Oxalic Acid
This week a beekeeper in the Antipodes posted on social media about making oxalic acid and glycerine strips at home for treating Varroa destructor (varroa). They apply continuous treatments without a break using this method of varroa control and I wondered if they understood the implications and risks around using oxalic acid like this—but suspected not.
Organic Acids
Organic acids are found naturally at low levels in honey, and oxalic acid is present in everyday foods that we eat such as carrots and rhubarb. However, when they are deliberately used as acaricides to kill varroa mites they are given in much higher doses than you will find in carrots. Organic acids are pretty good at killing varroa, and oxalic acid and formic acid are highly effective at disinfecting laboratories due to their very strong antibacterial properties1. This gives us a clue about how they will behave in a hive. Acids will not only kill off some varroa but important other little mites and microorganisms that work symbiotically with honey bees to maintain their unique hive ecosystems.
We know that organic acids are irritants and harmful to human health as well as to mite life inside the hive. That’s why we wear protective equipment when we administer treatments. Therefore, there is no reason to imagine that honey bees go completely unscathed by these treatments. Some of us like to think that organic chemicals are better and less harmful than using the synthetic pyrethroids and artificial acaricides. ‘Organic’ certainly sounds harmless but these acids are not benign chemicals They may be preferable and better in some ways to ‘heavy duty’ chemicals, but only if the directions for use are strictly adhered to and the bees are not overdosed. As Randy Oliver (www.scientificbeekeeping.com ) often says, “The label is the law”, and it is a good principle to follow.
Essential Antennae
Bartek Maleta1 draws attention to the dangers of organic acids in his thought-provoking and useful book, Beekeeping in Harmony with Nature that I read through over again from time to time because it is impossible for me to remember it all. https://www.northernbeebooks.co.uk/products/beekeeping-in-harmony-with-nature-maleta. He talks about speaking to natural beekeepers (nature-based beekeepers) at conferences and hearing how they often find mature honey bee antennae on the floor after treatments with acids. I was interested in this because we know from work by Dr Mondet2 in France that functional antennae are key to varroa sensitive hygienic behaviour and the detection and removal of mites by honey bees detecting brood pheromones and mite kairomones. The sensilla found on honey bee antennae are part of their central nervous systems and involved with detecting and recognising odours so damage to them leads to communication problems for the bee.
Literature Review
Erik Tihelka’s3 literature review on the effects of all acaricides makes a good read. In fact, we owe it to our bees to know everything there is to know about everything we put inside their hives. Nothing that we put inside a hive is natural apart from the bees themselves, and every other addition comes with some side effect. I encourage you to download this paper to keep for reference. All the information in the paper has been gleaned by Tihelka from numerous articles describing peer reviewed research and I have highlighted some of the key facts. He covers the side effects of all known varroa treatments which gives an individual much better insight to all the risks to make better informed choices of varroa treatments.
Tihelka points out that there are far more papers on varroa research than there are on honey bee health, especially in relation to varroa treatments and side effects. The subject is somewhat controversial due to the pharmaceutical companies and beekeeping suppliers wanting us to keep on buying their products.
It is quite alarming really that even recommended treatments and correct doses can lead to high honey bee deaths and shortened lives in some cases. Synthetic acaricides coumaphos and fluvalinate are more toxic to older bees than younger ones and if the outdoor temperature is higher then the bee mortality is also higher. Coumaphos causes behavioural changes reduced trophallaxis (food exchange) which can potentially affect food transfer in a colony and reduce energy levels for the colony. Apistan (fluvalinate) and Apivar (amitraz), commonly used in UK, are known to impair honey bee metabolism, and reduced proteins, carbohydrates, lipids and some enzymes were found in the haemolymph of 0-7 days old and 21 day-old worker bees.
Thymol is apparently almost harmless when outdoor temperatures are between 5-9°C but there is increased bee mortality above 27°C. Thymol and formic acid induce stress and increased fanning at the entrance. Apivar Life contains thymol which can cause decreased brood and colony development as well as reduced food intake and reduced sealed brood. Empty cells are found near the tablets which the bees remove most of within ten days. This is true for all thymol products that are quickly removed. What I found most alarming was that thymol has been shown to reduce the sperm count in drones and also reduce the lifespan of stored sperm in queens. Apiguard has an increased larval and pupal mortality and a reduced sealed brood area on combs. Older worker bees try to avoid it and contact via honey bee antennae induces fanning. We wonder why we have so many problems with queen mating and getting good long lasting queens.
Formic acid has been shown to negatively affect the number of sensilla in bee antennae and in high doses impairs metabolism and reduces oxygen intake in younger bees. In high doses of 85% formic acid, increased shock proteins were found in the bee brain as a result of stress, and there was some gut cell death. At 65% concentrations, irritated bees increased fanning and there was a reduction in worker population and worker brood population along with reduced brood survival and antennal sensory organs.
At 30% concentrations of formic acid there were increased heat shock proteins in the brain, and the area of sealed brood cells decreased. The bees cleared out the brood cells close to the absorbent pads of acid and this treatment was associated with queen loss.
Formic Pro, which is commonly used in the UK, is 42.25% by weight.
When oxalic acid was applied to bee bodies in lab conditions, rather than ingested, it penetrated the keratin of the exoskeleton and traveled to the internal organs where it was found in the digestive tract, rectum and haemolymph (bee blood) and the damage raised adult bee mortality levels. There was increased brood removal and decreased amount of brood in the nest. When oxalic acid is dissolved in sucrose the bees will eat more and with higher doses mortality increases. The pH of honey drops after oxalic acid treatments and stored honey can become more acid over the seasons. Shock proteins and bee mortality also increase and colony strength decreases in adult bees. Larval gut and salivary glands are damaged by oxalic acid solutions. It seems that vaporising or sublimating is more effective at killing mites, and less risky for bees than trickling or spraying in solution form. A little riskier perhaps for beekeepers administering it though than using a liquid.
Conclusion
If this is all a bit scary and depressing it is worth remembering that you can keep your bees safer if you follow integrated pest management principles and only treat for varroa if you have to because the varroa levels are above the safe threshold. I covered this topic some time back, so here below are the key facts about IPM that relate to any aspect of responsible livestock management. It is also good for the bees if we know all about each acaricide and when best to use each one. That way the choices we make on behalf of the bees are thoughtful and measured ones. https://www.beelistener.co.uk/?s=integrated+pest+management
Integrated Pest Management
Integrated Pest Management (IPM) is a practical, environmentally responsible approach to controlling pests. Instead of relying mainly on pesticides, IPM combines several methods to prevent pests, monitor their activity, and control them only when necessary.
Key principles include:
- Prevention: Remove food, water, shelter, and access points that allow pests to thrive.
- Monitoring: Regularly inspect and use traps or other monitoring methods to detect pests early.
- Identification: Correctly identify the pest and understand its life cycle and behaviour.
- Control: Use the least-risk effective method first, such as hygiene, proofing, traps, biological controls, or targeted pesticides.
- Evaluation: Check whether the treatment worked and adjust the approach if needed.
- The aim of IPM is effective pest control with the minimum possible risk to people, animals, property, and the environment.
- Prevent → Monitor → Identify → Control → Review →Prevent → Monitor → Identify → Control → Review…
References:
1 Maleta, B., Beekeeping in Harmony with Nature: The Evolutionary Solution to The Varroa Problem, 2025. Northern Bee Books.
2Mondet, F., Alaux, C., Severac, D., Rohmer, M., Mercer, A.R. and Le Conte, Y., 2015. Antennae hold a key to Varroa-sensitive hygiene behaviour in honey bees. Scientific reports, 5(1), p.10454.
3 Tihelka, E., Effects of Synthetic and Organic Acaricides on Honey Bee Health: A review. 2018 Slov Vet Res 2018; 55 (2): 119-40 DOI 10.26873/SVR-422-2017 UDC 638.157:615:661.164.2:
Discover more from The Beelistener
Subscribe to get the latest posts sent to your email.
Ann, you have perfectly summarised a large amount of wide-ranging research and conclusions that I would not have found it easy to plough through; thank you.
The overall direction of thought expressed gives fact-based support to the instinctive disinclination that many beekeepers have towards the use chemicals in the control of varroa mites.
Hello, Peter. Thank you for your positive comments and contributing to the discussion. Best wishes, Ann.
Hi Ann,
It’s unfortunate that everything comes with risks, but that’s life isn’t it? At least formic and oxalic don’t seem to hang around for ages in bees wax like the synthetics.
And in real-life beekeeping it’s obvious that a treated low-mite colony is healthier than one heavily infested (bigger colonies, no obvious DWV, less defensive, overall seem happier). Then there’s higher winter losses in untreated colonies – shown recently in a BBKA survey of members (not quite science, but not nothing).
At scale (bee farmers) it is very time consuming and of questionable value for every hive to be continually monitored to try to find and breed resistance. Losing 50%+ of colonies is an existential threat to commercial beekeepers, so most treat and hopefully follow the instructions properly. If it did not work well it would soon change. Many focus on honey production and leave queen breeding to others who specialise in that (there is only so much time).
At a hobby level, of course people can spend more time on colonies so it is wise to practise IPM and treat once a mite threshold is passed, but not before.
If queens were available in large numbers that can produce colonies of bees that thrive without treatment, that’s what everybody would use. But it’s still dreamland for most, despite decades of work by some breeders.
So there is still very much a place for mite treatments, at least for commercial beekeepers. Sure, there are losses, but that’s always the case with livestock. I think the biggest threat to most people’s bees is not treating for mites, but I know many disagree. And high levels of varroa+virus also affect drone sperm viability.
As always, there are plenty of challenges and areas to improve in the wonderful world of beekeeping.
Best wishes, Steve
” it’s obvious that a treated low-mite colony is healthier than one heavily infested ”
Sure it is!
but that’s not the issue and the point here.
It’s obvious that it’s better to treat lung cancer with chemicals, than not (people – as the bees – are generally happier when they are alive than when they are dead 😉 ), but that does not mean we should smoke, inhale all the dusts, eat unhealthy and generally live unhealthy etc.
The issue here is that knowing that there are side effects of different pesticides, amateurs and commercials still won’t do everything to keep bees in healthy and sustainable way, breed for resistance and/or let bees breed for resistance by allowing free mating and natural selection. Because there is a solution to the varroa problem, and people stubbornly don’t follow that solution. One of the many solutions is following IPM with treating only when the bees need it and choosing the more resistant bees for further breeding. The alternative that it’s either quiting treatments at once and letting more than 50% bees die or treating and having healthy and happy livestock – this is just false alternative. [I guess the best evidence is that in US in one of the last years they lost over 60% of bees while treating].
best wishes
Hello, Bartek. Thank you for contributing to this discussion. I can’t help thinking that what you said, “One of the many solutions is following IPM with treating only when the bees need it and choosing the more resistant bees for further breeding” is probably the best way forward here in the UK to help beekeepers gain more confidence with managing varroa. At the moment, conventional beekeeping teaching and government advice mostly perpetuate the idea that routinely treating every colony every winter with oxalic acid is a good management strategy, but it is not integrated pest management.
Onwards and upwards.
Best wishes, Ann.
Hello, Steve. Thank you for contributing to the discussion from a bee farmer’s perspective and sharing your thoughts with us. I think that it is the challenges in beekeeping that hold our attention and make us want to dig in deep to try to find answers. One of the other things that contributes to the enjoyment for me is engaging with beekeepers from other places and experiences in a sort of collaborative way as beekeeping can be a solitary activity.
Re the BBKA survey, I think that they could improve that by asking a few more questions such as: does the beekeeper practice IPM, or do they routinely treat with oxalic acid in winter whether the bees need it or not, does the non-treating beekeeper feed the bees, are collected swarms counted in the losses, does the beekeeper practice migratory beekeeping? and more to give a more balanced and meaningful survey. I am aware that people don’t want to spend ages answering questions but I think that the survey would be more useful if constructed differently.
Best wishes, Ann.
What a great article Ann you really do your research and seem to really understand the bees needs.
Unfortunately honey production by large commercial bee keepers is like farming and the bees are seen as Steve said livestock and with livestock, losses are expected. Thank goodness there are people like you out there with the expert knowledge and years of research to guide beekeepers to look at bees in a much more sustainable way.
Hello Susan, thank you for being such a loyal supporter of the blog despite not being a beekeeper. Well I suppose there will always be dichotomy when it comes to the management of animals that are used in food production for a profitable living, and those same animals that are kept by hobbyists largely for pleasure, interest and a few jars of honey when the seasons are good.
Thank you Ann for highlighting the damage caused to honey bees by organic treatments. This does need to be heard at the NBU, DEFRA and the BBKA.
Concurrently, there is a need for a national educational program for beekeepers to identify and understand varroa-resistant traits. This could build on the work of BIBBA (https://bibba.com/varroa-resistance/) who, of the organisations, have led the way.
Importing over 30,000 queens into the UK in 2025, which only a small % are checked, represents an unacceptable bio security risk by the commercial sector. In addition, these will be non-resistant genetics that lead to large numbers of mites in unsuspecting beekeepers’ colonies. Stewardship is required.
Hello, Steve. Thank you for contributing to this important discussion. Like Ray Baxter’s ‘Bottom up Beekeeping’, this is really ‘bottom up’ varroa management with BIBBA being the base in the UK and ideas pushing upwards from there. The BBKA seems open to change and Karl Coyler’s excellent articles on varroa resistance have been a welcome addition to BBKA News this year. Maybe, just maybe, strong stewardship will come out of this.
Best wishes, Ann.
I feel that, in orthodox beekeeping teaching, and in the innumerable sources of information for beekeepers, far too little emphasis is placed on IPM as an integral part of a varroa management strategy. In the minds of many beekeepers, the process of developing varroa resistance is perceived as too a long, painful, painstaking journey with little guarantee of success. There is no reliable way in Britain of buying VSH stock unlike in the US where there are suppliers with exotic sounding stocks like Russians and Ankle Biters. Many have colonies in areas heavily populated with managed honey bee colonies and there is no concerted action in their area to populate the DCAs with ‘good’ drones so any effort they make is likely to be nugatory. Add this to the received wisdom that allowing colonies to follow their instinct by swarming is bad beekeeping so they don’t benefit from a natural brood break. All these headwinds, combined with the pressure from Big Pharma, make it only too easy to resort to using treatments as a routine to be followed regardless of any other alternative.
An interesting line of research is on the effect of varroa treatments on the endocrine system. In one study thymol caused a delay in the normal developmental peak of vitellogenin expression ( Gael Charpentier et al 2014). In another study oxalic acid was found to produce a short-term increase in adult haemolymph Vg, whereas OA-glycerine exposure was associated with reduced Vg expression in prepupae (Aupinel et al 2014) In view of the apparent increase in winter colony losses, this might add a further insight into the effect of treatments on the transition from summer to winter bees and back again the following spring.
Hello, Alan. Thank you for that useful contribution to the discussion. It is really a minefield with so many things to consider.
Best wishes, Ann.