Organic Acid Varroa Treatments Come With Risks Too.

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: