Tag Archives: Api-Bioxal

Repeated oxalic acid vaporisation

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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.

If Carlsberg did vaporisers …

… they’d make the Sublimox sublimator.

I wonder how many beekeepers have one of these on their wish list for Santa?

Sublimox vaporiser

Sublimox vaporiser …

From being a bit of an imported oddity (I had to import mine from Icko Apiculture in France three years ago) they’re now becoming mainstream … over the last year or so they’ve been sold by an independent importer, then Abelo and – just this month – the ‘Big Daddy’ of UK beekeeping suppliers, E. M. Thorne (Rebecca at Thorne’s kindly asked if they could include links in their December newsletter to previous posts here on the Sublimox and vaporising Api-Bioxal).

The Sublimox is also now being discussed more widely on the online forums, with much of this discussion emphasising the price (they are expensive). Aside from this disadvantage, I think there are a number of significant advantages of this design of sublimator (vaporiser) which are worth emphasising.

The general principle of active and passive sublimators

Sublimators (vaporisers) are designed to heat oxalic acid (OA) crystals sufficiently that they sublimate i.e. go through a phase transition from a solid into a gas without an intervening liquid phase. As readers of this site should know, oxalic acid is highly effective against the Varroa mite – both by sublimation and trickling in solution, which has recently been covered in excruciating detail so won’t be elaborated on here.

Varrox

Varrox

Many sublimators (vaporisers) are passive. You add the OA to a pan, slide it into the hive entrance, apply a current to heat the pan, allow the OA to sublimate, withdraw the pan and start again. You usually need to cool the pan in water before addition of the next dose. This heat, rinse and repeat cycle takes time, but is very effective and the pan-type vaporisers are relatively inexpensive (£35-£100).

The Varrox vaporiser is one of the original and best known models, though there are any number of much less expensive copies sold by beekeeping suppliers and on eBay. Most require a 12V supply of some kind.

In contrast to these, the Sublimox is ‘active’, as is the US equivalent machine, the ProVap 110. In these the pan is pre-heated, the OA is ‘dropped’ into the pan in such a way that the vapour is generated in an enclosed space which it then escapes under pressure through a nozzle.

Probably the best sublimator in the world …

In terms of speed, convenience and ease of use I’d argue that the Sublimox is hard to beat.

Quick

Delivery of a single dose takes no more than 45 seconds from inserting the Sublimox nozzle into the hive. It’s appreciably faster than the pan-type passive sublimators. There’s a preliminary warming up period before use, as the machine reaches operating temperature. After that it’s simply treat, refill, treat, refill ad infinitum. The rate-limiting step is sealing the hive and refilling the small plastic ‘cartridge’ with OA.

For one person, a hive-a-minute is just about possible – if you have 10-20 closely spaced hives, sufficient entrance blocks and buy additional white plastic cartridges. But, to achieve this you’ll be rushing about like a mad thing and it’s not realistically achievable1.

But speed isn’t the major benefit.

Convenient

Sublimox vaporiser

Sublimox vaporiser

It’s convenient because you can ‘squirt’ the gas through a small hole in hive. You don’t need to open the hive and so it works with any hive type or entrance. My favoured kewl floors cannot be used with a sublimator that needs to be pushed through the entrance. Instead, all my hives have a neat 6mm hole drilled through the sidewall of the floor which is usually plugged with a small foam bung or twist of grass2. Vapour quickly permeates throughout the hive, ensuring all surfaces are reached.

The only exception are the all-poly hives, such as the garishly coloured Abelo’s I’ve been using for work this season. The nozzle of the Sublimox gets hot and melts polystyrene (been there, done that 🙁 ). With poly hives I usually use a simple shallow eke with a Sublimox nozzle-sized hole in one side. In this case you do need to remove the roof and crownboard, add the eke, replace the roof (upside down), treat and then close up the hive.

Foam bung ...

Foam bung …

The alternative is to simply squirt the gas through the open entrance. This isn’t ideal as some of the gas will escape, potentially exposing the beekeeper and definitely not exposing the bees/mites. One way to avoid this loss of gas is to use a wooden block with a small hole through the middle held over the poly hive entrance.

It’s important to have a clear space into which the gas is ‘squirted’. If you don’t the OA tends to not permeate through the hive properly.

The Abelo hives have a hole in one face of the brood box. This is usually plugged. I think it’s intended as an upper entrance. I’ve yet to try poking the Sublimox nozzle through this hole to deliver the OA … this might not work as it may be too close to the frame, so impeding the spread of gas through the box. Time will tell†.

Safety considerations

There’s an additional benefit of the way the Sublimox is used. Operator exposure to OA should be minimised. The gas isn’t generated until the machine is inserted and inverted and takes no more than ~45s to deliver. If the hive is sealed properly there’s very little exposure to the gas.

… but all this comes at a cost

The Sublimox is three-times the price of a Varrox vaporiser. Is it worth it? That’s up to the purchaser to decide, based on the number of hives to be treated, the depth of their pockets, the perceived benefits of the speed, ease of use, convenience and safety etc.

Many will baulk at the cost. Some build their own. Others don’t bother vaporising, but solely dribble OA solution in midwinter. This is the cheapest and fastest way to treat colonies. I estimate it costs about 0.5p/hive to trickle treat if you buy OA in 25kg bags. Dribbling is probably even faster than using the Sublimox. However, dribbling is really only practical for broodless colonies – swarms and for midwinter use – and should only be done once per season. In contrast, repeated sublimation is tolerated well by colonies.

Would I buy another one?

Almost certainly.

Before you splash the cash

Generator

Generator …

Be aware … the Sublimox requires a 240V supply. One of my apiaries has mains power which is the best solution, but unlikely in the corner of a farmers field. You can use an inverter from a car battery which is fine if you can get your car close enough to the hives. Alternatively – and this is what I do – you can use a portable generator. I’ve got an 700W Impax one from Screwfix which works perfectly.

But that’s another £190  🙁

You’ll also need to periodically buy replacement sealing rings for the OA “cassette”. These wear out or perish rather fast. Icko sell them for a daft price, but they’re now available in the UK.

Single use ...

Single use …

You should also be aware that Api-Bioxal, the VMD-approved oxalic acid-containing miticide, has glucose in it which caramelises in the pan of the Sublimox (and other vaporisers) and is a bit of a nightmare to clean out properly. I’d go further and suggest that Api-Bioxal is unsuitable to use with a Sublimox. If the nozzle is blocked the gas has to escape and there could be inadvertent exposure of the operator.

Safety first

I’ve touched on safety above. However, just because OA vapour is generated for a very short and well defined time doesn’t make it safe. There are still exposure risks which must be taken seriously.

You’ll need PPE – personal protection equipment – to prevent exposure to the OA vapour when treating colonies. This includes eye protection and a suitable vapour mask. Don’t skimp on this and assume you’ll just stand downwind. If the hive is poorly sealed, swirling gusts of wind will expose you to vapour and it’s – at best – very, very unpleasant.

Entrance block

Entrance block …

Unlike passive vaporisers, the Sublimox generates a very large volume of gas immediately the OA is added. There’s no opportunity to ‘stand well back’ as the pan warms up as you can with the passive machines. You have to be holding the Sublimox to invert it and drop the OA into the pan. You’ll be bent over the hive and unable to avoid the swirling fog of acidic vapour if it escapes. To help minimise this seal the hive thoroughly. I use a full-width entrance block and tightly fitting Varroa tray. Even then, particularly on ageing cedar hives, there are all sorts of little gaps from which the OA vapour can be seen escaping.

Entrance block in use

Entrance block in use …

Finally, as if gassing yourself with an evil smelling acidic cloud of OA wasn’t enough, remember you’re using a 240V supply outside, probably on damp grass … or possibly even in the rain. Don’t get electrocuted. Make sure you use an RCD (residual current device) that’s been tested and you know works.

Take care.

Share the costs

At the time of writing the Sublimox costs over £300. Perhaps competition will force the price down a bit? When you consider that these machines are used relatively infrequently during the season it makes sense to consider purchasing them as an Association item (or with a group of friends), in the same way that some associations have extraction equipment available.

When compared to a reasonable extractor the Sublimox doesn’t seem so outrageously priced.

Of course, like an extractor, everyone needs it at about the same time of the season (at least in midwinter). However, unlike an extractor, it’s generally needed for a relatively short period, is easy to transport and easy clean after use. There should be no reason it couldn’t be shared by association members.

I appreciate that many associations don’t have shared equipment, or many beekeepers don’t belong to their local association. All I’m doing is suggesting a way in which a good quality and highly effective piece of equipment could be purchased so that many can benefit.

Whether you’ve got one of these on your list for Santa or not …

… Happy Christmas!

1 The rate-limiting step is probably having enough entrance blocks. If removed too soon you’ll lose lots of vaporised OA goodness. Leave it a good 5 minutes if at all possible, which is easily enough time to treat another 5 hives. See what I mean?

2 When treating a hive for the first time I’ve even drilled this hole through an occupied hive.

† Actually, this hole isn’t suitable. It opens onto the face (rather than end) of a frame, and the vapour is therefore restricting from spreading. Don’t bother.

‡ I’ve regularly treated colonies in the dark. Sometimes the only time I can get to the apiary is after work. The bees are all ‘at home’ but you can easily seal the hive up and treat them. Use a headtorch with a red bulb. Since bees can’t see red, any that escape won’t directly target your head and you can probably work safely without a veil.

Colophon

If Carlsberg did … is one of the most recognisable advertising campaigns of all time. Originally created in 1973 it has achieved near-universal recognition and remains in daily use, though predominantly these days as internet memes. The opening three words of the adverts were as recognisable as the closing seven … probably the best lager in the world.

There are any number of comedic If Carlsberg did ‘advertising’ campaigns, including some from Carlsberg itself … Probably the best poster in the world, featured the distinctive swirly underscore, colour scheme and font, together with a real tap dispensing lager installed in Londons Brick Lane.

If Carlsberg did ...

If Carlsberg did …

Carlsberg revamped the advertising campaign in 2015 (the poster above was part of this), over 40 years after it was first used. You can view these adverts on the Carlsberg website.

I’ve used a variant of the If Carlsberg did … phrase previously, when describing the large Dadant smoker. It’s still a great smoker.

Trick(le) and treat

Tools of the trade

Tools of the trade

This is the third and final post on why, with what, when and how to minimise mite levels in colonies in midwinter.

In the first post I explained why midwinter mite treatment makes sense. In the second I described how oxalic acid-containing solutions should be prepared and stored.

“Oxalic acid-containing” solutions includes both Api-Bioxal, the VMD approved treatment, and the unadulterated chemical. All three posts focus on trickling or dribbling – I’ve covered sublimation previously and both are essentially equally effective. Sublimation or vaporisation is currently very fashionable … but trickling is simplicity-itself and requires almost no special equipment.

In this post I’ll discuss how to administer the oxalic acid-containing solution.

For readability I’ll use the term OA solution to mean any oxalic acid-containing solution. About 50% of the readers of this site are from outside the UK; local rules may determine what you are or are not allowed to administer to your bees.

Trickling or dribbling

You’ll hear both terms used interchangeably1. The general principle is that you directly administer 5ml of a 3.2% w/v solution of oxalic acid in thin (1:1) syrup per seam of bees in the colony.

‘Directly‘ because you administer the OA solution to the seam of bees. You don’t count the seams and then simply pour it into the hive. You don’t spread it across the top bars. The idea is that the bees at the top of the seam get coated in the solution and that it dribbles down through the colony, being passed from bee to bee as they feed and groom and move about.

Two seams of bees

Two seams of bees …

During this process any phoretic mites will also get exposed to the oxalic acid. Since mites are readily damaged by the OA solution they fall off and gradually drop out of the bottom of the cluster. Gradually, as it takes a few days for gravity to deliver all the corpses.

You can therefore determine whether mites were present and killed by placing a Varroa tray underneath the open mesh floor of the hive. Note that this doesn’t tell you how effective the treatment has been … for that you’d need to know the mite infestation level before treatment as well.

When to treat

In many ways this is the critical decision. As described previously, maximal benefit occurs when the colony is broodless. Ideally you want an extended cold period late in the calendar year. The colony will cluster tightly and brood rearing will slow down or stop completely.

If the cold period has lasted 2-3 weeks, even better. This will mean that some or all of the brood present will have emerged. The more sealed brood present, the less effective trickling OA solution is as a means of controlling mites.

Choose a calm, cool or cold day. I usually wait for a day with temperatures between 0 and 5°C. Much warmer than that and the cluster starts to break up or the bees are more likely to fly about as the crownboard is lifted. Windy or wet days disturb the bees (at least when you prise the crownboard off), so it’s best to avoid those.

I prefer to treat before the year end, rather than after, if I can. From a few irregular midwinter peeks into the cluster I think queens start laying earlier than most beekeepers think.

It pays to be prepared …

Trickle 2 - £1

Trickle 2 – £1

… Aesop (~620-560BC) was right, though he wasn’t talking about beekeeping. Before treating your colonies there is some preparation needed. Do this properly and it’s a doddle.

Purchase a Trickle 2 container from Thorne’s. These are a measly quid each. You’ll only need one.

Practice with the Trickle 2 container (see below).

Gently warm your pre-prepared OA solution to about 25°C. If you made it up in advance and stored it at 4°C in the fridge this will take an hour or two. The easiest way is to stand the container (preferably thin-walled … I use a well-rinsed milk carton) in a basin of warm water.

Pour the pre-warmed OA solution into a well-labelled vacuum flask. You can buy these from Tesco for £2.50 with a capacity of 1 litre. The aim here is to take everything you need ready-prepared to the apiary so the treatments take the minimum time possible.

Remember that OA is toxic. Label everything carefully, make sure children can’t get near it and don’t use it again for food/drink purposes.

That’s it … you’re ready. You’ll need a hive tool, a bee suit, thin gloves (to protect you from the OA, not the bees), your vacuum flask of OA solution and the Trickle 2 bottle. By all means take your smoker, but you shouldn’t need it.

I’ve got a 5 ml (or 25 ml) syringe … won’t that do?

Yes … but no.

A Trickle 2 bottle holds 100ml of prepared OA solution. It takes two hands to fill the bottle, but only one hand to use it. That 100ml is sufficient for 20 seams of bees i.e. two completely full colonies (assuming an 11 frame National box). In midwinter the colony is unlikely to occupy 10 seams. A Trickle 2 bottle is also pretty accurate, reproducibly dispensing about 4.6-4.8ml of liquid. That’s close enough to 5ml.

In contrast, a syringe also takes two hands to fill (and refill). However, unless it’s a 5ml syringe, it’s difficult to accurately and reproducibly dispense liquid without using two hands. A 5ml syringe gives you the necessary accuracy, but needs refilling for every seam of bees. This takes time … during which the crownboard is off and the colony is getting chilled.

I’ve done both and can assure you that the Trickle 2 bottle is much better. Just buy one. It’s only £1 and it’ll last ages if one of your association members doesn’t borrow it … or doesn’t return it.

How to use a Trickle 2 bottle

  • Remove the cap and fill to the top of the lower chamber with liquid (practice with water).
  • Replace the cap.
  • Hold the bottle with your thumb and fingers on opposite sides of the lower chamber, with the external ‘pipe’ to the upper chamber next to your palm.
  • Undo the spout about a turn.
  • Gently squeeze the lower chamber. Liquid is forced up the pipe into the upper chamber. Hold it against the light to observe this.
  • Once the upper chamber is full, stop squeezing. Excess liquid drains back into the lower chamber.
  • If you are right handed turn the Trickle 2 bottle anti-clockwise2 using your wrist and gently squeeze the bottle to dispense the liquid in the upper chamber from the spout. If you’re left handed you need to turn the bottle clockwise.

And in practice

The single-handed operation for the Trickle 2 container really pays dividends when treating a colony. You can gently prize up one side of the crownboard, hold it in one hand, administer the OA solution to each seam with the other hand and gently lower the crownboard back down … all in less time than it took me to write that.

Like this:

This is a reasonably sized colony being treated in the second week of January 3 years ago. The video is 1’45” long, but the crownboard is only open for about 50 seconds. And I was chatting with Mick Smith off camera, so could have perhaps gone a bit faster if I’d concentrated … 😉

Here’s a more detailed view of treating a small colony:

33 seconds of warmed, acidic goodness to slaughter the mites and give the colony the best possible start to the upcoming season.

Cautions and considerations

Discard any OA solution that’s not been used. Warming it will have raised the HMF levels and this may be toxic for your bees. However, read footnote 3 for another way to avoid HMF buildup3.

Wash everything carefully – the Trickle 2 bottle, the vacuum flask, gloves etc. Since the OA solution was in syrup everything gets sticky and gummed up. Clean stuff up, make sure it’s labelled and not going to be used in the kitchen and put it away until next year.

Oxalic acid kills mites, but it’s also toxic for unsealed brood. This is perhaps unsurprising considering it has a pH of 1 (i.e. very acidic) and ‘naked’ larvae aren’t protected by the tough exoskeleton that adult bees have. This is another reason to treat during a broodless period in midwinter.

In summer, swarms can also be treated with trickled oxalic acid-containing solutions before they have sealed brood. If a swarm arrives in bait hive, let it settle and start drawing comb on the foundationless frames. A day or so later treat it with oxalic acid by trickling. When I’ve done this I usually wait until late afternoon or early evening, so most of the bees are in the box. The colony obviously won’t be clustered, but the principle is the same – 5ml of syrup down each seam. Easy peasy. Effective.

Swarms have a significant mite load, so it’s well worth treating them before they rear brood and give the phoretic mites somewhere to breed.

Finally, it’s often recommended that a colony is only treated once per year with oxalic acid by trickling or dribbling. I’m not sure where this advice originates, but it’s probably wise.

‘Vaping’ vs. trickling

The discussion forums are awash with recommendations to ‘vape’ the colony, rather than trickle. Vaporisation, or more correctly sublimation, is a widely used method and has been in use for two decades. It’s currently very fashionable. I’ll write a more substantial comparison sometime in the future, but the following brief notes might be of interest.

Sublimation can be done repeatedly with brood present (though there’s no peer-reviewed evidence of efficacy) and is both well-tolerated by the colony and is not toxic to unsealed brood. It requires specialised and potentially expensive equipment, both for delivery and personal protection. You can build your own vaporiser, but shouldn’t skimp on protection for the operator. With a well designed vaporiser and hive there’s no need to open the colony to administer treatment.

In contrast, trickling requires only the Trickle 2 bottle and vacuum flask described here. Personal protection is a pair of latex gloves. It should only be conducted when the colony is broodless, should probably only be conducted once and does require the hive to be opened (albeit briefly).

You’ll be told that vaporisation is faster. It isn’t. Watch the videos above. Even my Sublimox – probably the fastest ‘active’ vaporiser on the market – takes well over a minute per colony if you take into account sealing the box, moving the generator about, unsealing the hive etc.

There are reports that sublimation is more effective, but the difference is marginal, and possibly not statistically significant. There is also a report that colonies are stronger in the Spring after sublimation, though this may be due to toxicity to open brood by trickled OA solution. If the colony is broodless this shouldn’t be an issue.

I’ve used both many, many times without a problem. Across the UK I suspect more beekeepers trickle OA, rather than ‘vape’ (a word I dislike), though the vocal ones on the discussion forums currently favour vaporisation.

What’s more important than how you deliver the oxalic acid, is that you do treat. Trickling OA solution is so easy and inexpensive that there’s no reason not to … and your colonies will be much healthier for it.

Get dribbling 😉

1 If the beekeeper is of a certain age you’ll hear these terms used in a different context. We’re restricting discussions here to delivering OA 😉

2 If you are left handed you need to turn the Trickle 2 bottle clockwise. Actually, to be pedantic, if you are left handed and holding the bottle in your left hand, turn it clockwise. It’ll make sense once you try.

3 In the previous article on preparing oxalic acid solutions Calum posted a comment on preparing the OA in water and only adding and dissolving the required amount of sugar just before use. This has the advantage that there will be no HMF buildup. OA solution in water should be perfectly stable. I’ve not done it this way, but it makes sense and might be worth trying.

Colophon

The title of this article is a twist on the term Trick or treat. This is not entirely inappropriate as Trick or treating is a Halloween (31st October … just a few days away) custom dating back – in various forms – centuries.

The modern usage, essentially North American, dates back to the 1920’s and refers to children in costumes going house to house threatening to play a trick unless the homeowner provides a treat, usually sweets or toys. In Britain these traditions date back to the 16th Century, both of children going house-to-house asking for food and of dressing up in costumes at Halloween.

Closer to home, ‘guising‘ – children in Scotland going from door to door in disguise asking for food, coins or chocolate  – dates back at least a century.

The term Trick or treat only entered common usage in the UK in the 1980’s.

 

Oxalic acid preparation

This is the second of three articles on midwinter treatment of colonies with oxalic acid to minimise Varroa levels. In a recent post I explained why a midwinter treatment was necessary, even if you’d treated three months earlier. Essentially this is because:

  • there will still be some residual Varroa, particularly if you treated in late summer rather than early autumn (and this post explains why early treatment is preferable)
  • midwinter is the time when brood levels are at a minimum, so most mites will be phoretic and readily accessible to the miticide treatment

Midwinter is the time to use oxalic acid-containing treatments. It can be delivered in a variety of ways; by sublimation (vaporisation), spraying or trickling (dribbling).

Trickling or dribbling

This post is about the preparation and storage of oxalic acid-containing solutions for trickling. Sublimation is covered elsewhere and spraying is not approved or widely used in the UK.

The process for trickling is very straightforward. You simply trickle a specific strength oxalic acid solution in thin syrup over the bees in the hive. The oxalic acid kills the mites. How isn’t entirely clear – it’s thought to corrode the mouthparts and soft tissue. It’s more than 90% effective in killing phoretic mites when used like this.

Beekeepers have used oxalic acid for years as a ‘hive cleaner’, as recommended by the BBKA and a range of other official and semi-official organisations. All that changed when Api-Bioxal was licensed for use by the Veterinary Medicines Directorate (VMD).

Oxalic acid and Api-Bioxal, the same but different

Spot the difference ...

Spot the difference …

Api-Bioxal is the VMD-approved oxalic acid-containing miticide. It is widely available, relatively inexpensive (when compared to other VMD-approved miticides) and very easy to use.

It’s very expensive when compared to oxalic acid purchased in bulk.

Both work equally well as both contain exactly the same active ingredient. Oxalic acid.

Api-Bioxal has other stuff in it (meaning the oxalic acid content is a fraction below 90% by weight) which actually makes it much less suitable for sublimation.

How much and how strong?

To trickle or dribble oxalic acid-containing solutions you’ll need to prepare it at home, store it appropriately and administer it correctly.

I’ll deal with how it is administered next time. This is all about preparation.

The how much is easy. You’ll need 5ml of oxalic acid-containing solution per seam of bees. In midwinter the colony will be reasonably well clustered and its likely there will be a maximum of only 8 or 9 seams of bees, even in a very strong colony.

Hold on … what’s a seam of bees?

Two seams of bees

Two seams of bees …

Looking down on the colony from above, a seam of bees is the row visible between the top bars of the frames.

Remember to prepare ~10% more than you think you need. You’ll inevitably spill some when using the Trickle 2 bottle to administer it to the colony. It’s not that expensive, so don’t risk running out.

And the how strong? The recommended concentration to use oxalic acid at in the UK has – for many years – been 3.2% w/v (weight per volume) in 1:1 syrup. This is less concentrated than is recommended in continental Europe (see comments below on Api-Bioxal).

My advice – as it’s the only concentration I’ve used – is to stick to 3.2%.

Listen very carefully, I shall say zis only once†

A bit of basic chemistry coming up. Skip to the warning in red below and then the recipes if you want, but this explains some important things about working out how much to use.

The molecular formula of oxalic acid is C2H2O4. The molecular weight of oxalic acid is 90.03 g/mol. However, the oxalic acid you purchase – including Api-Bioxal – is the dihydrated form of oxalic acid.

Di as in two, hydrated as in water.

The molecular formula of oxalic acid dihydrate is C2H2O4.2H2O and oxalic acid dihydrate has a molecular weight of 126.07 g/mol.

Therefore the weight of oxalic acid in 1 g of oxalic acid dihydrate is 90.03/126.07 = 0.714 g.

Caution

Oxalic acid is toxic

  • The lethal dose for humans is reported to be between 15 and 30 g. It causes kidney failure due to precipitation of solid calcium oxalate.
  • Clean up spills of powder or solution immediately.
  • Take care not to inhale the powder.
  • Store in a clearly labelled container out of reach of children.
  • Wear gloves.
  • Do not use containers or utensils you use for food preparation. A carefully rinsed plastic milk bottle, very clearly labelled, is a good way to store the solution prior to use.

Recipes : oxalic acid

The standard recipe is 100 g water plus 100 g white granulated sugar. Mix well and then add 7.5 g of oxalic acid. The final volume will be 167ml i.e. sufficient to treat over 30 seams of bees, or between 3 and 4 strong colonies (including the 10% ‘just in case’).

This final concentration is 3.2% w/v oxalic acid … (7.5 * 0.714)/167 * 100 = 3.2. Check my maths.

0.01 g to 500 g

0.01 g to 500 g

If you have more colonies to treat, or have trouble weighing 7.5g, scale everything up ten-fold. Or buy a small, accurate set of digital scales – like these for £9 which work very well. 1 kg of sugar plus 1 kg (1 litre) of water requires 75 g of oxalic acid and makes 1.67 litres … enough to treat all the colonies in the association apiary.

Which is not such a bad idea. Make it up carefully once and share it with your fellow beekeepers. Storage details are provided below.

Recipes : Api-Bioxal

Warning – the recipe on the side of a packet of Api-Bioxal makes up a much stronger solution (4.4% w/v) of oxalic acid than has historically been used in the UK. Stronger isn’t necessarily better. The recipe provided is 35 g Api-Bioxal to 500 ml of 1:1 syrup. By my calculations this recipe makes sufficient solution at a concentration of 4.4% w/v to treat 11 hives. 

To make a 3.2% Api-Bioxal-based oxalic acid-containing solution using the 35 g pack of Api-Bioxal you need to mix the entire contents of the pack with 691 ml of 1:1 syrup.

Here’s the maths:

  • 35 g of Api-Bioxal contains only 22.14 g of oxalic acid. 88.6% of the 35 g is oxalic acid dihydrate (the remainder is cutting agents like glucose and powdered silica) and so the oxalic acid content is ((35 * 0.886) * 0.714) = 22.14 g.
  • To calculate the volume of syrup you need to divide it by the final percentage required i.e. (22.14 / (3.2/100)) = 691 ml. I don’t know the exact amount of sugar and water needed to make this amount … it’ll be about 430 g of each (I think).

A 35 g packet of Api-Bioxal is therefore sufficient to treat about 15 colonies (assuming 5 ml per seam, 8 seams per hive and 10% ‘just in case’) at the recommended concentration of 3.2% w/v.

Api-Bioxal is sold in three pack sizes (35 g, 175 g and 350 g). If you are wealthy enough to be able to purchase the larger pack sizes you’ve probably got your own beekeeper (or mathematician). Relax on your yacht while they do the calculations‡ for you 😉

On the other hand … if you have a smaller number of colonies either make a full 35 g packet up and share it, or use accurate scales and the following table:

Api-Bioxal recipes for 3.2% OA trickling

Api-Bioxal recipes for 3.2% OA trickling

Storage

Storage of oxalic acid syrup at ambient temperatures rapidly results in the acid-mediated breakdown of sugars (particularly fructose) to generate hydroxymethylfurfural (HMF). As this happens the colour of the oxalic acid-containing solution darkens significantly.

This breakdown happens whether you use oxalic acid or Api-Bioxal.

Stored OA solution and colour change

Stored OA solution and colour change …

HMF is toxic to honey bees at high concentrations. Studies from ~40 years ago showed that HMF concentrations below 30 mg/l were safe, but above 150 mg/l were toxic1. HMF buildup is one way overheated honey is detected.

At 15°C HMF levels in OA solution can reach 150 mg/l in a little over a week. At room temperature this happens much faster, with HMF levels exceeding 150 mg/l in only 2-3 days. In the dark HMF levels build up slightly less quickly … but only slightly 2,3.

Only make up OA solutions when you need them.

If you must store your oxalic acid-containing syrup for any length of time it should be in the fridge (4°C). Under these conditions HMF levels remain well below toxic levels for at least one year. However, don’t store it for this long … use it and discard the excess. Don’t use discoloured oxalic acid solutions as they’ve been stored incorrectly and may well harm your bees.

Please re-read the comments above about the toxicity of oxalic acid. If you are going to store it in the fridge it must be very clearly labelled and there must be no chance that children can reach or open the container.

Conclusions

Api-Bioxal is the least expensive VMD-approved miticide and powdered oxalic acid is much, much cheaper. Both contain the same active ingredient, oxalic acid, which is highly effective against phoretic mites.

In midwinter, with very low levels (or no) of brood, a single oxalic acid-containing treatment minimises mite levels for the coming season.

Oxalic acid-containing solutions are easy to prepare. I recommend you make sufficient for your own colonies and those of your beekeeping friends and association members. My previous BKA used to distribute litres of the stuff for use in midwinter. Use this solution in midwinter and then discard any that is unused.

Oxalic acid-containing solutions are inexpensive and easy to administer by trickling. As I shall demonstrate next time.

Please re-read the safety instructions highlighted in red above.

Michelle Dubois

Michelle Dubois

† Listen very carefully, I shall say zis only once was a catchphrase used by “Michelle of the Resistance” in the 1980’s comedy ‘Allo ‘Allo! Michelle (Dubois) was rarely seen without a trench coat and beret, had a corny French accent and was played by Kirsten Cooke.

‘Allo ‘Allo! ran for 85 episodes in the decade from 1982 on BBC one. It was about a café in Nazi-occupied France and the French Resistance, just about. It mixed bawdy humour with gross stereotypes (posh British twits, sex-obsessed French) and was a parody of ITV’s series Secret Army (’77-’79).

Early episodes had obvious and rather dull titles. In the later series the individual episodes had some quite good puns like Awful Wedded Wife.

Michelle – Listen very carefully, I shall say zis only once

René – Well, in that case, could you please speak slowly?

You had to be there … 😉

‡ Oh alright then, since you insist. The 175 g pack of Api-Bioxal (~£39) needs to be made up in 3.459 litres of 1:1 syrup and the 350 g pack (~£65) 6.919 litres of 1:1 syrup. Determining how much water and sugar to mix to make these amount is, as they say, an exercise for the reader. Assuming a 3.2% solution and 8 seams of bees per colony Api-Bioxal costs between 63p and 41p per hive (see note below), depending upon the pack size you purchase. I know that beekeepers moan on and on about the outrageous cost of Api-Bioxal (as do I), but is 63p per colony really an unreasonable amount to spend on VMD-approved medicines to keep your colony as clear of Varroa as possible? I don’t think so.

Note – the costs in the paragraph were calculated using the lowest prices I could currently find for Api-Bioxal. C Wynne Jones has the 35g packets for £9.50 and Maisemores have the 350g packets for £64.79. Prices correct on 9/10/17.

1 Jachimowich T., El Sherbiny G., Zur Problematik der verwendung von Invertzucker für die Bienenfüttering, Apidologie 6 (1975) 121-143.

2 Bogdanov S., Kilchenman V., Chamere J.D.. Imdorf A. (2001) available online.

3 Prandin, L., Dainese, N. , Girardi, B., Damolin, O., Piro, R., Mutinelli, F. A scientific note on long- term stability of a home-made oxalic acid water sugar solution for controlling varroosis Apidologie, 32:) 451-452

 

Kick ’em when they’re down

Out, damn'd mite ...

Out, damn’d mite …

Why bother treating colonies in midwinter to reduce Varroa infestation? After all, you probably treated them with Apiguard or Apivar (or possibly even Apistan) in late summer or early autumn.

Is there any need to treat again in midwinter?

Yes. To cut a long story short, there are basically two reasons why a midwinter mite treatment almost always makes sense:

  1. Mites will be present. In addition, they’ll be present at a level higher than the minimum level achievable, particularly if you last treated your colonies in late summer, rather than early autumn.
  2. The majority of mites will be phoretic, rather than hiding away in sealed brood. They’re therefore easy to target.

I’ll deal with these in reverse order …

Know your enemy

DWV symptoms

DWV symptoms

The ectoparasite Varroa feeds on honey bee pupae and, while doing so, transmits viruses (in particular DWV) that can completely mess up the development of the adult bee. Varroa cannot replicate anywhere other than on developing pupae. It’s replication cycle, and the resulting mite levels in the colony, are therefore tightly linked to the numbers and availability of hosts … honey bee pupae.

If developing brood is available the mite can replicate. Under these conditions, newly emerged adult, mated, female Varroa spend a few days as phoretic mites, riding around the colony on young bees. They then select a cell with a late-stage larvae in, enter the cell and wait until pupation occurs. If developing worker brood is available each infested cell produces 1 – 2 new mites (drone cells produce 3+) and mite numbers increase very rapidly in the colony.

In contrast, if there’s no developing brood available, the mites have to hang around waiting for brood to become available. They do this as phoretic mites and can remain like this for weeks or months if necessary.

Therefore, when brood is in abundance and the queen in laying freely mites can replicate to very high levels. In contrast, when brood is limiting and the queen has reduced her egg laying to a   v  e  r  y     s  l  o  w     r  a  t  e     the mite cannot replicate and must be predominantly phoretic.

When does this happen?

Lay Lady Lay … or don’t

Ambient temperature, day length and the availability of nectar and pollen likely influence whether the queen lays eggs. When it’s cold, dark and there’s little or no pollen or nectar coming into the hive the queen slows down, or even stops, laying eggs.

About 8 days after she stops laying there will be no more unsealed brood in the colony. About 13 days after that all the sealed brood will have emerged (along with any Varroa). Therefore, after an extended cold period in midwinter, the colony will have the lowest level of sealed brood … and the highest proportion of the mite population will be phoretic.

Under normal (midsummer) circumstances about 10% of the mite population is phoretic. It’s probably unnecessary to state that, if there’s no brood available, 100% of the mites must be phoretic.

All licensed miticides work extremely well against phoretic mites†.

Caveats, guesstimates, global warming and the Gulf Stream

Global warming

Global warming …

Whatever the cause, the globe is warming (irrespective of what Donald Trump tweets). Long, hard winters are getting less common (or perhaps even rarer, as they were never particularly common in the UK). In Central, Southern or Eastern Britain it’s possible that the colony will have some brood present all year. In parts of the West, warmed by the Gulf Stream, I’d be surprised if a colony was ever broodless. Only in the North is it likely that there will be a brood break in midwinter.

Most of the paragraph above is semi-informed guesswork. I don’t think anyone has systematically analysed colonies in the winter for the presence of sealed brood. Sure, many (including me) have opened colonies for a quick peek. Others will have peered intently at the Varroa board to search for shredded wax cappings that indicate emerging brood. The presence of brood will vary according to environmental conditions and the genetics of the bees, so it’s not possible to be dogmatic about these things.

However, it’s safe to say that in midwinter, sealed brood – within which the mites can escape decimation by miticides – is at a minimal level.

Reducing mite levels and minimal mite levels

Within reason, the earlier you apply late summer miticides, the better you protect the all-important overwintering bees from the ravages of viruses, particularly deformed wing virus. This is explained in excruciating detail in a previous post, so I won’t repeat the text here.

However, I will re-present the graph that illustrates the modelled (using BEEHAVE) mite levels‡.

Time of treatment and mite numbers

Time of treatment and mite numbers

The gold arrow (days 240-300 i.e. September and October) indicates when the winter bees are being reared. These are the bees that need to be protected from mites (and their viruses). Mite numbers (starting with just 20 in the hive on day zero) are indicated by the solid coloured lines. The blue, black, red, cyan and green lines indicate modelled mite numbers when the colony is treated with a miticide (95% effective) in mid-July, August, September, October or November respectively.

The earlier you treat, the lower the mite levels are when the winter bees are being reared. Study the blue and black lines.

This is a good thing.

In contrast, by treating very late (the cyan and green lines) the highest mite numbers of the season occur at the same time as the winter bees are being reared. A bad thing.

But … look also at mite numbers after treatment

Look carefully at the mite numbers predicted to remain at the end of the year. Early treatment leaves higher mite levels at the start of the following year.

This is simply because mites escaping the treatment at the end of summer have had an opportunity to reproduce during the late autumn.

This is why the additional midwinter treatment is beneficial … it kills residual mites and gives the colony the best start to the new calendar year§.

Kick ’em when they’re down

Early treatment protects winter bees but risks exposing bees the following season to unnecessarily high mite numbers. However, in midwinter, these residual mites are much more likely to be phoretic due to a lack of brood in the colony. As I stated earlier, phoretic mites are relatively easy to target with miticides.

So, give the mites a hammering in late summer with an appropriate and effective miticide and then give those that remain another dose of the medicine in midwinter.

But not another dose of the same medicine

Since the majority of mites in a colony with little or no brood will be phoretic, you can easily reduce their numbers using a single treatment containing oxalic acid. This can be administered by sublimation (vaporisation) or by trickling (dribbling).

There’s no need to use any treatment that needs to applied for a month. Indeed, many (Apiguard etc.) are not recommended for use in winter because they work far less well on a largely inactive colony.

Trickle 2 - £1

Trickle 2 – £1

I’ve discussed sublimation previously. However, since this requires relatively expensive (£30 – £300) specialised delivery and personal protection equipment it may be inappropriate for the two hive owner. In contrast, trickling requires almost no expensive or special equipment and – reassuringly – has been successfully practised by UK beekeepers for many years. I did it for years before I bought my Sublimox vaporiser.

Therefore, in two further articles this autumn (well before you’ll need to treat your own colonies) I’ll describe the preparation and storage of oxalic acid solutions and its use.

Be prepared

If you want to be prepared you’ll need to beg, borrow or steal the following – sufficient oxalic acid (or Api-Bioxal), a Trickle 2 bottle sold by Thorne’s, a cheap vacuum flask (Tesco £2.50), granulated sugar and a pair of thin disposable gloves.

Do this soon. Don’t leave it until midwinter. You need to be ready to treat as soon as there’s a protracted cold spell (when brood will be at a minimum). Over the last few years my records show that this has been anywhere between the third week in November and the third week in January.

More soon …

† Only MAQS is effective against mites sealed in cells. This is why most miticides are used for extended periods in the late summer or early autumn … the miticide must be present as Varroa emerge from sealed cells.

‡ I’ll repeat the caveat that this is an in silico simulation of what happens in a beehive. Undoubtedly it’s not perfect, but it serves to illustrate the point well. It’s freely available, runs on PC and Mac computers, and is reasonably well-documented. In the simulations shown here the virtual colony was ‘primed’ with 20 mites at the beginning of the year. BEEHAVE was run using all the default settings – climate, forage etc. – with the additional application of a miticide (95% effective) in the middle of the months indicated. Full details of the modelling have already been posted.

§ The National Bee Unit recommend Varroa levels are maintained below 1000 throughout the season. Without treatment, 20 mites at the start of the season can easily replicate to ~750 in the autumn. If you start the season with 200 mites then levels are predicted to reach ~5000 in the following summer. The colony will almost certainly die that season or the next. There’s a more detailed account of the consequence of winter brood rearing and the level of mite infestation written by Eric McArthur and reproduced on the Montgomeryshire BKA website that’s worth reading.

¶ The cumulative (year upon year) effect of late summer treatment with no midwinter treatment has been discussed previously. I’ll simply re-post the relevant figure here – 5 years of bee (in blue, left axis) and mite (in red, right axis) numbers with only one treatment per season applied in late September. Within two years the higher mite numbers that are present at the start of the year reproduce to dangerously high levels.

Mid September

Mid September