Oxalic Acid: A Beekeeper’s ally against varroa

Oxalic acid is an organic acid compound, found in different plants and vegetables we eat every day, including the golden liquid our craft is famous for, honey.

National Honey Show 2022 write-up Reading Oxalic Acid: A Beekeeper’s ally against varroa 15 minutes Next The British Beekeeping Show- time to get a jump on the season!

By Andrew BrownNov 14, 2022

This is the article I wrote for Bee Craft which went into the November 2022 issue of the magazine. It gives an overview of oxalic and its uses for varroa control in bee keeping.

Oxalic Acid- What is it?

Oxalic acid is an organic acid compound, found in different plants and vegetables we eat every day, including the golden liquid our craft is famous for, honey. It is safe to say that Oxalic acid is abundant in our environment, and we consume oxalic acid in our diets at low levels all the time.

At one of Professor Ratnieks’ lectures at LASI (Laboratory of Apiculture and Social Insects) on oxalic acid sublimation I learned that you could extract enough oxalic acid to treat a colony (about 2g) from one bag of carrots. Obviously extracting said acid would be quite difficult but this highlights the fact that oxalic acid is present in food that we consume regularly.

Oxalic acid is used outside of bee keeping as an effective cleaner for many different things from buildings to swimming pools. Oxalic acid is also used for bleaching wood and in some tooth whitening products.

oxuvar oxalic acid application for varroa mite treatment

For us bee keepers, oxalic acid is used as a ‘soft’ acaricide as it is toxic to our primary pest Varroa destructor. Oxalic acid is a good option to treat varroa mites because of the way that the acid interacts with the mite. It is unlikely that varroa mites will develop a resistance to it however it is always advisable to rotate treatments of any pest using an integrated pest management strategy and oxalic acid is no different.

oxalic acid winter varroa treatment Bee Craft article

Problems treating the varroa mite

One of the main problems of treating the varroa mite is that mites exploit the honey bee’s natural behaviour. Honey bees swarm as a means of reproduction for the colony. They also rob from other colonies at certain times of the year and can drift from hive to hive within an apiary so will find themselves in different colonies from their own. Any adult female varroa mites riding on adult honey bees will be moved with the bee to the next colony.

If you move any bees or brood, you will be moving the mites. Varroa mites are moved between hives when a bee keeper manipulates colonies as a means of swarm control or to make nuclei for the coming season. Even removing frames to boost other colonies with brood will inadvertently move the varroa mite populations into the new hive along with the bees. The dispersal phase of the varroa mites’ life cycle has a considerable impact on the colony but the reproductive phase is equally as devastating as this occurs in the sealed brood cells.

varroa mite on bee larvae brood cells

The varroa mite moves into the brood cell from its dispersal phase into the reproduction phase when the honey bee egg hatches into a larva at around 5 days, poised to begin laying her first egg. She waits until the bees cap the cell before she lays her first egg; an efficient way to ensure her young are kept safe and well fed. By the time the bee has pupated into an adult each mature female has laid 5-6 eggs. Firstly, one male and then the rest are females which will reach maturity and are able to reproduce in 7 days.

Because the population of the varroa mite builds under the brood cap, it can be challenging for a bee keeper to access these mites. There are important points in the year when varroa treatment is best conducted that has the most impact on the population. For oxalic acid which does not penetrate the brood cap; winter is one of the most optimum times to treat. The colony is at its smallest and there is little to no sealed brood around the winter equinox. If the colony is small, the varroa population is small as well. The Oxalic acid will impact the varroa population when they are sapping the winter bees of their vital fat body tissues which sustain them through the winter. Once any given colony has swarmed is also another perfect time to use oxalic acid. All the varroa mites taken by the hive will be on the bees in the dispersal phase so an oxalic acid spray here will impact all the mites significantly.

Because the life cycle of varroa is intricately linked to its honey bee host it is difficult, almost impossible to completely eradicate varroa from the hive. There is a constant risk of reinfestation from other bee hives with varroa so even if your colony has little to no mite infestation after treatment, they could get re-infested with varroa quickly.

varroa mite population year round graph

Just like the honey bee, varroa mites can live 2-3 months in the active season and up to 5 months in the winter. The adult varroa mites live on the honey bee in the winter and feeds primarily on the fat body tissue of the bee, not the haemolymph as once thought. This ground breaking discovery by Samuel Ramsey in 2019 has shown that the varroa mites exploits the honey bee ‘liver’ which we know in humans is a highly important organ in the body for everyday functions. When given a choice between fat body tissue and haemolymph, Varroa will favour the fat body tissue. They live longer and lay more eggs which suggest that the fat is integral to the Varroa mite’s diet.

Varroa is problematic to manage for the beekeeper and if the varroa mite is left to multiply, within a 2–3-year period any colony can collapse and die. The likelihood of colony mortality goes up with higher varroa infestation, so treating for varroa is very important if you don’t want to risk losing your colony over winter.

varroa mite colony mortality graph

Graph from Figure 5 of the The German bee monitoring project: a long term study to understand periodically high winter losses of honey bee colonies by Elke Genersch et al. from Apidolgie 2010 https://doi.org/10.1051/apido/2010014

Why Oxalic Acid is good for treating varroa

Oxalic acid is toxic to the varroa mite; it forms crystals on the adhesive lobes of the varroa (its feet) and on the feeding mouthparts which are then go into the body of the mite. The adhesive lobes have an aqueous secretion which binds to the oxalic acid crystals and makes them grow. This damages the mite’s vital organs and kills it. The honey bee secretes a waxy coating on its feet and over its whole body which protects the bee from the oxalic acid meaning it does not succumb to oxalic acid in the same way but can transport the oxalic acid through the colony.

Oxalic acid is versatile and can be used to treat varroa in different forms and at various times of the year; the most widely used and well known is the winter trickle method. This has been used by bee keepers for a long time, but oxalic acid spray and vaporisation is now becoming another popular way to apply the treatment to the hive.

Oxalic Acid methods of application

Oxalic acid treatment in summer - Spray Method

Oxalic acid spray is a way to apply oxalic acid to the colony when the average temperature is above 8°C. This can be throughout the season from Spring to early autumn to swarms, splits, and brood-less colonies.

If your colony has a moderate to severe infestation (this would be 5-10 mites per 250 bees) Oxalic acid spray is an excellent way to combat the varroa quickly.

oxuvar oxalic acid application for varroa mite treatment

Water and oxalic acid are mixed to make a solution that is 3% strength. For example, our oxalic acid product Oxuvar is a pre-prepared solution and the 275g bottle needs 250ml of water to be added to make it 3%. Not as widely used in the UK, throughout Europe bee keepers use oxalic acid spray at the start of the year in Spring.

Instructions for oxalic acid spraying

Bees on combs are sprayed with a hand sprayer in an angle of 45°. 4-5 sprays (3–4 ml) are enough to moisten the bees on one side of the comb. Bees in clusters are sprayed with 20–25 ml per kg of bees. Best results are seen when the application is done when the bees remain inside the hive for a few hours following the treatment.

oxuvar oxalic acid spray application winter varroa treatment

Oxalic acid dihydrate mixed in a 3% solution is well tolerated by the bees and queen. Avoid spraying into open brood cells to avoid residues which is why it is recommended that you spray oxalic acid at a 45° angle. A fine mist on the bee is sufficient, there is no need to drench the bees in the spray as this may cause stress and risk adverse effects on the bees.

Oxalic acid is fast acting on contact and the varroa mites perish and fall off the bees soon after spraying. Since the solution does not contain any sugar, the water evaporates quickly leaving the oxalic acid dihydrate in the form of tiny, needle shaped crystals on the bee. These crystals are cleaned off the bee by themselves or they encounter the adult varroa mite in its dispersal phase. Oxalic acid dihydrate spraying solution should not be used more than twice on any one generation of worker bees as sublethal damages and stress may shorten the life span of the bees. Repeated use may cause damage to the queen; the last thing you want to happen.

Summary of Oxalic Acid Varroa Spray Treatment

This is a good option for treating swarms, nuclei or splits that are brood-less as it is simple to prepare and mix the spray and it can be administered as you work through an inspection.

Trickle Treatment

The most well-known use of oxalic acid in bee keeping is the trickle treatment. Applied to the colony when there is a brood-less period, a premixed solution is syringed and ‘trickled’ onto the seams of bees that have clustered for the winter. Due to the time of year the solution must be warmed to reduce the risk of shocking the cluster with cold solution. Oxalic acid is mixed with sugar at a ratio of 1:1 to create a sugary solution that is passed around the colony. The sugar extends the time that the oxalic acid is in liquid form and so the oxalic acid can encounter more varroa mites.

oxalic acid trickle treatment

Trickle Treatment Instructions

The solution is trickled over the bees clustering between the combs. In the UK and at low outside temperatures the bees will remain in the hive and a dose of 5–6 ml per row for National hives will be sufficient. When the colony remains in the hive but is in a loose cluster form, the oxalic acid trickle method gets the best results.

Summary OA Trickle

This treatment is quick and easy to apply to the hive and can impact the varroa when they are at their smallest so a great option for winter.

OA Vaporisation/Sublimation

Oxalic acid vaporisation is a technique of treatment that has gained popularity in the hobbyist and professional bee keeping world alike. There are a range of different oxalic acid vapourisers on the market, but they all do the same thing; take pure oxalic acid dihydrate and heat it up to disperse it into the hive. The oxalic acid vapours circulate and then settle on the bees as crystals themselves where they encounter the phoretic mites.

Oxalic acid dihydrate evaporates in two steps and breaks up at high temperatures into inactive vapours. Vaporisation of oxalic acid allows repetitive accuracy for each treated hive when the instructions of use are followed.

Oxalic acid vapour treatment instructions

The bee keeper adds the required amount of oxalic acid dihydrate crystal (usually 2g per hive for a national hive) to the dish or tray of the vapouriser of their choice.

varrox vapouriser application

The vapouriser is then inserted into the hive through the entrance or into an opening that has been prepared by the bee keeper. Ensure that the hive is well sealed up to keep the oxalic acid vapours in the hive. When the vaporisation is complete, seal the entrance or opening for another ten minutes after treatment. Best results are achieved on brood-less colonies that are in a loose cluster formation when outside temperatures are above 8 °C.

Safety when applying oxalic acid to the hive

Oxalic acid is an irritant so any solution or vapours can cause irritation of eyes and the respiratory system. Always wear protective clothing (acid resistant gloves, safety glasses and a respiratory mask when vaporising). Always vaporise your hives with oxalic acid upwind to reduce the risk of inhalation.

VMD oxalic acid registered products

Any oxalic acid treatments used on the hive in the UK should only be on the VMD-approved list.

VMD approved varroa treatments

Unregistered oxalic acid is easily available online and in shops but these are not on the VMD list. Using the VMD-approved treatments give you peace of mind that you are complying with regulation and you are treating your bees correctly.

VMD-registered treatments make it easier to apply the correct amount of oxalic acid to the hive; there is a fine line between toxic enough to kill the varroa mites and too toxic to adversely effect the bees. Time and research has gone into these products to remove the hassle of calculating the correct recipe and correct dosage for the bee keeper.

Oxalic acid in the future

Oxalic Acid Strips- these strips are not VMD-registered but in the USA, Randy Oliver has done some initial research on the efficacy and these can be bought as ‘hive sterilisers’ from Abelo in the UK. These have shown to be particularly efficient at impacting varroa mites so going forward I expect to see a product of the ilk registered in the UK by the VMD. The findings from Randy Oliver’s oxalic acid tests are always worth looking at.

Article Summary

Oxalic acid is a valuable string to the bee keeper’s bow. It is cheap used as a spray or trickled and is quite easy to apply to the hive. Oxalic acid vaporisation does cost more initially but if you band together with other bee keepers and share a vapouriser the cost comes down. If you have lots if hives to treat in the long-term vaporisation may be the better option. Or if you have a good honey year you can treat yourself to a nice new gadget!

The ability of the colony to tolerate oxalic acid well is advantageous, the bees can pass the acid around the hive and encounter the mites effectively. It is limited in that it cannot penetrate the brood cap like formic acid, so it is only the phoretic mites that are impacted. However, delivered when the colony is brood less it has as shown to be highly effective.

This is historically done in the winter at Christmas, but using oxalic acid in the spring/summer on swarms or colonies on a brood break is another great point in the year to use oxalic acid. Used as part of an IPM strategy oxalic acid is an excellent way to keep on top of the varroa population in your hives.

Further useful information

Bee Craft - https://www.bee-craft.com/