Category Archives: Diseases

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.

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.

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.

 

Mellow fruitfulness

Synopsis : Final colony inspections and some thoughts on Apivar-contaminated supers, clearing dried supers, feeding fondant and John Keats’ beekeeping.

Introduction

The title of today’s post comes from the first line of the poem ’To Autumn’ by John Keats:

Season of mists and mellow fruitfulness

The poem was written just over 200 years ago and was the last major work by Keats (1795-1821) before he died of tuberculosis. Although it wasn’t received enthusiastically at the time, To Autumn is now one of the most highly regarded English poems.

The poem praises autumn, using the typically sensuous imagery of the Romantic poets, and describes the abundance of the season and the harvest as it transitions to winter.

That’s as maybe … the last few lines of the first verse raises some doubts about Keats’ beekeeping skills:

And still more, later flowers for the bees,
Until they think warm days will never cease,
 For summer has o’er-brimm’d their clammy cells.

It’s certainly true that there are late summer flowers that the bees can forage on 1. However, he’s probably mistaken in suggesting that the bees think in any sense that involves an appreciation of the future.

And what’s all this about clammy cells?

If there’s damp in the hive in late summer then it certainly doesn’t bode well for the winter ahead.

Clammy is now used mean damp; like vapour, perspiration or mist. The word was first used in this context in the mid-17th Century.

‘Clammy’ honey

But Keats is using an earlier meaning of ’clammy’ … in this case ’soft, moist and sticky; viscous, tenacious, adhesive’, which dates back to the late 14th-Century.

And anyone who has recently completed the honey harvest will be well aware of how apt that definition is 😉 … so maybe Keats was a beekeeper (with a broad vocabulary).

And gathering swallows twitter in the skies

That’s the last line of ’To Autumn’ (don’t worry … you’ve not inadvertently accessed the Poetry Please website). The swallows are gathering and, like most summer migrants, already moving south. Skeins of pink-footed geese have started arriving from Iceland and Greenland.

Skein of geese over Fife

My beekeeping over the last fortnight has been accompanied by the incessant, plaintive mewing of buzzards. These nest near my apiaries and the calling birds are almost certainly the young from this season.

A few nights ago, while hosing the extractor out in the bee-free-but-midge-filled late evening, I was serenaded by tawny owls as the adults evicted their young from the breeding territory in preparation for next season.

These are all signs, together with the early morning mists, that summer is slipping away and the autumn is gently arriving.

Morning mist clearing over the loch

The beekeeping season is effectively over and all that remains is preparing the colonies for winter.

Supers

All the supers were off by the 22nd of August. There was still a little bit of nectar being taken in but the majority was ripe and ready. As it turns out there was fresh nectar in all the colonies when I checked on the 10th of September, but in such small amounts – no more than half a frame – that it wouldn’t have been worth waiting for.

At some point you have to say … enough!

Or, this year, more than enough 🙂 .

Most of the honey was extracted by the end of August. It was a bonanza season with a very good spring, and an outstanding summer, crop. By some distance the best year I’ve had since returning to Scotland in 2015.

Of course, that also meant that there were more supers to extract and return and store for the winter ahead.

Lots of lifting, lots of extracting and lots of buckets … and in due course, lots of jarring.

Storing supers wet or dry?

In response to some recent questions on storing supers wet or dry I tested ‘drying’ some.

I’ve stored supers wet for several seasons. I think the bees ‘like’ the heady smell of honey when they are added back to the hives for the spring nectar flow. The supers store well and I’ve not had any problems with wax moth.

However, this year I have over two full carloads of supers, so – not having a trailer or a Toyota Hilux 2 – I have to make multiple trips back to put them in storage 3. These trips were a few days apart.

I added a stack of wet supers to a few hives on the 1st of September and cleared them on the 9th. All these supers were added over an empty super (being used as an eke to accommodate a half block of fondant – see below) topped with a crownboard with a small hole in it (no more than 2.5 cm in diameter, usually less).

Converting wet supers to dry supers – note the crownboard with a small central hole

When I removed the supers on the 10th they had been pretty well cleaned out by the bees. In one case the bottom super had a very small amount of fresh nectar in it.

So, 7-8 days should be sufficient for a strong colony to clean out 3-4 supers and it appears as though you can do it at the same time as feeding fondant … result 🙂 .

Feeding fondant

I only feed my colonies Baker’s fondant. I add this on the same day I remove the honey-laden supers. I’ve discussed fondant extensively here before and don’t intend to rehash the case for its use again.

Oh well, if you insist 😉 .

I can feed a colony in less than two minutes; unpacking the block, slicing it in half and placing it face down over a queen excluder (with an empty super as an eke) takes almost as much time to write as it does to do.

Take care with sharp knives … much easier with a slightly warm block of fondant

But speed isn’t the only advantage; I don’t need to purchase or store any special feeders (an Ashforth feeder costs £66 and will sit unused for 49 weeks of the year). I’ve also not risked slopping syrup about and so have avoided encouraging robbing bees or wasps.

I buy the fondant through my association. We paid £13 a block this year (up from about £11 last year). That’s more expensive than making or buying syrup (though not by much) and I don’t need to have buckets or whatever people use to store, transport and distribute syrup. Fondant has a long shelf life so I buy a quarter of a ton at a time and store what I don’t use.

All gone! 12.5 kg of fondant added on 22/8/22 and photographed on 9/9/22

And, contrary to what the naysayers claim, the bees take it down and store it very well.

What’s the biggest problem I’ve had using fondant?

The grief I get when I forget to return the breadknife I stole from the kitchen … 😉 .

Apivar-contaminated honey and supers

Last season I had to treat a colony with Apivar before the supers came off. This was one of our research colonies and we had to minimise mite levels before harvesting brood.

I’ve had a couple of questions recently on what to do with supers exposed to Apivar … this is what I’ve done/will do.

Apivar

The Apivar instructions state something like ’do not use when supers are present’ … I don’t have a set of instructions to check the precise wording (and can’t be bothered to search the labyrinthine VMD database).

Of course, you’re free to use Apivar whenever you want.

What those instructions mean is that honey collected if Apivar is in the hive will be ’tainted’ and must not be used for human consumption.

But, it’s OK for the bees 🙂 .

So, I didn’t extract my Apivar-exposed supers but instead I stored them – clearly labelled – protected from wasps, bees and mice.

This August, after removing the honey supers I added fondant to the colonies. In addition, I added an Apivar-exposed super underneath the very strongest colonies – between the floor and the lower brood box.

I’ll leave this super throughout the winter. The bees will either use the honey in situ or will move it up adjacent to the cluster.

In spring – if I get there early enough – the super will be empty.

If I’m late they may already be rearing brood in it 🙁 … not in itself a problem, other than it means I’m flirting with a ridiculous ’double brood and a half’.

Which, of course, is why I added it to the strongest double brood colonies. It’s very unlikely the queen will have laid up two complete boxes (above the nadired super) before I conduct the first inspection.

But what to do with the now-empty-but-Apivar-exposed supers?

It’s not clear from my interpretation of the Apivar instructions (that I currently can’t find) whether empty supers previously exposed to Apivar can be reused.

WARNING … my reading might be wrong. It states Apivar isn’t to be used when honey supers are on but, by inference, you can use and reuse brood frames that have been exposed to Apivar.

Could you extract honey from brood frames that have previously (i.e. distant, not immediate, past) been Apivar-exposed?

Some beekeepers might do this 4.

It’s at this point that some common sense it needed.

Just because re-using the miticide-exposed supers is not specifically outlawed 5 is it a good idea?

I don’t think it is.

Once the bees have emptied those supers I’ll melt the wax out and add fresh foundation before reusing them.

My justification goes something like this:

  • Although amitraz 6 isn’t wax-soluble a formamidine breakdown product of the miticide is. I have assumed that this contaminates the wax in the super.
  • I want to produce the highest quality honey. Of course this means great tasting. It also means things like wings, legs, dog hairs and miticides are excluded. I filter the honey to remove the bee bits, I don’t allow the puppies in the extracting room and I do not reuse supers exposed to miticides.
  • During a strong nectar flow bees draw fresh comb ‘for fun’. They’re desperate to have somewhere to store the stuff, so they’ll draw out comb in a new super very quickly. Yes, drawn comb is precious, but it’s also easy to replace.

Final inspections

I conducted final inspections of all my colonies in Fife last weekend 7.

For many of these colonies this was the first time they’d been opened since late July. By then most had had swarm control, many had been requeened and all were busy piling in the summer nectar.

Why disturb them?

The queen had space to lay, they weren’t likely to think about swarming again 8 and they were strong and healthy.

Midsummer inspections are hard work … lots of supers to lift.

If there’s no need then why do it?

Of course, some colonies were still busy requeening, or were being united or had some other reason that did necessitate a proper inspection … I don’t just abandon them 😉 .

I don’t just abandon them … introducing a queen to a nucleus colony

But now the supers were off it was important to check that the colonies were in a suitable state to go into the winter.

I take a lot of care over these final inspections as I want to be sure that the colony has the very best chance of surviving the winter. 

I check for overt disease, the amount of brood in all stages (BIAS; so determining if they are queenright) and the level of stores.

And, while I’m at it, I also try and avoid crushing the queen 🙁 .

Queenright?

I don’t have to see the queen. In fact, in most hives it’s almost impossible to see the queen because the box is packed with bees. If there are eggs present then the queen is present 9.

But, there might not be a whole lot of eggs to find.

Firstly, the queen is rapidly slowing down her egg laying rate. She’s not producing anything like 1500-2000 eggs per day by early autumn.

A National brood frame has ~3000 cells per side. If you find eggs equivalent in area to one side of a brood frame she’s laying at ~1000/day. By now it’s likely to be much less. At 500 eggs/day you can expect to find no more than half a frame of eggs in the hive.

Remember the steady-state 3:5:13 (or easier 1:2:4) ratio of eggs to larvae to pupae? 10

Several of my colonies had about half a frame of eggs but significantly more than four times that amount of sealed brood … clear evidence that the laying rate is slowing dramatically.

The shrinking brood nest – note the capped stores and a little space to lay in the centre of the frame

Secondly, the colony is rapidly filling the box with stores, so reducing the space she has to lay. They’re busy backfilling brood cells with nectar.

Look and ye shall find …

So I focus carefully on finding eggs. I gently blow onto the centre of the frames to move the bees aside and search for eggs.

In a couple of hives I was so focused on finding eggs that – as I prepared to return the frame to the colony – I only then saw the queen ambling around on the frame. D’oh!

Some colonies had only 3-4 frames of BIAS, others had lots more though guesstimating the precise area of brood is tricky because of the amount of backfilling taking place.

I still need to check my notes to determine whether it’s the younger queens that are still laying most eggs … I’d not be surprised.

Stores

Boxes are now heavy but not full. All received (at least) half a block of fondant in late August and more last weekend. There’s also a bit of late nectar. The initial half block was almost finished in a week.

Once the bag is empty I simply peel it away from the queen excluder. If you’re doing this, leave the surrounding super in place. It acts as a ‘funnel’ to keep the thousands of displaced bees in the hive rather than down your boots and all over the floor.

Although the bees were flying well, the bees in and around the super were pretty lethargic. I’ve seen this before and am not concerned. I don’t know whether these are bees gorged with stores, having a kip or perhaps young bees that don’t know their way about yet. However, it does mean that any bees dropped while removing the bag tend to wander aimlessly around on the ground.

I’d prefer they were in the hive, out of the way of my size 10’s.

If you look at many of the frames in the hive they will be partially or completely filled with stores. The outer frames are likely to be capped already. 

An outer frame of capped stores

These frames of stores are heavy. There’s no need to look through the entire box. I simply judge the weight of each frame and inspect any that are lighter than a full frame of stores.

Closer to the brood nest you’ll probably find a frame or two stuffed, wall-to-wall, with pollen. Again, a good sign of a healthy hive with the provisions it needs to rear the winter bees and make it to spring.

Disease

The only sign of disease I saw was a small amount of chalkbrood in one or two colonies. This is a perennial situation (it’s not really a problem) with some of my bees. Quite a few of my stocks have some (or a lot of) native Apis mellifera mellifera genes and these often have a bit of chalkbrood.

I also look for signs of overt deformed wing virus (DWV) damage to recently emerged workers. This is the most likely time of the year to see it as mite levels have been building all season and brood levels are decreasing fast. Therefore, developing brood is more likely to become infested and consequently develop symptoms.

Fortunately I didn’t see any signs of DWV damage and the initial impression following the first week or so of miticide treatment is that mite levels are very low this season. I’ll return to this topic once I’ve had a chance to do some proper counts after treating for at least 8-10 weeks (I use Apivar and, since my colonies all have medium to good levels of brood, the strips need to be present for more than the minimum recommended 6 weeks).

Closing up

Although these were the last hive inspections, they weren’t the last time I’ll be rummaging about in the brood box.

At some point during the period of miticide treatment I’ll reposition the strips (adjacent to the ever-shrinking brood nest) having scraped them to maximise their effectiveness.

Apivar scratch and sniff repositioning studies

However, all that will happen in a month or so when I can be reasonably sure the weather will be a lot less benign. Far better to get the inspections out of the way now, just in case.

So, having added the additional fondant (typically half a block) I closed the hives, strapped them up securely and let them get on with making their preparations for the coming winter.

Goodbye and thanks for the memories

There’s a poignancy about the last hive inspections of the season.

The weather was lovely, the colonies were strong and flying well, and the bees were wonderfully placid. It’s been a great season for honey, disease levels are low to negligible and queen rearing has gone well 11.

But it’s all over so soon 🙁 .

Hive #5 (pictured somewhere above … with the empty bag of fondant) was from a swarm control nuc made up on the last day of May (i.e. a 2021 queen). It was promoted to a full hive in mid-June. At the same time, while the hive they came from (#28) was requeening I’d taken more than 20 kg of spring honey from it. The requeening of #28 took longer than expected as the first was almost immediately superseded. Nevertheless, the two hives also produced almost 4 full supers (conservatively at least 40 kg) of summer honey.

Good times 🙂 .

My notes – for once – are comprehensive. Over the long, dark months ahead I’ll be able to sift through them to try and understand better 12 what went wrong.

That’s because – despite what I said in the opening paragraph of this section – there were inevitably any number of minor calamities and a couple of major snafu’s.

Or ’learning opportunities’ as I prefer to call them.

Last light over Rum and Eigg … not a bad view when visiting an out apiary

But that’s all for the future.

For the moment I have a sore back and aching fingers from extracting for days and the memory of a near-perfect final day of proper beekeeping.

It’s probably time I started building some frames 🙁


 

Shook swarms and miticides

Synopsis : Combining a shook swarm with miticide treatment removes most mites in the colony and dramatically reduces DWV levels. The application of this strategy for practical beekeeping is discussed.

Introduction

Why does Varroa have such a devastating impact on colony health?

Feeding on haemolymph – or the abdominal fat body – by Varroa is probably detrimental. Furthermore, during feeding the mite induces immunosuppressive responses which make the bee both more susceptible to bacterial infections and compromises its nutritional status (Aronstein et al., 2012 1 ).

But if that wasn’t enough, the real damage is caused by transmission of viruses – in particular deformed wing virus (DWV) – from the mite to the developing pupa (and adult worker, as mites probably also feed on newly eclosed workers during the misnamed phoretic stage of the life cycle).

In the absence of Varroa, DWV is seemingly inconsequential for honey bees. Varroa-free colonies – including mine on the remote west coast of Scotland – carry DWV, but virus levels are very low and there is never any overt disease.

But Varroa infested colonies, particularly at this time of the season, often have very high levels of DWV.

Individual pupae parasitised by Varroa can develop stratospherically high DWV levels – reaching over a million times higher levels than seen in unparasitised bees (which can be similar to those recorded in Varroa-free bees). In the mite-exposed pupae the virus levels can kill the developing bees, or result in the characteristic symptoms (primarily deformed wings but also stunted abdomens and discolouration) that give the virus its name.

Worker bee with DWV symptoms

Worker bee with DWV symptoms

But bees not directly exposed to Varroa also have higher DWV levels in mite-infested colonies, particularly as the season progresses. Presumably this is due to horizontal transmission of the virus during larval feeding or trophallaxis.

What happens to these elevated virus levels after the removal of Varroa using a miticide such as Apivar?

Who cares? … I mean, Why could that matter?

The clue is in the section above.

Here it is again:

But bees not directly exposed to Varroa also have higher DWV levels [ … snip … ] presumably this is due to horizontal transmission of the virus during larval feeding or trophallaxis.

If you remove mites the virus levels in the treated adult bees are often surprisingly high 2. That makes sense because the miticide is only removing the vector for the virus … the bees with high levels of virus infection are unaffected.

If, during larval feeding or trophallaxis, these elevated levels of DWV result in yet more bees acquiring high DWV levels then the health of the colony will remain compromised.

The real reason that DWV is a problem for honey bees is that high levels of the virus result in the reduced longevity of bees. This isn’t an issue for the short-lived summer foragers 3. However, reducing the longevity of the winter bees – the so-called diutinus bees – can be fatal for the colony. These are the bees that support the queen in winter, thermoregulating the hive and that rear the first brood of the following season.

Their importance to successful overwintering cannot be overemphasised.

So, the question remains. What happens to the virus levels in the hive after the removal of Varroa?

Of course, the reason I’m posing this question is that we now know … 😉 .

Two easy-to-understand potential outcomes

It seemed to us that there were at least two likely outcomes.

  1. The virus levels in the hive drop very quickly after mite removal (red dashed line, below) and return to some sort of basal level. How quickly and to what basal level? We didn’t know.
  2. Virus levels remain elevated for a long period after Varroa is removed (red solid line, below). How long and to what elevated level? Yes – you guessed it – we didn’t know 😉 .

Of course, biology isn’t binary. There are any number of alternative outcomes … it’s just that those two seemed the most likely.

Two possible outcomes for virus levels after mite removal (black vertical dashed line)

What’s more, they’re the easiest to understand … and to explain.

Why might virus levels remain high if Varroa are removed?

Surely the short lifespan of adult bees means these would soon be lost from the colony … particularly if they have reduced longevity?

Yes, but …

We published a paper a couple of years ago that clearly demonstrated that honey bee larvae fed high levels of DWV became infected with the fed virus. The latter, which we could distinguish from any DWV already present in the larvae, replicated to similar high levels seen in a mite-infested hive (Gusachenko et al., 2020).

This observation perhaps suggested that the second scenario outlined above could occur. All the mites are slaughtered, but the remaining bees with high levels of DWV feed developing brood which consequently also go on to develop high levels of DWV.

Although it’s always good to remove mites this would not be the best outcome for the colony.

Virus quantification

Before I explain how we tested which, if any, of these two possibilities is correct I need to say a few things about virus ‘levels’.

For a variety of reasons I don’t have time, space or energy to explain, we don’t actually count viruses, instead we count copies of the virus’s genetic material (the genome).

All the magic happens in one of these machines – a Bio-Rad CFX96 Touch Real Time PCR system.

The virus genome is made of ribonucleic acid (RNA) and we can therefore use fantastically expensive sensitive and accurate diagnostic methods to measure how many copies are present in a particular sample – for example, in a worker bee, or a developing pupa.

Still with me?

Good.

To complicate things a little, we can’t meaningfully express the number of virus genomes present as an absolute number (like one million, or 2,478) because bees are different sizes; larvae are tiny, pupae are bigger, drones are larger still.

In addition, different workers are different sizes, larvae grow etc.

Therefore we express it as genomes per unit of total RNA extracted from the sample. That’s a bit of a mouthful, so we abbreviate it to GE / μg 4.

Phew!

And finally, to put some numbers on the low and high levels of DWV I discussed earlier, a bee from a Varroa-free colony contains ~1,000 – 10,000 GE / μg (103 – 104) of DWV whereas a pupa parasitised by Varroa regularly has 10,000,000,000 to 1,000,000,000,000 GE / μg (1010 – 1012).

That’s a lot of virus 🙁 .

The experiments

Experiments plural because we did these studies in both 2018 and 2019. ‘We’ are Luke (a then PhD student and now post-doctoral fellow in my laboratory, and the first author on the paper) together with our friends and collaborators, Craig, Ewan and Alan (in Aberdeen) and Giles (in Newcastle). The work was published a few days ago in the journal Viruses and is ‘open access’ (Woodford et al., 2022). This means that anyone feeling particularly masochistic or suffering from sleep deprivation can read all the gruesome details at their leisure.

Not ‘breaking rocks in the hot sun’ … but it sometimes feels like that

The paper covers more than just the one experiment I’m going to discuss here. We also looked at how the virus population changes when mite-free bees become infested with Varroa.

I’ll save that for another post 5  … it’s a good story in its own right.

Most mites are in capped cells

It’s been known for at least three decades that the majority of the Varroa population in a brood rearing colony are within capped cells, feasting on developing pupae.

Nom, nom, nom!

Precisely what percentage of the population is the majority varies a bit 6, but a figure of 90% is often quoted as typical for midseason.

% of mites in capped cells

The percentage of mites in capped cells (this is predicted, not actual data)

We reasoned that the best way to quickly remove all 7 the Varroa in a colony was to combine treatment of the phoretic mites with removal of all the brood … where the majority of the mites are lurking.

And to remove the brood (and associated mites) we conducted a shook swarm.

The shook swarm

Many beekeepers will be familiar with the technique called a shook swarm.

Shook swarm setup. Note Apivar strips in the open hive. Returning foragers already clustering at the entrance

This involves shaking all the adult bees into a new hive with frames containing fresh foundation. All the old frames and brood from the original hive are discarded.

We modified this by including Apivar strips in the hive into which we shook the adult bees.

Shook swarmed colony strapped up for transport … we wait for all the bees to enter the hive before moving it

The ‘shook swarm and miticide’ experiment – which we conducted in May – therefore involved the following steps (we used three strong double brood hives per season, each containing similar amounts of bees and brood):

  1. We quantified DWV in emerging brood in hives in which no Varroa management was conducted.
  2. The queen was removed, caged and kept safe for a few hours.
  3. All adult bees were shaken into a new brood box containing 11 frames of fresh foundation and two strips of Apivar 8.
  4. The shook swarms were relocated to a quarantine apiary.
  5. The queen was returned to the shook swarmed colonies and they were fed ad libitum with syrup to encourage them to draw fresh comb.
  6. Mite drop was recorded at 5 day intervals, increasing to longer intervals, until October when brood rearing ceased.
  7. DWV levels were quantified on a monthly basis from June to October.

As you can see, a very simple experiment.

The results

The mite levels in the ‘donor’ hives were much higher in 2019 than 2018. It’s not unusual to see this type of year to year variation in mite levels. In this instance the mean temperature in February and March 2018 had been several degrees colder than 2019 (remember the Beast from the East?).

The Beast from the East ...

The Beast from the East …

This almost certainly reduced early season brood rearing and so delayed mite replication. Brood rearing was strong by late Spring, but the mite levels in 2018 had yet to catch up.

The results of the experiment in both years were essentially the same. However, for clarity I’ll just present the 2019 data as the mite infestation numbers were so dramatic.

Mite drop after conducting the shook swarm

The cumulative mite drop from Apivar-treated shook swarms ranged from ~500 to ~3000 in the first 5 days. After that the daily mite drop remained at extremely low levels until recording stopped in October.

Mite drop following shook swarm and Apivar treatment

If you assume that only 10% of mites were phoretic at the time we conducted the shook swarm, this means that the total number of mites in some of these colonies was about 30,000. Even the colony with the lowest mite drop may have been hiding an additional 4,500 mites in capped cells.

Remember … the National Bee Unit guidance states that if mite levels exceed 1,000 then treatment is strongly recommended ’to avoid Varroa causing significant adverse effects to the colony’.

I think this part of the study shows just how effective Apivar is. After the first 5 days of treatment the cumulative drop – the Apivar strips still were left in place for 8 weeks – was extremely low for each fortnightly sampling period.

Of course – other than the very high numbers – none of this was particularly surprising. We know Apivar kills Varroa.

Perhaps you’re thinking ”My hives drop more Varroa during the autumn treatment, and for longer.”

When you treat a colony with brood present the mite drop is high in the first few days, but then often remains significant over the next 2-3 weeks while the mite-infested brood emerges. 

In our case, all the mites were on adult bees. By killing these mites in the first few days before there was new sealed brood in the colony we ensured the majority of the new brood did not become infested.

Virus levels before and after the shook swarm

In each colony we sampled a dozen emerging workers, once before the shook swarm and then on a monthly basis until brood rearing stopped. By testing emerging brood we could be certain they had been reared in the test colony, rather than drifting in from elsewhere. 

Before the shook swarm virus levels ranged from 105 to 1010 per worker, with an average of around 5 x 107 GE / μg. For those of you unfamiliar with scientific notation that is 50 million virus genomes.

Virus quantification in individual workers from colonies before and after the shook swarm and Apivar treatment

Strikingly, from the June sample onwards, virus levels dropped to an average of about 104 GE / μg (10,000 virus genomes, a 5,000-fold reduction). This average obscured a range of individual levels, from about 102 to 106.

These reductions are statistically significant … always reassuring 😉 .

The 2018 data showed a similar marked reduction in virus levels. The pre-treatment levels were marginally lower (remember, it was a ’low Varroa’ season), but the levels dropped to an average of only 1,000 GE / μg, a slightly higher fold-reduction and again highly statistically significant.

If you remove the majority of the Varroa the virus levels drop very fast to levels seen in mite-free colonies, or colonies with very low mite counts.

Tough love?

Some beekeepers consider that a shook swarm is tough on the colony. 

I’m not sure I agree.

How and when the shook swarm is done matters a lot.

It can be tough, but it shouldn’t be.

The bees need to draw new comb. For this they need ample feeding, lots of bees and warm weather. By conducting shook swarms on strong colonies in late May and giving them a few gallons of syrup we achieved all this.

‘I know I put that caged queen down here … somewhere’

Doing a shook swarm on a weak colony, too early (or late) in the season or omitting feeding is a recipe for disaster. The colony will struggle to draw comb, its brood rearing will be limited and it will be playing ’catch up’ for the remainder of the year.

Our shook swarmed colonies were booming by late July and entered the winter very strong. All overwintered successfully.

I’d argue that a shook swarm is a lot less tough on a colony than the disease burden caused by thousands of mites … 🙁 .

Why Apivar?

It’s worth emphasising that this was a scientific experiment to investigate the consequences for the virus population of removing almost all of the Varroa.

It was not designed as an example of how a beekeeper would necessarily choose to manage a honey production colony.

Our choice of Apivar was considered and deliberate. Application is straightforward, toxicity – at the levels we used – is undetectable and, critically for these studies, it remains active for weeks.

Apivar strip on wire hangar

Of course, Apivar cannot be used when there are honey supers on the hive 9. Any supers added for the summer nectar flow were not extracted.

Additionally, feeding gallons of syrup when there are honey supers present is also not recommended 😉 .

What else could we have used?

The two obvious choices were MAQS or oxalic acid. Both are effective against phoretic mites, though perhaps less so than Apivar. However, both are only active for a short period in the hive; the treatment period for MAQS is 7 days and the activity of oxalic acid – trickled or vaporised – is probably less than a week.

Neither could be relied upon to slaughter the maximum number of mites, a necessity to produce an understandable result 10. We were additionally concerned about problems with queens or absconding had we used MAQS (both of which would have invalidated the study), and we were keen to avoid the need for repeat treatments with oxalic acid (not least because this is not an approved application method).

With thousands of mites we wanted to ensure that the majority were killed quickly … and, as important, that any that survived the first few days of miticide treatment were also more than likely to be killed later 11.

Application to practical beekeeping

The main aim of this experiment was to investigate the levels of DWV in the colony after the majority of Varroa are removed. However, we were also mindful that the method may be useful for a beekeeper who discovers his/her colony has damagingly high mite levels mid-season, or for someone who inherits abandoned hives with high mite loads.

In these scenarios, assuming there are sufficient bees, some nice warm weather and lashings of syrup available, the combination of a shook swarm and simultaneous miticide application is probably the fastest way to restore colony health.

I am not suggesting that beekeepers routinely conduct a shook swarm and miticide application mid-season. It might not be tough on the colony, but that doesn’t mean it’s not very disruptive. If it’s not needed (because mite levels are well controlled, for example) then it’s a waste of brood … and syrup.

However, there are times when I could imagine it might be useful.

If your primary crop is heather honey you’ll know that the hives sometimes don’t come back from the hills until late-September. That’s late to be applying miticides to protect the winter bees. In an area with an extended June gap (which often starts in May) it might be possible to effectively rid the hives of Varroa in June and have a strong colony to take to the moors in early August.

This is probably a better approach than using a half dose of Apivar in June (as some do) which probably doesn’t kill all the mites anyway, risks contributing to amitraz resistance in the mite population and may result in Apivar strips being left in the hive during the heather flow 12.

Conclusions

Miticides kill mites … big deal.

However, it’s the viruses – in particular deformed wing virus – that kill colonies.

We have now shown that removing the majority of the mites from a colony (including those associated with sealed brood) results in the levels of DWV in the hive dropping very quickly.

The speed with which this happens – four weeks or less – is probably accounted for by the lifespan of the adult bees in the colony following the shook swarm.

This suggests that high levels of virus are not horizontally transmitted or (and this is subtly different) that horizontal transmission, through feeding, of large amounts of virus does not result in elevated levels of virus replication in the recipient bee (larva or adult).

All sorts of questions remain. Would oxalic acid be a suitable replacement for Apivar? How much virus is transferred from a worker to a larva during brood rearing, or between workers during trophallaxis? Is this below a threshold for efficient infection? Do virus levels drop as dramatically when treating a broodless colony (e.g. after caging the queen for three weeks)?

In the meantime just remember that ”the only good mite is a dead mite” … and, if you kill the mites, you also quickly reduce virus levels to a level at which they do not damage the colony.

And a straightforward way to achieve that is to combine a shook swarm with an effective miticide.

Result!


References

Aronstein, Katherine A., Eduardo Saldivar, Rodrigo Vega, Stephanie Westmiller, and Angela E. Douglas. ‘How Varroa Parasitism Affects the Immunological and Nutritional Status of the Honey Bee, Apis Mellifera’. Insects 3, no. 3 (27 June 2012): 601–15. https://doi.org/10.3390/insects3030601.

Gusachenko, Olesya N., Luke Woodford, Katharin Balbirnie-Cumming, Ewan M. Campbell, Craig R. Christie, Alan S. Bowman, and David J. Evans. ‘Green Bees: Reverse Genetic Analysis of Deformed Wing Virus Transmission, Replication, and Tropism’. Viruses 12, no. 5 (May 2020): 532. https://doi.org/10.3390/v12050532.

Woodford, Luke, Craig R. Christie, Ewan M. Campbell, Giles E. Budge, Alan S. Bowman, and David J. Evans. ‘Quantitative and Qualitative Changes in the Deformed Wing Virus Population in Honey Bees Associated with the Introduction or Removal of Varroa Destructor’. Viruses 14, no. 8 (August 2022): 1597. https://doi.org/10.3390/v14081597.