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?


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.


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.


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.

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 😉


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.


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:
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:
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:
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:
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:


  1. No, I don’t think rhubarb leaves under the crownboard will do your colony much good – or harm for that matter – but if you really want to try then use parsley instead, as this contains over 1.7% oxalic acid.
  2. From the Latin mordere ‘to bite’ … reflecting the attachment of the dye to the fabric.
  3. Sorry readers, I can’t do this for other countries.
  4. After a rather cursory search admittedly.
  5. Confused, you soon will be.
  6. Just to keep the pedants happy.
  7. Or create a tightly-fitting tinfoil lining before use, which you can simply discard afterwards.
  8. In case you can’t tell, the graph above is a cartoon – not real data.
  9. As an aside, I think the API-Bioxal instructions are incorrect – I calculate that, using the maximum dose (which you should – see below) – there are 0.26 g of crap to be scrubbed out of the vaporiser.
  10. Or my eyes are too tired to determine.
  11. Where colonies were known to be rearing brood the figures were surprisingly (inexplicably and with no attempt made to justify them that I can see) high … let’s move on.
  12. To me almost incomprehensible
  13. Which is not the same as not broodless … if there is a period when they are broodless then ACT. Don’t decide to treat on a particular date if there’s a chance they will be broodless on a later or earlier date.

38 thoughts on “Repeated oxalic acid vaporisation

  1. Helen

    Hey David

    Great article…maybe one of your best.

    All I know is that the series of 9 OAS treatments I did in August (5 days apart) is unsustainable for me, the beekeeper. Beesuit, full-face respirator and in 100° moist heat is exhausting. My bees seem fine, and are healthy, but it wore me out.

    I usually do Apivar strips but decided to try summer OAS instead. I may return to using Apivar next July with my additional (and customary) OAS series at Thanksgiving. I use the InstantVap which is, so far, my most user-friendly method; it’s quick and I’m able to administer a 4 gram dose in one shot.

    Thank you, again, for your informative post. I look forward to the next one to aid in my own personal endeavor in finding a sustainable (for me) mite-managing regimen.

    1. David Post author

      Hi Helen

      As I was writing I was thinking about the interesting contrast between a 45 second ‘delivery’ using your InstantVap or the Sublimox and the preparation; PPE on, sealing the hives, unsealing the hives, PPE off etc. that is essential if it is to be done safely and effectively.

      If repeated sublimation worked really well it might be acceptable three or four times at 5 day intervals, but I’m not convinced (or perhaps ‘have yet to be convinced’ because I think a workable strategy should exist) that it’s currently the best option.


      1. Helen

        Yes, David. I concur. For me, not using Apivar this summer, was an experiment.

        My mite loads in October, using an OAS. “wash”, were extremely high, indicating that the summer treatment, with brood, was almost ineffective.

        As a mid-to late-November treatment, though, it is currently irreplaceable in its efficacy.

        1. David Post author

          Hi Helen

          You highlight another point that I didn’t have the space, time or energy to expand on … the amount of brood in the hive (and proportion of drone to worker as they vary in attractiveness to mites) may well influence the efficacy of treatment. I would expect that treatment would improve with low levels of available brood as individual mites would have less choice, but I could well be wrong. The phoretic phase of the life cycle is very important for ‘fitness’ and is often stated as an average of 6 days, but we know it can vary significantly.

          I’d absolutely agree that a mid/late November single treatment with vaporised or trickled oxalic acid on a broodless colony is ‘the business’ 🙂


          1. Helen

            Yes, David, it’s certainly impossible to cover everything, but you came darn close!

            I can’t recall the study but I’d read that the average time for a mite to feed outside of a capped cell is 4.4 days and that the healthier the nurse bee is the less time said foundress mite needs to feed on her “fat” body.

            I waited until mid July to treat with OAS when there was very little drone brood. However, the mite drop was still high when “washed” in October.

          2. David Post author

            Hi Helen

            One of the recent papers that has studied the impact of the phoretic stage on the life cycle of the mite is Piou et al., [Piou, V. et al. (2016) ‘Impact of the Phoretic Phase on Reproduction and Damage Caused by Varroa destructor (Anderson and Trueman) to Its Host, the European Honey Bee (Apis mellifera L.)’, PLOS ONE, 11(4), p. e0153482. Available at: though this was published a few years before Ramsey’s studies on the fat body. There’s another study that links mite fecundity with the duration of the phoretic stage, but I’ll need to look that up (and don’t think I’ve previously discussed it).

            If it is 4.4 days I’d expect that the repeat frequency of treating would need to be less (than 5 days) unless oxalic acid stays active for longer than I think after administration. I’m currently sitting in front of a spreadsheet trying to get some sensible numbers on this.


  2. Greg Young

    Hi David

    Great article as always. However, I think you should have prefaced the thread with safety measures for the uninitiated with Oxalic Acid; nitrile gloves must be used along with a full face fitting organic acid mask as it is such dangerous stuff to breath in. Just a thought.



    1. David Post author

      Hello Greg

      It’s not an article on how to apply oxalic acid …

      However, PPE is by definition ‘personal’. I don’t recommend the type of beesuit to use, the type of gloves you should wear or the footware that will keep you safe. I do talk about what I use, where it’s appropriate. I do use nitrile gloves but don’t use the sort of mask you describe as – in my view – it’s not necessary. But I do use eye protection and a proper mask that eliminates acid vapours.

      It’s important that users follow the current instructions provided with the product, rather than something – potentially out of date (or just plain wrong) – on the internet written by me. The API-Bioxal instructions have recommendations for PPE. Users must follow them, or know the potential consequences of not doing so.

      But I agree 100% that it’s unpleasant stuff … particularly for mites.


  3. Patrick Gibb

    Thank you for taking the time to carefully analyse experimental data. So the recommendations from some apparently trustworthy sources are inconsistent and not very robust. I know that happens in other fields too, and in the medical world similar findings led to the birth of “evidence-based medicine” some thirty years ago.

    One very picky point: If you treat every 5 days for 35 days then that would be 8 treatments, assuming you treated on both day 0 and day 35.

    1. David Post author

      Hello Patrick

      I just think, like so many things to do with beekeeping, there are a bunch of assumptions made and ‘facts’ stated that aren’t actually underpinned with evidence. Generally this doesn’t matter too much, after all “They’re only bees”, but unthinkingly following a treatment regime with the expectation that it will work might well lead to the loss of the colony. In which case “They were only bees” 🙁

      Regarding the treatments and days, the precise wording in the paper was: Colonies were randomly assigned to one of two treatment groups: (1) vaporized with 1 g/super of OA every 5 d for seven applications (=7 treatments spread over 35 d) or (2) an untreated control group. The seven application regimen on days 0, 5, 10, 15, 20, 25, & 30 was chosen in order to capture both worker (21 d) and drone (24 d) developmental times. I think their analysis/quantification was on day 35.


  4. Vincent Poulin

    David a rest day for me in the high Himalaya at 16,500′. Such a great post I wish I were at my desk. I’d love to summarize those daily drops.. Day 1/2 as you show maybe Day 3 hit them the hardest after that effectiveness is virtually so low assume nil. Repeat treatments essential sometimes for me out to 4-5 or more. Its nice when at sub-10 but that means still present. I think vaporization is significantly effected by air flow in a hive. often I get smaller drops even on Day 1-2 and they can extend over a broader period. Meaning often random results. Also amazing this year was how rapid mites returned to hives forced queen less! OA administered after release got some hold-0ut mites but within 3 weeks or so drops while lower were about half those not trapped. It just shows how saturated our environments are with this pest.

    1. David Post author

      Hi Vince

      I had to use a hand drawn graph of mite drop post treatment as it’s so variable. If you think what’s happening in the box it’s not surprising; you’ve got new infested worker cells being uncapped as variable numbers per day and you’ve got an ever-reducing amount of ‘active’ oxalic acid remaining. I’m almost tempted to get out a spreadsheet and produce a little model 😉

      What would be interesting would be to take a brood-rearing hive that has been repeatedly treated (and therefore considered ‘cleansed’) and then add Apivar … that would be long-lasting and kill all emerging mites over the next 8-10 weeks. How many mites remained after the OA treatment? I think this might produce some surprising and rather depressing results … it would certainly show how effective the repeat treatment had been. Of course, you’d need to account for mites acquired exogenously from drifting and robbing, so not entirely straightforward.

      Enjoy the Himalaya …


  5. Kate Atchley

    Hi David

    Many thanks for this. I’ve been vaporising during bloodless (or minimal brood periods) for a while and was asking myself this same Q … about repeat treatments.

    The Gas-Vap kit I use came with a recommendation from John ? for repeat treatments. He may have more data for you if you haven’t been in touch: website



    1. David Post author

      Hi Kate

      Several manufacturers promote repeat treatments. The Berry study used a Pro-Vap which is like my Sublimox. The Pro-Vap instructions include a recommendation for four treatments at 5 day intervals but – as Jennifer Berry tersely states in the introduction to her paper – “the protocol has not been shown effective”.

      I’d be interested to see the recommendation from the Gas-Vap manufacturer (there’s nothing on the website) on the dose/treatment regime … and, just as importantly, how it was determined.

      Hope your bees survived the downpour yesterday!


  6. Wendy Gregory

    Thanks David for another excellent article. But for most of us with just a few hives (I have 6) I really don’t want to pay out for all the kit to do OA vaporisation. Any advice on repeat treatments using the trickle method?

    1. David Post author

      Hello Wendy

      My advice would be don’t.

      Wait until the colony is broodless then treat once. If the colony is never brood less move to Scotland choose the time when the colony has the absolute minimum amount of brood, treat once and then monitor mite levels carefully in the following season. If necessary exploit the broodless period during swarm control – there should be one – to treat.


  7. Iain Dewar

    Hi David, another excellent post covering the single most important aspect of keeping honeybees today, beyond all the other stuff we shouldn’t forget! It’s good to keep it on the seasonal radar to remind us not to get complacent. After reading an article in the October 2016 Beefarmer, research by Francis LW Ratnieks and Hasan Al Toufailia of Sussex University on this very subject, I changed to their recommended sublimation method and have had better success ever since.
    On the same theme, some time ago I came across an old 1920’s photograph of a local beekeeper in a village where I grew up. His hives are placed against a south facing wall on the edge of a field of rhubarb. The question is, was it just coincidence or did he know that the rhubarb leaves contained high levels of Oxalic Acid and did he also know that it was having a beneficial effect on the health of his colonies by keeping pest levels down?
    Coincidence, or did he know something long forgotten?!
    Cheers, Iain

    1. David Post author

      Hello Iain

      The work by Ratnieks and Al Toufailia should be more widely read as I think it’s definitely one of the more considered studies of OA treatment that have been conducted. It’s available open access, so free to view (and one of the 3 or four papers are linked from the article above, to save me retyping it 😉 ).

      Your 1920’s beekeeper would have had to be extremely prescient as Varroa didn’t arrive in the UK for another 72 years 😉 . However, I think the clue was the ‘south facing wall’ … likely to be both warm and sheltered, so an ideal spot for some hives. I’m looking at something very similar up here for next season.


  8. Ian King

    Hi David
    Yet another great article.
    I’ve only vapped my 25/30 colonies once a year – around about xmas/new year, I haven’t touched Apivar or any other expensive treatment for 10 years plus.
    I’m not brave enough to non treat – although several BK’s here on the Isle of Wight have taken that step.
    I’m thinking after reading your last two articles to step into the “two vap team” – starting in about two/three weeks time with a Five day separation period.

    1. David Post author

      Hello Ian

      If you’ve got a system that works, why change? However, I’d be surprised if your colonies are broodless at Xmas/New year that far south, so perhaps too many are escaping treatment.

      It’s strange … Apivar only feels expensive when I walk out of Thorne’s clutching 3 packets and being £90 worse off. When I compare it to the value of the bees and honey it is protecting it’s a very small investment indeed, perhaps a bit less than I sell a single 227 g jar of honey for, per hive per year.

      I can live with that 😉


  9. John Eaden

    Thank you David for posting this article.
    I found the results you referenced on the effectiveness of repeated vaping treatments interesting but confusing at the same time. It’s not unusual in beekeeping to find contradictory sets of data but I am left with many more questions after reading the article!

    Thanks particularly for highlighting the key factors to consider – dosage, frequency, duration and adverse effects.

    I had assumed that the 2.3g oxalic acid dosage had some empirical basis – perhaps that’s not really the case with the USA adopting such a different figure. I will stick with 2.3g in my practice – I don’t want to muddy the water.
    I liked your hand drawn graph showing the daily mite drops following a treatment- that seems convincing enough to suggest the 5 day interval should be effective at delivering sufficient active agent to the phoretic mites ( the spellchecker wanted phonetic mites!).

    Given that I am currently trying to deal with a large mite load in two of my colonies (which occurred despite an Apivar treatment started in August) while there is currently at least 4 or 5 frames of brood – judging by the biscuit coloured callings on the inserts – I will do 3 treatments on each colony now to get the mite load down as much as possible. If the mite levels remain high I intend to repeat this after Christmas. I have not hitherto noticed any adverse effects of repeated vaping.

    One factor which seems crucial is that the largest and most productive colonies are the one with the biggest mite problem. More brood = more mites I assume.

    As I mentioned last week in the comment on Broodless this season’s Apivar treatment has been less effective than in previous years. The treatment appears to be 2 strips per colony regardless of size – could that be a limiting factor in effectiveness?

    1. David Post author

      Hi John

      Don’t take the scrawled graph as an accurate representation of real data … it was meant as a rough approximation of the sort of pattern I’ve seen. I deliberately made the units on the vertical axis rather crude.

      My recollection from the Apivar instructions is that treating a double brood box is not recommended (or words like that). I take advantage of the fact that overdosing with Apivar is difficult (it is well tolerated) and add two strips per box. I also reposition the strips to maximise their efficacy midway through the treatment period.


  10. Iain Dewar

    Hi David, wasn’t meaning varroa! 🤣 Still, the difference between treating or not treating for the wee blighters was brought home this morning when removing a 2 year old nest from a dormer window roofspace in an old house adjacent to our apiary. It was a surprisingly large nest with an equally substantial quantity of honey, small amounts of eggs and brood all stages including capped drone cells, but only a small amount of working bees. Amongst them was the still marked 4 year old queen, but also many with DWV indicating a heavy varroa load. As a lost prime swarm they clearly got off to great start, building up well, but without varroa treatment they were doomed to collapse from the get go, and sooner than later, likely succumbing over this winter despite their ample stores.
    A timely reminder!
    Cheers, Iain 😉

    1. David Post author

      Hi Iain

      A timely reminder indeed 🙁

      It would be interesting to know the infestation level of the pupae. It’s very late in the season to have drone brood; perhaps the queen is failing or it might be abandoned brood that the bees stopped ‘incubating’ (this sometimes happens at the edge of the broodnest as the bee numbers contract). One of the things that puzzles me is how colonies with only small amounts of brood don’t have it all parasitised by Varroa if the mite levels are high in the winter.

      One of the very many things I still don’t properly understand about those damned mites.


  11. Dave Kelleher

    David, thanks for the treatise on OA, I am confused as a relative newbie about whether it is ok to use plain OA crystals the active ingredient in one of the products mentioned as opposed to the product itself for treatment by sublimation. The question arises out of having read Food Safety for Bee Keepers: Andy Pedley: Northern Bee Books. Pages 12/13 under the heading of “Treating the bees” which seems to me to suggest that treatment by sublimation with the active ingredient “plain OA crystals” cannot be used as it not licensed. I would welcome you views and those of our bee keeping colleagues.

    PS. The Food Safety for Bee Keepers book says the same thing regarding treating for wax moth with Acetic Acid vapours

    On a separate but related topic; To put the OA issues to bed for once or for all, would it not be useful for the BBKA to sponsor a PhD with a research questions(s) to answer the OA effectiveness and regime question in UK conditions, or alternatively but less desirable from my perspective a Meta Analysis of all extant studies to get a collective view. That would seem like money well spent for the UK’s Beekeepers subscriptions.

    1. David Post author

      Hello Dave

      I tried to indicate what’s allowed in the text above … API-Bioxal is the only approved powdered oxalic acid-containing miticide listed in the VMD database. I don’t know the book you refer to, but the VMD should be considered the definitive guide. Of course, that’s not the same as saying that ‘”plain OA crystals” cannot be used’, but that is not me endorsing using anything other than API-Bioxal.

      API-Bioxal is – as I describe – a highly effective but decidedly sub-optimal product.

      Don’t underestimate the cost (or difficulty) of determining the best treatment regime. For starters you’d need approval to NOT use the miticide as the label states. You’d then need to work with sufficient number of hives to generate statistically meaningful data – they used ~100 in the Berry study, though fewer in the Jack et al., studies (though these are less compelling statistically). Then repeat it in another season, and ideally do repeats in areas where more or less winter brood is present and/or with higher or lower winter temperatures. It quickly gets to be a big and expensive experiment. There are few, if any, labs in the UK set up to do this type of study. I don’t think I know any that run 100 colonies in their research apiary. The logical place to conduct the study would have been the National Bee Unit, but I think their remit has changed since Mike Brown retired as Head of the unit. My lab have done geographically coordinated treatment studies with collaborating beekeepers (a simpler study altogether – we didn’t have to maintain the large research apiary) and it was a lot of work.

      I’ve read most of the papers on the topic and I suspect there are too many variables, too little overlap and some inconclusive stats to make a meta analysis useful (though I’d be delighted to be shown wrong … 🙂 ).


  12. Iain Dewar

    Hi David, yes, interesting it is.
    The eggs and brood all looked perfectly normal, maybe as you’d expect this time of the year, except for the presence of sealed live and healthy drone brood and a distinctly low number of adult bees. A healthy but old queen present and no signs of supercedure, except for the sealed drone cells.
    More investigation tomorrow and perhaps a trip to SASA.
    Cheers, Iain

    1. David Post author

      ‘perhaps a trip to SASA’ … perfect, ask Mairi or Fiona to check the mite numbers in each capped cell (worker and drone), ideally quantifying adult mites and progeny separately, together with infestation levels of workers. Oh yes, and the DWV levels … or then again, perhaps not 😉


  13. Archie McLellan

    Hi David

    I’ve heard that Pete Little’s final recommendation on repeat treatments was 3 x five days followed by a fourth treatment after four days. Sounds like the sort of thing you’d come up with after thinking about something A LOT!

    Although one of the studies you mention says that doses of OA up to 8g didn’t cause visible harm to bees, I’ve read that LASI found that the most effective dose is in fact 2.5g. It seems ridiculous that the dose limit in the US is 1g (ineffective), and interesting that they now permit OA treatments while honey supers are on hives. (Not that it’s difficult to remove the supers to treat with OA and then return them.)

    Bob Binnie and colleagues in a 3-part YouTube discussion mentioned the difficulty caused by not knowing when the mites re-enter the cells. 5-14 days is usually quoted but they asked, what if the mites re-enter after four days? That is an issue for timing of repeat OA treatments. I see you’re already discussing this in the comments above. I wonder if the long-standing recommendation for 3 (or 4) repeats at 5-day intervals comes from the idea that the mites won’t re-enter sooner than that. Maybe we should be repeating at 4-day intervals!

    (The YouTube link is

    I tried OA in August for the first time this year. It was hard work with too many mites still falling from a few colonies after 7 repeats! I resolved to return to Apivar next season. Unfortunately, I know of others who did the opposite: tried Apivar for the first time in August, were very disappointed with the result, and resolved to return to OA next year.

    After a relatively mite-free year last year (monitoring after treatments), this year is payback time. After all these OA repeats, I put Apivar in some hives that still had significant loads and stopped monitoring. I’ll treat with OA again for a winter treatment soon and then cross my fingers. This is the first year that I feel I’m not on top of the problem.

    I realise that all of this doesn’t affect you in the NE (and much less in the NW!) because your Apivar + OA regime works for you. Sorry if this is tiresome to read and thanks for getting this far.

    1. David Post author

      Hello Archie

      I’m not sure how much Pete ‘thought’ about it … as I understand it he determined the 5 days x 3 repeats empirically (a much more compelling solution than any of my arm waving speculation). Of course, the caveat would be that the conditions he found were optimal were those on Exmoor (that year). I’m not suggesting he didn’t think about it, just that he actually got on and did it 😉

      There are several studies on OA toxicity. The LASI people did high doses, Radetzki (I think in stuff cited on Randy’s site) looked at repeated treatments, week after week after week, and it’s been used for years in the UK and Europe with no hint of resistance … it really is an excellent solution to the mite problem. At least it would be IF we had a good quality commercial product AND knew the best treatment regime for ‘real world’ beekeeping (by which I mean a real warming up world, where many beekeepers never have a broodless period during the winter). We don’t have either of these.

      I spent a few hours last night struggling with a spreadsheet looking at mites emerging, phoretic mite numbers and OA residual activity. I’ve yet to build in the re-infestation rate, not least because of the issues with the duration of the phoretic phase you allude to.

      My recollection is that if this phoretic phase is reduced the fecundity or fitness of the mites reduces. They need a phoretic phase of about 6 days to mature. However, I couldn’t find the reference and my struggles with RANDBETWEEN and NORMDIST functions nearly made my head explode, so I’ve given up for a few days. I’ll have another stab at it in the ever-lengthening winter nights.

      Clearly the 1 g dosage allowed in the US is nonsensical … I’ve no idea how that happened (but look at the stupidity of our own VMD approval for the repeated usage of trickling and single use of vaporisation, where the former is more harmful to the colony … D’oh!). However, since treatment is allowed with supers present it’s perhaps good that the dose allowed is rather low … what would be the consequences of repeated vaporisation of 4 g of OA every 5 days with a stack of supers present?

      I think my current opinion is that we have to work with what we’ve got and that most beekeepers only THINK their colonies are rearing brood all the time. I kept bees in the Midlands for many years and was well aware the colonies usually had a broodless phase at some point in the winter. Recognising it takes work, but that’s a worthwhile thing to do as we know how effective OA is when the colonies are broodless. It’s lazy to just assume there’s brood there and look for a – seemingly less effective – repeat treatment regime, when there might be a better time to treat. I guess that’s all part of the “if you’re going to do something you might as well do it properly/well” principle … but the reality is it’s cold, dark and wet outside and many beekeepers want a working solution they can just apply.

      I’ll bookmark those videos to watch at some point in the future … many thanks.

      Hope tings are all good with you and your bees.


  14. John

    Hi David,

    Many thanks for the wonderful information on your site and emails.

    As very much a beginner beekeeper, I am concerned that I cannot ‘safely’ (without excessive or inappropriate disturbance of the colonies) inspect the hives to ascertain precisely when the colonies are broodless. Checking the correx sheets is a guide but for me, it’s difficult to know exactly when emergence has ended and even if this is known, if the queen has not started laying again.

    I know this isn’t officially recommended and not the topic here, but what would be the risks and maybe benefits of applying liquid Oxalic Acid treatment (dribbling warm solution down the seams) on four occasions, (day 1, day 7, day 14 and finally day 21) such that there should be no doubt that all bees are treated ?

    1. David Post author

      Hello John

      Have a look at Broodless? again (or for a first time if you’ve not read it before). You should be able to tell when the colony stops having capped brood (because it’s all uncapped), but you can’t tell whether the Q has started laying again.

      But you don’t need to know the latter. You only need to know if there is capped brood present. Then treat. Once.

      The problem with repeated OA dribbling is:

      • assuming the Q is laying you will kill or damage multiple cohorts of open brood – this will weaken the colony
      • there’s at least a theoretical chance of damaging the Q – I don’t think there are any long-term studies done on the fate of the Q in response to repeated OA solutions, but I would certainly try and avoid this if I could

      When I routinely used to use OA trickle treating in the winter – for many years when I lived in the Midlands – I never applied multiple treatments, and never thought I needed to if the colony was either broodless, or as close to broodless as could be achieved.


  15. andrew brough

    Hi David
    I treat all swarms in summer within 6 days with sublimated oxalic acid (commercial) my crucible is built into a board with a steel center section and the hive open mesh floor put over it. The hive entrance and inspection board slot are blocked with foam. It works with nucs as well.
    Interested in the chemical reaction products of oxalic acid with chromium and nickle used in the heating crucible? Because this acid cleans (removes) the surface of many metals.

    1. David Post author

      Hello Andrew

      I also treat swarms within a few days (and certainly before there’s any sealed brood, for obvious reasons) with an oxalic acid-containing miticide. I even do this for casts where the Q needs to get out and mate. I treat late in the evening to avoid disturbing her. I don’t remember having any problems with her getting mated.

      You need to talk with a chemist about how oxalic acid works … but this might be a start.


  16. Isabel

    Dear David, I read with interest your ( peer-reviewed publishable) article. As a scientist, I have been making myself ‘test & control’ varroa experiments with my (very patient, and very understanding) bees, and agree with all of your statements / recommendations on timings, dates, frequency and quantity (as well as all the problems with the products sold not being just Oxalic Acid). I am finding myself now (with my fellow physicist and engineer beekeepers, here in Switzerland), with an additional problem. We are now not exactly sublimating, but transferring the OxAc through cavitation, which poses an additional problem regarding quantities of effectively transferred active ingredient. We started with this new method last year, and had mixed results – not sure yet why. Happy to discuss about it, would you consider it of interest. Best regards from Switzerland

    1. David Post author

      Hello Isabel

      I don’t know ‘cavitation’ … I wonder if the translation is correct? I’m always interested to see what other beekeepers do to control mites. Perhaps email me something to the link in the page footer?


  17. Larry Krengel

    Greetings David
    I enjoy reading what you write. Thinking of the use of OA, I am concerned about the physical damage done to the adult bees. In a conversation with a well-spoken researcher I heard about damage he felt was done by OAV applications on the bee’s antennae. The suggestion was that this damage would handicap the bees by diminishing the effectiveness of the wide variety of senses located on the antennae. The damage would be compounded with repeated applications.

    I have chosen other mite control methods and avoided OAV largely because of this… yet over the years I have seen no research confirming or refuting the concern about the physical damage done to the bees by oxalic acid. (It is rather tough stuff!) What I continue to hear is, as you have said, oxalic acid is extremely well-tolerated by the bees. Is there research to support that?

    Thanks for your thought-provoking writings.

    1. David Post author

      Hello Larry

      I’m not aware of any research showing damage to adult bees. However, there is lots showing that OA is well tolerated by the bees, and that vaporised OA causes less harm to the colony than dribbled OA-containing solutions. Not clear which evidence you are asking for in your comment … if the latter (i.e. no harm) then look back through my posts on sublimation. The studies include work by Radetzki (see above) and Al Toufailia (ditto).



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