Category Archives: Problems

Repeated oxalic acid vaporisation

Synopsis : Does repeated oxalic acid vaporisation of colonies rearing brood work sufficiently well? Is it as useful a strategy as many beekeepers claim?

Introduction

Oxalic acid is a simple chemical. A dicarboxylic acid that forms a white crystalline solid which dissolves readily in water to form a colourless solution. It was originally extracted from wood-sorrels, plants of the genus Oxalis, hence the name. In addition to the wood-sorrels it is present in a wide range of other plants including rhubarb leaves (0.5% oxalic acid 1 ), the berries and sap of Virginia creeper and some fruits, such as starfruit. Additionally, fungi excrete oxalic acid to increase the availability of soil nutrients.

Oxalic acid is inexpensive to produce by a variety of processes and was possibly the first synthesised natural product. About 120,000 tonnes are produced annually and it is mainly used for bleaching wood (and often sold as ‘wood bleach’) and cleaning products – including teeth. It chelates iron and so is used for rust removal and is used as a dye fixative (or mordant 2 ).

Spot the difference ...

Oxalic acid and API-Bioxal … the same but different

It is also, when used properly, devastatingly effective against the ectoparasitic mite Varroa destructor.

And, even more importantly, when used properly it is extremely well-tolerated by honey bees.

Great!

Not so fast …

Unfortunately for beekeepers, some of the commercially available i.e. licensed and approved, oxalic acid-containing treatments either contain unnecessary additives and/or have limitations in their approved modes of administration that reduces their efficiency and use in real world beekeeping situations.

Oxalic acid-containing miticides and their use

A quick search of the UK’s 3 Veterinary Medicines Directorate snappily titled Product Information Database for ‘target species = bees’ and ‘active ingredient = oxalic acid’ yields three products :

  • Varromed (BeeVital GmbH) which is a solution containing formic acid and oxalic acid
  • Oxybee (DANY Bienenwohl GmbH) which is an oxalic acid solution PLUS a separate powder containing essential oils and sugar. As far as I can tell, Oxybee looks to be the same product as Dany’s BienenWohl powder and solution, which – although listed and licensed – I cannot find for sale 4 in the UK
  • API-Bioxal (Chemicals Laif S.P.A) which is purchased as a powder composed of 88% oxalic acid dihydrate together with silica and glucose

I’m going to largely ignore Varromed and Oxybee for the rest of this post. I’m sure they’re perfectly good products but I’ve not used either of them so cannot comment from personal experience.

Keeping your powder dry

More relevant to this post, Oxybee and Varromed are both liquids, and this post is about vaporising (aka sublimating) oxalic acid.

And vaporisation involves using the powdered form of oxalic acid.

Which neatly brings me to the methods of application of oxalic acid-containing treatments to kill mites.

I’m sure there are some weird and wonderful ones, but I’ll be limiting any comments to just three which – from my reading of the instructions – are the only ones approved (and then not for all of the products listed above) : 5

  • Spraying a solution onto the surface of the bee-covered frames
  • Dribbling or trickling a solution onto each seam of bees between the frames
  • Vaporisation or sublimation of powdered oxalic acid by heating it in a metal pan to convert it to a gas. This permeates the hive, settling on all the surfaces – woodwork, comb, bees – and remains active against mites for a period after administration

Broodless is best

Oxalic acid, however it is administered, does not penetrate brood cappings. Therefore all of the approved products are recommended for use when the colony is broodless.

Typically – though not exclusively – this happens in the winter, but the beekeeper can engineer it at other times of the season.

If the colony is broodless you can expect any oxalic acid-containing miticide to reduce the mite population by 90% or more. There are numerous studies that support this level of efficacy and it’s what you should be aiming for to give the colony the best start to the season.

I discussed at length how to determine whether a winter colony is broodless a fortnight ago in Broodless?

This post is a more extensive response to several comments (made to that Broodless? article) that recommended repeated vaporisation of oxalic acid at, either 4, 5 or 7 day intervals.

The idea is that this kills the phoretic mites present when the colony is first treated and the mites subsequently released as brood emerges.

How many repeats?

I’ve seen anything from two to seven recommended online.

I’ll discuss this further below, but I’d note that the very fact that there’s such variation in the recommended repeat treatments – perhaps anything from two, fours days apart to seven at weekly intervals (i.e. spanning anything from 8 days to 49 days) – suggests to me that we don’t know the optimal treatment schedule.

Which is a little weird as, a) Varroa is a globally-distributed problem for beekeepers and is more or less invariant (as is the brood cycle of the host honey bee), and b) repeated treatment regimes have been used for over 20 years.

Which brings me back to a crude comparison of vaporisation vs dribbling, or …

Sublimation vs. trickling

A hive can be sublimated with oxalic acid without opening the hive. The vaporiser alone is introduced through the hive entrance or – in the case of certain models – the vapour is squirted through a hole in the floor, brood box or eke. In contrast, trickling oxalic acid requires the removal of the crownboard.

In the video above I’m using a Sublimox vaporiser. The hive entrance is sealed with foam and the open mesh floor is covered with a tightly fitting slide-in tray. As you can see, very little vapour escapes.

Although oxalic acid is well tolerated by bees, and it has no effect upon sealed brood, a solution of oxalic acid is detrimental to open brood. Therefore, trickled oxalic acid weakens the colony – because the acidity kills some or all of the open brood – and repeated trickling of oxalic acid is likely to compound this (see Al Toufailia et al., 2015). In contrast, repeated oxalic acid vaporisations appear not to be detrimental to the colony (caveat … I’m not aware of any long-term studies of this, or for the impact on the queen).

API-Bioxal approved methods of administration

The instructions for API-Bioxal clearly state that only a single treatment by vaporisation is approved per year. The exact wording is:

Maximal dose 2.3g per hive as a single administration. One treatment per year.

In contrast, when used as a solution for trickling the instructions state:

Up to two treatments per year (winter and/or spring-summer season in brood-free colonies).

This seems nonsensical to me considering what we now know about oxalic acid – remember, API-Bioxal was licensed in the same year (2015) that Al Toufailia et al., demonstrated it was detrimental to open brood, and I’m reasonably sure this had been shown previously (but can’t currently find the reference).

But, it gets worse …

API-Bioxal contains oxalic acid with powdered silica and glucose. I presume the silica is to keep it free-running. I’m not aware that powdered silica kills mites and I’m damned certain that glucose has no miticidal activity 😉 .

Neither of these two additives – which I’ve previously called cutting agents – are there to increase the activity of the oxalic acid … and the presence of the glucose is a real problem when vaporising.

Single use ...

Caramel coated Sublimox vaporiser pan

When glucose is heated to 160°-230°C it caramelises (actually, this happens at 150°C 6 ), coating the inside of the vaporising pan. This needs to be cleaned out afterwards 7. The instructions state:

Cool down and clean the vaporizer after use to remove possible residue (max 6%, around 0.140 g).

However, I don’t want to focus on what I consider to be a very effective but decidedly sub-optimal product … instead I want to discuss whether repeat treatment with oxalic acid actually works when there is brood present.

Why is repeat treatment recommended?

Remember, it’s not recommended or approved by the manufacturers of API-Bioxal or the Veterinary Medicines Directorate. I really should have titled this section ’Why is repeat treatment recommended by those who advocate it?’

But that wouldn’t fit on a single line 😉 .

When you sublimate oxalic acid, the gas cools and the oxalic acid crystals settle out on every surface within the hive – the walls, the frames, the comb, the bees etc.. For this reason, I prefer to vaporise oxalic acid when the colony is not tightly clustered. I want everything to be coated with oxalic acid, and I particularly want every bee to be coated because that’s where most of the mites are.

Unless they’re in capped cells 🙁 .

And if they’re in capped cells, the only way the Varroa (released when the brood emerges) will come into contact with oxalic acid is if it remains present and active within the hive. Unfortunately, it’s unclear to me exactly how long the oxalic acid does remain active, or what accounts for a drop in its activity.

But it does drop.

If you treat a colony with brood present and count the mites that appear on the Varroa tray every day it looks something like this:

Mite drop per day before and after treatment

’Something like’ because it depends upon the phoretic mite levels and the amount and rate of brood uncapping. For example, you often see higher mite drops from 24-48 hours than 0-24 hours after treatment.

I know not why.

The drop in the first 48 hours – presumably almost all phoretic mites – can be very much higher than the drop from day three onwards 8.

The duration of activity after vaporisation

Some studies claim oxalic acid remains active for 2-3 weeks after administration. I’m a little sceptical that it’s effective for that long and my own rather crude observations of post-treatment mite drop (of brooding colonies) suggests it returns to background levels within 5-7 days.

I could rabbit on about this for paragraphs as I’ve given it a reasonable amount of thought, but fortunately the late Pete Little did the experiment and showed that:

The recommended dose for colonies with brood is three or four doses seven days apart, however I found out that this is not effective enough, and treated 7, 6, 5 4, 3, 2 days apart to find out the most effective which is 5.

It therefore makes sense that three treatments at five day intervals should be sufficient. This period comfortably covers a complete capped brood cycle (assuming there is no drone brood in the colony) which is 12 days long.

Repeated oxalic acid vaporisation treatment regime.

If there is drone brood present you would theoretically need four treatments at 5 day intervals to be sure of covering the 15 day capped brood cycle of drones.

But it turns out there are some additional complications to consider.

Dosage

In the UK the recommended i.e. approved, maximum dose of API-Bioxal is 2.3 g by vaporisation. Remember my comments about the other rubbish stuff API-Bioxal contains, 2.3 g of API-Bioxal actually contains a fraction over 2 g of oxalic acid dihydrate.

This is the active ingredient.

When comparing different experiments where some have used ‘plain’ oxalic acid dihydrate and others have used – or will use – API-Bioxal, it’s important to consider the amount of the active ingredient only 9 .

In the US, oxalic acid was registered as an approved treatment for Varroa in 2015. By vaporisation, the approved dosage is 1 g of oxalic acid dihydrate per brood box i.e. half that approved in the UK.

Remember also that a deep Langstroth is 5% larger (by volume) than a National brood box.

And Jennifer Berry and colleagues in the University of Georgia have recently determined whether repeated administration of vaporised oxalic acid to a colony rearing brood is an effective way of controlling and reducing Varroa infestations (Berry et al., 2021).

And the answer is … decidedly underwhelming

Here are the experimental details.

The paper doesn’t state 10 when the experiment was done but they measured honey production in the treated colonies and were definitely brood rearing, so I’m assuming late summer.

Colonies were treated with 1 g / box (double Langstroth deeps) vaporised oxalic acid every five days for a total of 35 days i.e. 7 applications. Mite infestation levels (percent of workers carrying phoretic mites) were measured before and after treatment. Almost 100 colonies were used in the experiment, in three apiaries, randomly split into treated and control groups.

Let’s get the easy bit out of the way first … there was no difference in brood levels, adult bees or food stores at the end of the study. The treated hives were not disadvantaged by being treated … but they didn’t gain an advantage either 🙁 .

Mite levels after treatment normalised to pre-treatment levels (dotted line = no change)

During the experiment the percent mite infestation (PMI) levels in the untreated control colonies increased (as expected) by ~4.4. This is an average and there was quite a bit of variation, but it means that an initial mite infestation level of 4 (average) increased to 8.4 i.e. over 8 mites on every 100 adult workers in the hive.

3% is often considered the cutoff above which treatment is necessary.

Overall, the PMI of treated colonies reduced over the duration of the experiment … but only by 0.7.

From a colony health perspective this is a meaningless reduction.

Seven treatments with the recommended (in the US) dose of oxalic acid stopped the mite levels increasing, but did not reduce them.

Repeated administration of the US-approved oxalic acid dose by vaporisation does not reduce mite levels in a way that seems likely to significantly benefit the colony.

🙁

Dosage, again

I’m not sure the primary data used to justify the US approved 1 g / box dosage. Early studies by Thomas Radetzki (PDF) showed a 95% reduction in mite levels using a dose of 1.4 g. This was a large study involving ~1500 colonies and a dose of 2.8 g was not significantly more effective. I’m quoting the figures for broodless colonies 11.

The Berry results were similar to two smaller previous studies by Jamie Ellis and colleagues (Jack et al., 2020, 2021) who demonstrated that 1 g oxalic acid vaporised three times at weekly intervals was ineffective in controlling mite levels.

However Jack et al., (2021) also applied a similar treatment schedule using different doses of oxalic acid.

Data from Jack et al., 2021 using different repeat doses of oxalic acid

Ignore the intermediate values in panel A, just look at the pretreatment and ‘3 weeks’ mite infestation values.

Mite levels increased in untreated controls and decreased in all treated colonies. However, there was a clear dose response where the more oxalic acid used the greater the impact on the mite levels.

Four grams of oxalic acid reduced the mite infestation rate significantly … from ~5% to ~2% (I’ll return to this). However, the intermediate levels of oxalic acid, whilst reducing mite levels, did not do so significantly from the next closest amount of oxalic acid. For example, 1 g wasn’t significantly more effective than no treatment (as already stated), 2 g was not significantly more effective than 1 g and 4 g was not significantly more effective than 2 g.

But wait … there’s more

I’m familiar with two other studies that look at dose and/or repetition and efficacy (there are more, but this isn’t meant to be an exhaustive review, more a ”Do we know enough?” overview).

Gregoric et al., (2016) published a 12 study that appeared to use combinations of treatments in multiple apiaries. The abstract claims 97% reduction using three 1 g vaporisations, though these are spread over a 57 day period (!) stretching from mid-August to late-November. Mite drop in November following treatment was ~75% (presumably broodless) , but only 10-20% in August. Interestingly I can’t find the figure 97% anywhere in the results …

Finally, Al Toufailia et al., (2015) investigated the dose response to vaporised oxalic acid, showing an 80% reduction in infestation at 0.56 g and 93-98% who using 1.125, 2.25 and 4 g of oxalic acid. All of these studies were determined using broodless colonies.

The Al Toufailia and Jack studies – as well as the Berry study – also reported on adverse effects on the colony. With certain exceptions vaporisation was well tolerated. Some colonies went queenless. Where the queen was caged in late summer to render it broodless (Jack et al.,) some colonies subsequently failed to overwinter successfully (though, look on the bright side, mite levels were reduced 😉 ).

Don’t do that at home … I presume they impacted the production of winter bees.

confused.com

I’m not sure there’s a compelling, peer-reviewed study that definitively shows that repeat treatments of vaporised oxalic acid administered to a brood rearing colony reduces mite levels sufficiently.

Yes, the Jack et al., (2020) showed a significant reduction in the infestation rate (using 4 g three times at seven day intervals), but it was still around 2%.

In late summer, with 20-30,000 bees in the box and 6 frames of brood, that’s still ~600 mites (and potentially more in the capped brood).

In midwinter with about 10,000 workers and much smaller amounts of brood in the hive a 2% infestation rate is still 200 mites.

That’s still a lot of mites for a nearly broodless colony … I treat my colonies when broodless (and assume I’m killing ~90% of the mites present) and am disappointed if there are 45 mites on the Varroa tray. 50 mites on 10,000 workers is an infestation rate of 0.5%.

I’ve waffled on for too long.

All those advocating – or using – repeated oxalic acid vaporisation on brood rearing colonies in late autumn/winter need to think about:

  • dosage … 1 g is clearly too little (at a 5-7 day interval, but what if it was at a 4 day interval?), 2 g is better and 4 g is well-tolerated and certainly more effective
  • frequency … which I suspect is related to dosage. The goal must be to repeat sufficiently frequently that there is never a period when oxalic acid levels fall below a certain amount (and I don’t know what that amount is). 1 g on a daily basis might work well … who knows?
  • duration … you must cover a full capped brood cycle with the repeats
  • adverse effects … inevitable, but can be minimised with a rational treatment schedule

Broodless is best

It really is.

But, if your colonies are never broodless 13 then I wouldn’t be confident that repeat treatment was controlling Varroa levels sufficiently.

I have treated repeatedly with oxalic acid. In the good old days before API-Bioxal appeared. It certainly reduced Varroa levels, but not as well as my chosen Apivar does these days.

Repeated oxalic acid vaporisation is regularly proposed as the solution to Varroa but I’m certainly not confident that the data is there to support this claim.

Take care out there 😉


Notes

In a future post I’ll revisit this … I’ve got a pretty clear idea of how I’d go about demonstrating whether repeated oxalic acid treatments are effective in meaningfully reducing mite levels i.e. sufficient to protect the colony overwinter and through to the following late summer.

References

Al Toufailia, H., Scandian, L. and Ratnieks, F.L.W. (2015) ‘Towards integrated control of varroa: 2) comparing application methods and doses of oxalic acid on the mortality of phoretic Varroa destructor mites and their honey bee hosts’, Journal of Apicultural Research, 54(2), pp. 108–120. Available at: https://doi.org/10.1080/00218839.2015.1106777.
Berry, J.A. et al. (2022) ‘Assessing Repeated Oxalic Acid Vaporization in Honey Bee (Hymenoptera: Apidae) Colonies for Control of the Ectoparasitic Mite Varroa destructor’, Journal of Insect Science, 22(1), p. 15. Available at: https://doi.org/10.1093/jisesa/ieab089.
Gregorc, A. et al. (2016) ‘Integrated varroa control in honey bee (Apis mellifera carnica) colonies with or without brood’, Journal of Apicultural Research, 55(3), pp. 253–258. Available at: https://doi.org/10.1080/00218839.2016.1222700.
Jack, C.J., van Santen, E. and Ellis, J.D. (2020) ‘Evaluating the Efficacy of Oxalic Acid Vaporization and Brood Interruption in Controlling the Honey Bee Pest Varroa destructor (Acari: Varroidae)’, Journal of Economic Entomology, 113(2), pp. 582–588. Available at: https://doi.org/10.1093/jee/toz358.
Jack, C.J., van Santen, E. and Ellis, J.D. (2021) ‘Determining the dose of oxalic acid applied via vaporization needed for the control of the honey bee (Apis mellifera) pest Varroa destructor’, Journal of Apicultural Research, 60(3), pp. 414–420. Available at: https://doi.org/10.1080/00218839.2021.1877447.

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.

 

Biological control with Varroa

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

Introduction

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

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

Explosion on the steamboat Jenny Lind near San Francisco, California

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

Californian Channel Islands map

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

Field station, nature reserves, pigs and bees

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

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

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

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

But what about the bees?

Good question.

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

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

But getting rid of the bees took a bit longer …

Save the bees, or not

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

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

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

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

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

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

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

Killer bees

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

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

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

Aerial view of Santa Cruz Island

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

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

Killing bees

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

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

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

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

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

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

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

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

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

Let there be mite

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

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

The approach they chose involved the introduction of Varroa.

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

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

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

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

Biological control

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

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

On Santa Cruz Island the pest was the honey bee.

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

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

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

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

Female cane toad (not jumping)

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

Oops.

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

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

Introduction of Varroa

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

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

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

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

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

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

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

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

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

Monitoring and confirmation of eradication

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

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

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

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

Lessons for beekeepers

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

There are (at least) three.

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

Varroa resistance

There wasn’t any.

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

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

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

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

Wenner estimates that only 3 colonies survived until 2001.

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

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

Varroa spread

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

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

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

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

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

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

One mite is all it takes

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

And one mite is all it takes.

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

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

Here endeth the lesson 😉


Note

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

References

Fries, I., Imdorf, A. and Rosenkranz, P. (2006) ‘Survival of mite infested (Varroa destructor) honey bee (Apis mellifera) colonies in a Nordic climate’, Apidologie, 37(5), pp. 564–570. Available at: https://doi.org/10.1051/apido:2006031.

Kevan, P.G., Laverty, T.M. and Denmark, H.A. (1990) ‘Association of Varroa Jacobsoni with Organisms other than Honeybees and Implications for its Dispersal’, Bee World, 71(3), pp. 119–121. Available at: https://doi.org/10.1080/0005772X.1990.11099048.

Kraus, B. and Page, R.E. (1995) ‘Effect of Varroa jacobsoni (Mesostigmata: Varroidae) on feral Apis mellifera (Hymenoptera: Apidae) in California’, Environmental Entomology, 24(6), pp. 1473–1480. Available at: https://doi.org/10.1093/ee/24.6.1473.

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

Williams, J.L., Danka, R.G. and Rinderer, T.E. (1989) ‘Baiting system for selective abatement of undesirable honey bees’, Apidologie, 20(2), pp. 175–179. Available at: https://doi.org/10.1051/apido:19890208.