this wasn’t covered in the paper – was that the colonies perished from either disease or starvation.

This week I want to consider the isolation – or otherwise – of ‘remote’ forests and the distance swarms travel from their origin. Inevitably this will involve some back of an envelope calculations and even outright guesstimates 1, so I’ll finish on a more familiar topic (to me) by briefly considering the pathogen loads of feral colonies.

Are these feral healthy and thriving, or riddled with disease?

Beekeepers and hives …

There are about 50,000 beekeepers in the UK and they manage about 250,000 hives.

That’s two ’abouts’ in one sentence, so the guesstimates have started already. The National Bee Unit reports there are 272,000 hives in the UK (2021 figures). They call this an ‘experimental statistic’ because ’several assumptions formed part of the calculations’ 2. This number is up from 247,000 in 2017.

I suspect some of these assumptions include extrapolating from the numbers of beekeepers/apiaries and hives registered on the National Bee Unit’s BeeBase. In 2013 this was 29,000 beekeepers managing 126,000 colonies.

That extrapolation is needed as not all beekeepers are registered on BeeBase 3, in the same way that not all beekeepers belong to a national or local association.

I’m going to ignore our commercial cousins, the bee farmers. There are only about 400 of them and only one or two of them manage more than 1,000 colonies.

The 2013 BeeBase numbers suggest that registered beekeepers manage 126,000/29,000 = 4.3 hives each. My opening sentence to this section would indicate that the average is perhaps about 5 hives. However, if the experimental statistic is correct but beekeeper numbers are still around 50k, then it’s a little over 5.4 hives per beekeeper.

Let’s keep the maths simple … on average, beekeepers manage 5 colonies 😉 .

… and apiaries

Unfortunately, I’m not aware of any publicly available statistics on hive density, but there is at least partial information available on apiary density.

If you are registered on BeeBase, two of the things you record are the apiary location and the number of hives in each apiary. Once an apiary is registered you can determine the ‘Apiary density within 10 km’ 4.

Beebase shows the ‘density’ of apiaries within 10 km

A radius of 10 km from your registered apiary encompasses 314 km2, so it is perhaps not surprising that there can be a large number of other apiaries in the neighbourhood.

When I lived in Warwickshire my two apiaries – separated by ~5 km – had 255 and 267 apiaries within a 10 km radius. This is a busy beekeeping area, with a very active local association (my alma mater, WLBK).

Hives in the corner of a Warwickshire field (almost every field!)

I don’t know how many hives there were in the surrounding environment, but it seemed as though almost every field margin or spinney contained a little row of hives balanced on old pallets.

Convenient assumptions

On the basis that I don’t have any other information, and in the interest of getting on with the article, I’m going to assume that each apiary contains an average of 5 hives. I think this is reasonable, though I’d be interested if anyone has any real figures.

With ~260 apiaries within 10 km, each containing an average of 5 hives, it suggests the hive density was 4.1 km2.

Coincidentally 5 this is almost exactly the same figure quoted in Kohl et al., (2022) last week.

And each year a significant proportion of these hives will attempt to swarm.

Swarms

With exemplary swarm control it is possible to avoid losing any swarms.

Of course, we do our beekeeping in the real world, and the reality is that we all lose swarms sometimes.

Hopefully not many and perhaps not even every year, but swarms are lost.

When I lived in Warwickshire I never failed to attract swarms to my bait hives each season. When I lived in Fife – where there were only ~45 apiaries within 10 km 6 – I caught four swarms in a bait hive in my back garden one season, and (again) never failed to attract swarms in the time I lived there.

Although I’d like to think this reflects the care I take in preparing my bait hives, I suspect it really means that – during the swarming season – a lot of queen cells are missed and swarms are lost.

Bivouacs and scout bees

Generally, though there are exceptions, the swarming process goes something like this:

  • the colony starts producing queen cells
  • on the first good day (warm, dry, fine etc.) after the first queen cells are sealed the colony swarms
  • the swarm bivouacs nearby, perhaps only 10-20 metres away
  • scout bees survey the environment for likely new nest sites, ‘dancing’ on the surface of the bivouac to persuade other scout bees to check out promising looking locations
  • a quorum decision is reached by the scout bees on the ‘best’ new nest site and they lead the swarm there 7

The scout bees survey at least a 2-3 km radius around the original hive; they probably start this process before the colony swarms, continuing it once the swarm has bivouacked. Since we can interpret the waggle dance, it is possible to observe the scout bees and infer from them the approximate distance and direction to the selected nest site.

By doing this, scientists have determined how far swarms usually travel (a relatively short distance) and how far they sometimes go (a long way).

Swarming distances

Most swarms relocate just a few hundred metres from their origin. Martin Lindauer did some of the first studies on swarming distances in the mid-50’s and Thomas Seeley and Roger Morse produced strikingly similar data in 1977 (Seeley and Morse, 1977).

Most swarms only travel a short distance to a new nest site

There are a number of related studies from the early 1980’s which demonstrate that, although scout bees may survey the environment from ~300 m to over 4 km away, at least 50% of swarms move no more than 1 km from their origin.

However, they can travel much further.

In recent studies José Villa studied swarming of bees in Louisiana (Villa, 2004). He studied swarm size (weight), nest volume preference and the timing of swarming. In addition, by interpreting scout bee waggle dancing, he recorded the distance 16 swarms travelled from their origin.

In this study a marked preference for relatively ‘local’ nest sites was not seen. Four swarms travelled less than 1 km, six from 1 to 4 km, five between 4 and 7 km and one ~10 km.

With three of the swarms, two that moved <500 m from the origin and one 2.2 km away, he confirmed their location by finding the uniquely tagged queen present in the original swarm.

Although I said ’they can travel much further’ it’s worth remembering that the distance travelled was inferred from the duration of the waggle run by scout bees on the surface of the bivouacked swarm (and specifically, the predominant dances being conducted 30 minutes before the swarm left the bivouac).

That’s not quite the same as proving that swarms may travel 5-10 km, but it is certainly suggestive that they do.

Isolated woodland in a bee-filled environment

Let’s do a bit more arm waving …

Assume there’s two to three thousand hectares of old native woodland, oak, beech, sycamore etc., rather than conifers. In the absence of black woodpeckers (see last week) some of these trees will still contain hollow cavities. They will have lost boughs or been hit by lightning, the rain will have rotted the exposed heartwood and a cavity will eventually form.

Voilà … a potential nest site for bees 🙂

A wood of 2500 hectares (or is that a forest?), if circular, fills a circle of 5.6 km diameter. Of course, it’s very unlikely to be circular, but it makes the maths easier so bear with me.

Assume this wood is in the middle of good quality mixed farmland, with early season oil seed rape, hedgerows filled with hawthorn and blackberry, and ample clover polka-dotted pasture.

In other words, a good environment for honey bees.

So the local beekeepers plonk a few hives in the corners of fields, or along field margins.

Eventually, the density of these hives reaches 4 km2 (as justified above).

Cartoon of woodland (green) and surrounding farmland (blue and red)

In the diagram above the inner (green) circle is the native woodland. The surrounding blue and red rings represent the surrounding farmland, in each case the area covered by an additional 1 km radius respectively from the centre.

The woodland contains no managed colonies and is 24.6 km2. The blue ring (excluding the central wooded area) has an area of 20.7 km2 and so contains – based upon all those assumptions above – 83 managed hives. Likewise, the red ring has an area of 27 km2 and contains 108 hives.

Define ‘isolated’

As shown above, 50% of swarms move no more than a kilometre to a new nest site, but some move further … and a few may move much further.

Any of the managed hives in the blue ring might produce a swarm that could reach the forest boundary. In addition, assuming the blue ring contained few suitable nest sites – and I’ll return to this point shortly – swarms issuing from hives in the red ring might well travel further and reach the forest.

In fact, if you overlay the roundel diagram with the swarm dispersal diagram – at the same scale – from the paper by Villa (2004) you can see that swarms from a very wide area are ‘in range’ of the hollow tree-filled forest.

Woodland (green) and surrounding farmland with – at the same scale – swarming distances from Villa (2004)

The swarm dispersal diagram shows the swarms starting from a central point, so you just need to imagine the arrows are reversed.

In fact, if you assume that swarms can travel up to 7 km (only one swarm studied by Villa may have gone further, but one third travelled 4-7 km) there could be as many as 517 potentially swarming colonies ‘in range’ 8.

Therefore, as far as migrating swarms are concerned, it’s quite possible that none of the forest is ‘isolated’.

Nest sites in farmland and forests

In the Kohl et al., (2022) paper I discussed last week, the majority of the woodpecker holes used by bees were in large beech trees. The average diameter of the trees was 55 cm when measured 1.5 m above ground.

These were substantial trees.

Trees of that size are common in old growth forest … but they’re rare in farmland.

Hedging, if it hasn’t been grubbed up, contains predominantly small trees. Many small copses and spinneys have also disappeared, all to make way for combine harvesters and subsidies.

Lots of forage but not a lot of mature trees

Of course, there are large trees in farmland, they’re just a whole lot less common than they are in old native woodland.

Therefore swarms issuing from managed hives on farmland – assuming they don’t end up in one of my bait hives – are more likely to gravitate to the forest as there will be more nest sites there.

Blenheim bees

I don’t know much about the widely-publicised ‘Blenheim bees’ that I briefly introduced last week.

However, I do know that the Blenheim Estate near Oxford has about 2500 hectares of woodland, and that there are a lot of beekeepers in Oxfordshire.

That 2500 hectares, if circular (which it isn’t) and centred on Blenheim Palace, would span from Combe to Oxford Airport, and completely covers the small market town of Woodstock 9.

This is a popular area for beekeeping. The National Bee Unit’s ‘BeeBase’ informs me that there are ~200 apiaries within 10 km of Blenheim Palace. Combe to the west has ~190 apiaries within 10 km.

If these apiaries have the expected number of hives in (i.e. 4 km2, and I see no compelling reason why not … for example, the countryside is similar to Warwickshire) then there are a very large number of colonies capable of producing swarms that are well within range of the forested area.

But let’s just revisit that figure of ~200 apiaries within 10 km.

How accurate it is?

Certainly some of the apiaries will have been ‘forgotten’ and are now vacant. I bet there’s a lot of redundant data on BeeBase.

Perhaps ~200 apiaries is not a very accurate figure?

I think it is probably inaccurate … but I strongly suspect it’s an underestimate rather than an overestimate of apiary numbers in the area.

Many beekeepers are not registered on BeeBase. Only the National Bee Unit knows 10 the proportion of beekeepers/apiaries/hives missing, but I’d be amazed if it was less than 25% and not at all surprised if it was 40%.

This is probably part of the ’fiddle factor’ used to extrapolate from BeeBase registrations and hive numbers to that ’experimental statistic’ of 272,000 hives in the UK.

Occam’s razor, the law of parsimony, and ‘isolated’ feral/wild bees

Are there self-sustaining populations of honey bees in the UK?

By self-sustaining I mean not dependent upon an annual influx of swarms from nearby managed colonies. These swarms compensate for the very high winter attrition rate seen in the Kohl et al., (2022) study which is likely due to pathogens and starvation (I’m going to deal with pathogens – briefly – next).

Well, are there?

I don’t know.

Based upon registered and predicted apiary and hive numbers, and the known distances swarms migrate, I think the simplest – and therefore most likely – explanation for feral colonies in ‘isolated’ locations are recent (< 1 year) swarms from nearby managed colonies.

Even assuming the National Bee Unit’s predicted 272,000 hives are evenly distributed over the entire UK (242,000 km2) that’s still >1 hive / km2. They’re obviously not evenly distributed; many areas are unsuitable or, at best, borderline for beekeeping.

I’d like to have been able to discuss the area of old growth forests in the UK and how isolated or otherwise it is. Unfortunately, I don’t have the data … or the GIS mapping skills to interrogate it.

Therefore I’ll close instead with something I know a little more about …

Feral colonies, pathogens and genetics

How healthy are feral colonies in the UK?

There aren’t a lot of published studies. Catherine Thompson and colleagues showed that the pathogen load – including Deformed wing virus (DWV), Black queen cell virus (BQCV) and Nosema (both apis and ceranae) – were similar or higher in feral colonies than in managed colonies (Thompson et al., 2014).

Pathogen levels in feral (F) and managed (M) colonies

Levels of DWV in feral colonies were significantly higher than in managed colonies, but they were similar to the levels seen in beekeeper’s hives not treated to control Varroa infestation.

We know – though many are still bitterly reminded every year – that colonies in which mite levels are high and uncontrolled usually perish overwinter.

Catherine Thompson also studied the genetic characteristics of feral colonies and compared them to managed colonies (Thompson, C. PhD. thesis, University of Leeds, 2010). Her results show that the feral colonies she studied were very similar – and effectively indistinguishable – to managed colonies when the overall level of genetic heterozygosity was analysed. This means that these feral colonies are not a distinct genetic race of bees.

That’s not the same as showing they were genetically related to (and so originated from) nearby managed colonies … those experiments still need to be done.

Are these wild bees self-sustaining, unique and ancient?

If a colony or two of bees (or even a hundred) are found in the woods I’d suggest the following tests need to be applied to convincingly demonstrate they are a unique and self-sustaining population.

  • how isolated are they really? Are there managed colonies within 5-10 km that could act as a source of swarms? Geographic isolation may be due to factors other than distance, for example an island population, or an isolated valley surrounded with mountains.
  • is the population truly self-sustaining? Do colonies regularly survive for sufficient time to reproduce? To be self-sustaining, annual colony losses must be less than or equal to new colonies established from the same feral bees.
  • are the bees genetically distinct from managed colonies within 10 km or so? If they are a well-established population you would expect this.

If the population is truly isolated, reproduces sufficiently to replace annual losses and is genetically distinct, then it may well be self-sustaining.

However, if it doesn’t meet any one of these three criteria then I suspect the population is dependent upon ‘spillover’ losses of swarms from neighbouring managed colonies.

Interesting perhaps, but not surprising, not unique and certainly not ancient.

Unsurprisingly, I’m sceptical about many of the claims made for long lost and unique strains of bees living in the woods (or anywhere else for that matter).

A glimmer of hope (?) … the Arnot Forest bees

The Arnot Forest is not dissimilar in size to Blenheim estate (17 km2 vs. 24 km2).

However, it is surrounded by lots more old growth forest (100+ years) and so is effectively more isolated. There are some managed colonies in the surrounding forests, but – when tested – they were genetically distinct from the Arnot Forest bees (Seeley et al., 2015). Finally, the colony survival characteristics (~1.5 years) and annual swarming of the Arnot Forest bees indicates that the population is self-sustaining. These Arnot Forest bees have adapted to live with Varroa through behavioural changes – frequent swarming, small colonies etc.

Clearly, self-sustaining populations of feral colonies can exist 11, but this is not the same as claiming that all feral populations are self-sustaining, unique or ancient.

Finally, it’s worth noting that the mechanisms that self-sustaining populations of bees have evolved to become Varroa tolerant (they are unlikely to be resistance) – small, swarmy, colonies – may make them unsuited for either beekeeping or pollination.


References

Kohl, P.L., Rutschmann, B. and Steffan-Dewenter, I. (no date) ‘Population demography of feral honeybee colonies in central European forests’, Royal Society Open Science, 9(8), p. 220565. Available at: https://doi.org/10.1098/rsos.220565.

Seeley, T.D. et al. (2015) ‘A survivor population of wild colonies of European honeybees in the northeastern United States: investigating its genetic structure’, Apidologie, 46(5), pp. 654–666. Available at: https://doi.org/10.1007/s13592-015-0355-0.

Seeley, T.D. (2017) ‘Life-history traits of wild honey bee colonies living in forests around Ithaca, NY, USA’, Apidologie, 48(6), pp. 743–754. Available at: https://doi.org/10.1007/s13592-017-0519-1.

Seeley, T.D. and Morse, R.A. (1977) ‘Dispersal Behavior of Honey Bee Swarms’, Psyche: A Journal of Entomology, 84, pp. 199–209. Available at: https://doi.org/10.1155/1977/37918.

Thompson, C. (2010) The health and status of the feral honeybee (Apis mellifera sp) and Apis mellifera mellifera population of the UK. phd. University of Leeds. Available at: https://etheses.whiterose.ac.uk/5211/ (Accessed: 19 October 2022).

Thompson, C.E. et al. (2014) ‘Parasite Pressures on Feral Honey Bees (Apis mellifera sp.)’, PLOS ONE, 9(8), p. e105164. Available at: https://doi.org/10.1371/journal.pone.0105164.

Villa, J.D. (2004) ‘Swarming Behavior of Honey Bees (Hymenoptera: Apidae) in Southeastern Louisiana’, Annals of the Entomological Society of America, 97(1), pp. 111–116. Available at: https://www.researchgate.net/publication/232681544_Swarming_Behavior_of_Honey_Bees_Hymenoptera_Apidae_in_Southeastern_Louisiana.

Feral facts and fallacies

Synopsis : Are feral colonies recently lost swarms or a self-sustaining ‘wild’ honey bee population? The latter must reproduce faster than they perish. Measuring rates of colony loss and nest occupancy provides a good indicator of the likely origin and independence of feral populations.

Introduction

Most colonies try to swarm every year. Most – not all – but if your colonies are strong and healthy they are likely to swarm. That’s why swarm prevention and subsequent swarm control are such important skills for the tyro beekeeper to master. Without swarm control the majority of the workforce is ‘lost’, the residual colony will be left temporarily queenless and the potential honey crop is probably much reduced.

A small swarm

A small swarm …

It is not difficult to become competent at swarm prevention and control. However, any beekeeper who claims to never lose swarms is probably being ‘economical with the actualité’ as the late Alan Clark once said.

What happens to those ‘lost’ swarms?

Some forward-thinking beekeepers set out bait hives. Any swarms that end up being attracted to these ‘swarm traps’ will eventually find their way back to a managed apiary. Some swarms end up in the church tower where ‘there have always been bees’, according to local parishioners.

Others move into the roof space above the entrance to the local nursery school, causing fascination, irritation and consternation in equal measure. Their fate depends upon whether the head teacher contacts a beekeeper or a pest controller … but their arrival reinforces the importance of swarm control and the use of bait hives.

A bait hive deployed in mid-April in good time for the swarming season ahead

And other swarms disappear over the apiary fence, across the field and into the local woods, eventually establishing a new colony in a suitable hollow tree.

No risk, no reward

Swarming is a risky business. The swarm leaves with the majority of the flying bees and the mated queen. However, it takes more than that to establish a functional colony. They need to draw comb, rear brood and collect sufficient stores to get through the winter.

That’s a tall order and most swarms fail.

Data from Thomas Seeley in The Lives of Bees suggests that only about 23% of swarms survive the winter.

In contrast, the swarmed colony has about an 80% chance of survival. They’ve got drawn comb, stores, eggs and larvae … ‘all’ they need to do is rear a new queen.

And then they’re likely to swarm again the following year 1. In fact, without swarm control, the average number of swarms produced by a colony is two per year – presumably a prime swarm (headed by the old queen) and a cast (headed by a virgin queen).

So, swarming is risky, but those swarms that succeed in establishing a new colony and overwintering can themselves attempt to reproduce again the following year.

That’s the reward.

Where are all these bees?

Even taking account of the exemplary swarm control by the UK’s ~25,000 beekeepers 2 I’m reasonably certain that a lot of swarms are lost every year.

Where do all these bees go?

I’ve been told of lots of churches or schools or trees with resident bees.

Quiet churchyard

A swarm magnet … or just an old church?

However, it’s certainly not every church, or school or hollow tree that’s occupied. Even when there’s a surplus of suitable nest sites, those that are occupied by a colony are the exception, not the rule.

The main reason of course is Varroa.

In the absence of intervention to reduce the mite population, the developing winter bees get parasitised by Varroa, and the resulting high levels of deformed wing virus (DWV) reduces the longevity of these necessarily 3 long-lived bees.

Consequently the winter cluster shrinks in size, from that of a football (early October) to a honeydew melon (late December) to a large orange (early February).

And then it freezes to death during a cold snap 🙁 .

The apiary in winter ...

The apiary in winter …

Numerous studies have shown that untreated colonies, in the absence of any natural resistance or tolerance to Varroa or DWV (though the latter is rarely discussed, and even less frequently tested for), almost always perish within a year or two of Varroa infestation.

Look back at the recent post on Biological control with Varroa for a reminder of the devastation wreaked on an island population of honey bees after the introduction of mites.

Wild? They’re livid feral …

Technically, swarms lost by beekeepers (that become established in the environment) are probably best termed feral colonies.

Originally feral meant simply ‘wild or untamed’, but the more common usage these days means ’animals or plants that have lapsed into a wild form from a domesticated condition’.

Bees aren’t domesticated, but I think feral conveniently encompasses their origin.

However, I’m more than happy to accept that a colony, initially feral, that becomes well-established in the church tower and throws off a swarm or two every year, that requeens every two or three seasons, surviving without intervention or management, must be considered ‘wild’ at some point.

It’s not worth discussing when a colony transitions from feral to wild.

It’s semantics, though I think the distinction between ‘recently arrived from a swarmed managed colony’ and ‘self-sustaining’ is an important one.

Notwithstanding the ravages of Varroa, whether feral or wild, there are colonies in the environment – churches, schools, trees – and probably rather more than many beekeepers are aware of.

The missing bees

Periodically there’s a little flurry of interest in the press about ‘long lost’ or ‘missing’ wild bees discovered in the woods.

Late last summer there were articles in all the newspapers about bees found on Blenheim Estate. The Observer reported this discovery with the headline ”No one knew they existed”: wild heirs of lost British honeybee found at Blenheim.

‘Blenheim bees’ article in the Observer, 7-11-21

As an aside – as this isn’t the real topic for discussion today – there are at least three challenging claims made in that headline; how can you be sure that no-one knew they existed? Is the British honey bee (it is not honeybee) actually lost? How do you know that these bees are their heirs?

Pedantic is my middle name.

But the 2500 hectare Blenheim Estate 4 isn’t the only location with apparently self-sustaining populations of honey bees. There are trees, churches and (I dare say) even nursery schools up and down the country that appear to have a ‘resident’ colony or two of bees.

Periodically they’re observed swarming. Sometimes things seem a bit quiet in the spring, but perhaps it’s too cold for the bees to be flying strongly anyway.

By May there’s a lot of activity so all must be well.

Right?

Perhaps 😉

Citizen science

These wild/feral colonies are infrequent but widely distributed. They are therefore difficult for one person to regularly observe. As a consequence there are several ‘citizen science’ projects monitoring some of these sites. Magnus Peterson regularly reports in The Scottish Beekeeper on the one he coordinates for the University of Strathclyde.

The criticism of these types of studies – certainly not Magnus’s specifically – but any study the largely relies upon infrequent observation by volunteers, is that stuff gets missed. A visit doesn’t happen because it’s raining hard. Or it does happen in heavy rain and no activity is observed and the colony is recorded as dead.

Or worse, recorded as alive, but not flying because of the heavy rain.

With more systematic observation, though not necessarily more frequent, you can have increased certainty that the site that was occupied last autumn is still occupied this spring.

The timing of these observations is important. Three per season is probably the minimum, early, mid and late, but they have to be at particular times of the season – see below.

Crowdfunding

So, let’s assume a colony is found in the autumn and the same hollow tree is occupied in late-April the following year … yippee, the colony is still alive.

Feral – or are they now wild? – bees living successfully with Varroa (at least presumably living with Varroa if they’re almost anywhere in mainland UK).

Perhaps they’ve evolved to have some interesting and useful trait(s) that renders the colony resistant to or tolerant of the dreaded parasitic mites?

These are valuable bees.

They are an important genetic resource.

They must be protected at all costs.

Perhaps it’s time to set up a web colony cam to record their activity? That’s going to cost a pretty penny, so some crowdfunding is needed.

A website is created … a dozen mini-nucs are purchased for the ambitiously planned queen rearing programme and – inevitably – there’s a misquoted article or two in the Guardian.

But hold on …

Are they really the same colony in April that were there the previous autumn?

How can you be sure?

How can you be certain that it’s not an unseasonably early swarm that was missed by the – usually eagle-eyed – local beekeepers? 5

It’s not unusual to find the odd charged queen cell during the first colony inspection of the season. At least, I’ve sometimes found queen cells during that first inspection. I’m sufficiently experienced to not go rummaging about in the boxes too early in the season, and so I am sometimes surprised at how well developed the colony is when I open the box.

Charged queen cell

But what if it had been raining, so I’d postponed the inspection?

On the next warm spring day – well before I was able to return to the apiary – the colony could swarm.

I’ve regularly seen April swarms in Scotland and there are many reports of even earlier swarms on social media every year.

Perhaps the active ‘overwintered’ colony is nothing of the sort.

Maybe it’s just been occupied by a very early swarm?

To be sure it’s the same colony you need to do some genetic testing. If the colony is the same the genetic testing will show identity. If the testing shows significant variation then it’s a different colony.

And, if you combine some genetic testing of overwintered colonies with three carefully-timed visits – late season, very early season and mid-season – to a large number of wild/feral colonies, or likely sites that they would occupy, you can determine their longevity and whether they are a self-sustaining population.

Bee trees

And I wouldn’t have given that long and rambling introduction if there wasn’t a recent scientific paper where they’ve done exactly that (Kohl et al., 2022). I’ll describe it briefly as it’s a nicely written and compelling story. The paper is open access, so you can read it if you want to check my interpretation of the data.

Importantly, I think it provides a very good guide to both the quality and quantity of data that are needed to be sure a population of bees are truly wild and self-sustaining 6 … or just regularly boosted by careless local beekeeping!

Feral colonies are few and far between. It’s hard work walking around the woods looking for hollow trees that may (but probably won’t) contain a colony. You find lots of trees with holes, but they need to lead to a suitably-sized cavity to be of any use to a colony of bees. Binoculars help (the holes are often 15 metres off the ground) … but perhaps there are better ways of doing this?

A bee tree?

Bee-lining – as described by Seeley in Following the Wild Bees – is an effective way of tracking down wild colonies, but needs good weather, good forage and ample time. It works well when locating a few colonies, but probably takes too long if you want 100+ to produce a statistically compelling set of results.

But what if you also wanted to record how many new nest sites are occupied? You would need to know where the empty cavities were before they were occupied. That’s not something you can determine by bee-lining, so you’re back to traipsing around the woods with a pair of binoculars.

Woodpeckers

But in Germany they have some very large woodpeckers.

The black woodpecker (Dryocopus martius) is a crow-sized bird that excavates correspondingly large holes for nest sites in old-growth forests. The average volume of a black woodpecker nest is about 10 litres, smaller than optimal for a swarm, but appreciably larger than most ‘natural’ tree cavities.

Black woodpecker

Conveniently, there are high-resolution maps of (historical) woodpecker nesting trees in old-growth forests in Swabian Alb, Weilheim-Schongau and the counties of Coburg and Lichtenfels. 98% of these woodpecker nest sites are in large beech trees, most are 10-12 metres above ground and with an entrance of ~10cm diameter (again, not optimal, but better than no nest site for a swarm).

Kohl and colleagues surveyed about 460 of these ‘cavity’ trees three times per season; in July (after the main May/June swarming season) to determine peak occupancy rates, in mid/late September to determine late summer survival and in early/mid April to determine winter survival.

‘Occupancy’ was determined by visual inspection and regular forager activity and/or pollen loads (i.e. they ignored scout bees checking empty cavities). In addition, for some colonies, a dozen or so workers were collected for genetic analysis.

With these data, the mathematical calculation of annual survival rates could be determined, as could the prediction of the annual numbers of swarms needed per colony for the population to be self-sustaining 7. In addition, it was possible to determine the average lifespan of a colony.

There were a bunch of perfectly reasonable assumptions made, based upon the known biology of honey bees – all are listed in the paper.

Yo-yoing colony numbers

The scientists counted colony numbers, but could also determine colony densities per km2. By making observations over a 3-4 year period it was strikingly obvious that the largest number of ‘cavity’ trees were occupied after swarming in summer, but that numbers dropped dramatically overwinter. This ’recurring temporal pattern of population fluctuations’ is very obvious in the major data figure in the paper.

Temporal population fluctuations of feral honey bee colonies in Germany; A) occupancy rates, B) population density

The average maximum occupancy rate and population density was 11% and 0.23 colonies per km2. This ‘dropped massively’ over the winter to just 1.4% and 0.02 colonies per km2.

The majority of nest sites (n = 112) occupied in late summer were unoccupied the following spring, before swarming started. 90% of colonies survived the summer (from July until late September), but only 16% of colonies survived the following winter.

The spring survival rate was calculated as 74% based upon genetic testing of colonies in early spring and mid-summer

Knowing the summer, winter and spring survival rates enables the annual survival rate to be calculated.

This was a sobering 10.6%.

Therefore, to maintain a stable population, each surviving colony would need to produce an average of 8.4 swarms per season.

That’s an unachievable amount of swarming.

The average lifespan of a feral colony in these three German forest regions was just 0.619 years … a little over 32 weeks.

Clearly, these honey bee populations are not self-sustaining.

Are these German forests typical?

There are two other regions where similar quality data exists for wild/feral honey bee populations. These are the Arnot forest in the USA, studied for decades by Thomas Seeley, and Wyperfield National Park in Australia.

There are striking differences between these two regions and the German forests, both in terms of colony lifespan and swarm numbers needed to be self-sustaining.

For the Arnot forest and Wyperfield National Park, lifespan was calculated as 1.34 and 1.53 years respectively (cf. 0.62 years for Germany), with annual survival of ~50% (cf. 11% in Germany). Annual swarm numbers per colony for the population to be self-sustaining was 0.94 and 0.85 for the the Arnot forest and Wyperfield National Park respectively (cf. 8.43 for the German forests).

Other than these obvious differences in the related figures for survival/longevity and ‘swarms needed’ the other significant difference between self-sustaining populations (like the Arnot forest and Wyperfield National Park) is the colony density.

In areas where feral/wild honey bees are self-sustaining the colony density is at least 1 per km2. In contrast, in Germany and a large number of other studied feral populations in other parts of Europe (including Ireland, Spain, Serbia, Poland and other regions of Germany), the colony density is usually much lower, at 0.1-0.2 per km2.

So, these German forests are seemingly typical of honey bee populations that are not self-sustaining. These are regions where the feral population is boosted annually (and is essentially dependent upon) an influx of swarms that become temporarily established in natural nest sites.

Environmental colony density

Where do all these swarms come from?

The average managed honey bee colony density in the areas of Germany studied is 4 per km2, appreciably higher than either the Arnot forest or Wyperfield National Park. Precise figures for these two were not quoted, but in both locations the feral colonies (remember, these were at ~1 per km2) outnumber managed colonies.

It therefore seems very likely that managed colonies from farmland areas surrounding the German forests acts as the source for swarms, and the latter – because of the paucity of suitable nest sites in the arable land (relatively few buildings, few mature trees etc.) – gravitate towards the forests looking for suitable nest sites.

Feral and managed colonies may therefore be spatially separated, though not very widely. In contrast, in urban environments – where nest sites are probably common – it might be expected that feral and managed colonies are intermixed in the environment.

A by-product of the study by Kohl and colleagues is that they could also calculate the difference in the relative attractiveness of woodpecker nests that had previously, or had never, been occupied by bees. When new colonies occupied woodpecker nest sites there was a strong preference of 5 to 15-fold for sites that had previously been occupied by bees.

This, of course, is why it makes sense to include a single old, dark comb in your bait hives.

That seems like a good place to stop …

I think this German study is interesting. It shows the quantity and quality of data needed to make a compelling case that a location has a self-sustaining population of feral/wild honey bees.

Such locations are likely to exhibit colony densities of at least 1 per km2 and to be physically separated from higher density managed colonies. This physical separation could be in the form of simple geographic isolation – just a long way from other apiaries – or something more complex like being surrounded by high hills or water etc.

Self-sustaining wild/feral populations are likely to exhibit >50% annual survival rates, to live for an average of ~1.5 years and to produce about 0.8-0.9 swarms per colony per year 8.

If survival rates are lower, or the life expectancy of a colony is much less, then the number of swarms needed to maintain the population rapidly becomes so high that they are unattainable.

In which case, large numbers of feral/wild colonies cannot be self-sustaining, but instead must be present because the area acts as a ‘sink’ for lost swarms from nearby managed colonies.

This post is already longer than my self-imposed-but-regularly-exceeded 3000 word limit so I’ll save further discussion of the Blenheim bees and other feral colonies for another post.

However, I hope the study shows that a healthy scepticism is perhaps sensible when considering any claims made about self-sustaining feral colonies.

That church tower in which ‘there have always been bees’ may well have had bees in it every year.

But that’s not the same as having the same bees in it.

In fact, with an ~90% attrition rate of feral colonies annually it’s very unlikely to be the same colony in successive years.


Note

In the final stages of completing this post – very, very late at night – I re-discovered an article (Moro et al., 2018) on citizen science and feral colonies that I’ll return to sometime in the future.

References

Kohl, P.L., Rutschmann, B. and Steffan-Dewenter, I. (2022) ‘Population demography of feral honeybee colonies in central European forests’, Royal Society Open Science, 9(8), p. 220565. Available at: https://doi.org/10.1098/rsos.220565.

Moro, A. et al. (2021) ‘Using Citizen Science to Scout Honey Bee Colonies That Naturally Survive Varroa destructor Infestations’, Insects, 12(6), p. 536. Available at: https://doi.org/10.3390/insects12060536.