Time to Treat? Oxalic Acid Reviewed.

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Treatment Temperature.

How many of you take the opportunity of a broodless, or nearly broodless period, in the Northern Hemisphere to give the bees an oxalic acid treatment? It’s time for me to get Varrox Eddy out of his box for his winter work but the outside ambient air temperature is hovering around 0 degrees C so the bees will be too tightly clustered for the vapours to percolate efficiently throughout the colony this week. The ideal temperature for this treatment is 4-16 degrees C so I’ll try for around 8 degrees C which means waiting till the weather warms up a bit. Eddy is all ready to go with a brand-new battery system installed under warranty by Andermatt Gardens in the summer.

Sublimation v Vaporisation?

Am I going to be sublimating or vaporising? This is a good question and I cannot tell you which because I have no way of knowing how Eddy heats up and what temperatures are reached. One assumes that we are vaporising since this is what is described in the Veterinary Medicines Directorate. There is no guidance on temperatures in the small print accompanying products for administering oxalic acid. Sublimating and vaporising are two different terms. We often use them interchangeably which is apparently incorrect as I’ve just discovered. During sublimation, oxalic acid dihydrate crystals are heated to 157 degrees C  when they change from a solid to a gas at once. However, with vaporisation, oxalic acid crystals are heated to 101 degrees C whereby they melt initially to a liquid before becoming a gas on further heating. In the US, beekeepers have only recently been permitted to use oxalic acid as a Varroa treatment, and they must only use 1 g per brood box using the vaporising method.

Worker bee coated in oxalic acid crystals. Photography by The Apiarist.

Oxalic acid works best in a broodless period because the vapours cannot penetrate the wax cell cappings. When gaseous oxalic acid permeates the colony, Varroa are killed on contact because their mouthparts are damaged and the adhesive lobes of foot parts are also affected causing them to starve and to lose their foothold on the bee. Increased bee to bee contacts with oxalic acid promotes grooming which also helps remove mites. You can see how the bee above is coated in fine crystals like frosting in winter.

Overheating & Degradation.

What happens when oxalic acid is heated above 188.8 degrees C? This might happen in uncontrolled situations where homemade equipment is used resulting in overheating and degradation of oxalic acid without effectively treating mites. Above this temperature oxalic acid decomposes at once to formic acid and carbon dioxide then it becomes carbon monoxide and water.

UK Dose.

In the UK we can use 2.3 g oxalic acid dihydrate crystals which is 2.5 ml and almost ½ teaspoon. Toufailia et al 1 studied different doses of oxalic acid during broodless periods and found that 2.25 g vaporised gave 97.7 % efficacy and that at least 2 g is needed to be an effective treatment. Doses of 4.5 g resulted in a slight increase in colony mortality and it was also uncertain how this dose might affect queen health.

Using Oxalic Acid When Brood is Present.

Although in the UK it is advised to use oxalic acid for broodless periods only, some beekeepers use oxalic acid sublimation/ vaporisation treatments during the active season when brood is present. Treatments might be targeted once a week for 3 weeks to cover a worker brood cycle and kill the emerging mites. Pro Vap 110 advises 4 treatments 5 days apart to kill successively emerging mites.

In one recent study, Berry et al 2 assessed repeated vaporisation of 1 g oxalic acid on 99 colonies with no adverse effects on brood and bees. 7 applications were applied separated by 5 days taking 35 days total for the study. However, there was no evidence that frequent applications reduced Varroa levels below treatment threshold and they couldn’t advise this method of treatment.

Different Doses.

Jack and colleagues3 trialled different doses on colonies rearing brood in their study published in 2021. The tested doses were; 1 g, 2 g, 4 g, and no g oxalic acid with 10 colonies per group treated once a week for 3 weeks. They also found that 1 g oxalic acid was ineffective but that doses from 2-4 g will significantly reduce varroa levels. They didn’t find any negative effects on brood and bees at repeated 4 g doses but they didn’t assess effects on the queen and conclude that further studies need to be carried out at this higher dose before recommending it for use routinely.

Varroa levels were assessed in both studies using alcohol wash tests on samples of 300 bees which are more effective than counting mite drops.

I hope that this information helps you make decisions about Varroa treatments now and later in the new season. You must be aware of current legislation for all honey bee treatments in your own country. In the UK, API-Bioxal powder 886mg/g is a permitted product for vaporisation. Dany’s Bienen Wohl powder is also available along with Oxybee powder. It is not permitted to use oxalic acid crystals available as hive cleaners and wood bleach. Manufacturer’s instructions must always be followed and using appropriate personal protection, including a face mask, is imperative. The face mask must be for use with organic acids otherwise you will not be properly protected and placing yourself at risk of lung damage. Check out the Veterinary Medicines Directorate for instructions on using API-Bioxal in UK.


1 Al Toufailia, H., Scandian, L., & 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 Apiculture Research, 54:2, 108-120 DOI:10.1080/00218839.2015.1106777

2Berry, J., Bartlett, L., Bruckner, S., Baker, C., Braman, S., Delaplane, K., & Williams, G. (2021) Assessing Repeated Oxalic Acid Vapourization in Honey Bee (Hymenoptera;Apidae) Colonies for Control of The Ectoparasitic Mite Varroa destructor. Journal of Insect Science, 2021, 22 (1) 0: 15; 1-6.

3Jack, C., van Santen, E., & Ellis, J. (2021) Determining the Dose of Oxalic Acid Applied Via Vapourization Needed for Control of the Honey Bee (Apis mellifera) Pest Varroa destructor. Journal of Apiculture Research, 2021, Vol. 60. No. 3, 414-420.

10 thoughts on “Time to Treat? Oxalic Acid Reviewed.”

  1. It’s really funny to see that they try to restrict oxalic acid as a pure organic acid as a varroa treatment, when there is the exactly same in all the other more expensive “tested” formulations..!
    On the other side nobody cares warning people about the much more toxic compounds like the insecticides (Amitraz) we still use this days for varroa treatment, that even ends up as it’s toxic metabolite in our honey we eat and in our wax we use again and again and will eventually select the mites for more resistance to our newest “chemical weapons” we have designed.

    1. I agree Roman that it is very hard to reconcile these conditions. We have no real control over policy and legislation. There are a lot of open minded beekeepers out there who are trying to change things for the better and move away from toxic varroacides. I’m fortunate enough to meet them and feel inspired to make changes to my own beekeeping management. Westerham beekeepers headed by Steve Riley springs immediately to mind. As someone said recently, “teamwork makes the dream work”.

  2. Thank you, Ann, for providing this evidence-based guidance for how to treat a colony with oxalic acid. The photo of a worker bee cloaked with oxalic acid crystals is shocking (at least to me), but I see that there is evidence that there is little or no damage to bees by this treatment if done properly. Good to know.
    Two questions: Is it known how much oxalic acid ends up in the honey produced by a treated colony? Does the use of oxalic acid mean that a beekeeper cannot claim that his/her honey is organic?

    1. Hello Tom. Thank you for commenting and raising these questions. Well, firstly the honey supers should not be in place when oxalic acid is used. If treated during a broodless winter period then that would not be a consideration, but during the season the supers must come off during treatment. Interestingly, the soil association permits the use of oxalic acid in organic honey production. See this site https://www.soilassociation.org/media/15931/farming-and-growing-standards.pdf
      It would be true to say that it is unlikely that any honey in UK is truly organic in such a small country with lack of control over foraging distances.

  3. I’m really grateful to you, Ann – for bundling up all the facts and presenting them in this way. I feel certain that many ‘everyday’ beekeepers don’t engage with the science because it is like wading through treacle! In this article, you’ve done the hard work for us. I have chosen to move to a delivery device that guarantees temperature and dosage – the accounts of carbon dioxide emissions due to incorrect dosage temperatures led me to do this. The last straw for me was when using the wand-type device last winter, the bowl dislodged from the stem and was lost inside the hive!!!

    1. Hello Liz. Coming from you this is praise indeed! Thank you for sharing your experience with the gimcrack equipment. I’ve had similar bad experiences myself with an unwieldy relatively cheap product that overheated and fell apart at the crucial moment. Bee buddy Cynthia will recall the winter air being blue with expletives and varroa levels higher than usual in spring! I have no regrets buying the Varrox Eddy except he does not fit into every hive entrance. I can lend it out knowing that is in one of the safest methods for the beekeeper on the market.

  4. Hi Ann, nice succinct summary….
    I would add that masks need to be able to deal with organic acids.
    My face mask for woodwork was not up to muster and apibioxal wafted through as if i had no mask at alI.

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