Oxalic acid in the broodless period: products, methods and what the studies say

In many apiaries November is the moment you wait for all season in the fight against varroa. Colonies move into broodlessness, the first frosts have drawn the bees into a cluster, and suddenly the mite's most effective hiding place, the sealed brood, disappears. It is the only window of its kind in the year, and oxalic acid is a tool made precisely for it. Let us lay it out properly: how it works, what to apply it with, which products we can legally use, and what the studies, rather than apiary folklore, actually say about each method.

How oxalic acid works and why only in the broodless period

Oxalic acid is a contact poison. It works on the varroa sitting on the bees, that is on mites in the phoretic phase, and it does not penetrate the cell capping. Interestingly, the exact mechanism by which it kills the mite is to this day not fully explained. The talk is of a strongly acidic reaction that damages the mite's mouthparts and the adhesive pads on its legs, and of an acidification that varroa tolerates worse than the bee. What is certain is that the bee tolerates it far better than the parasite, and the whole treatment rests on that difference.

From this follows an iron rule: oxalic acid works really well only when the varroa is on the bees and not hidden in the brood. And this is not apiary superstition, but hard numbers. In the study by the team from the Laboratory of Apiculture and Social Insects at the University of Sussex (Al Toufailia, Scandian and Ratnieks, 2015) the share of varroa hidden in the sealed brood changed over the year from about 70 per cent to as little as about 10 per cent, and that minimum fell in December. In other words: in the height of the season most of the mites are out of the acid's reach, while in the winter broodless period almost all of the mites come out onto the open stage.

The difference in outcome is enormous. A treatment with 97 per cent efficacy leaves about 3 per cent of the varroa, while a treatment at the 75 per cent level leaves around 25 per cent, that is eight times more. The authors note that the annual growth of a varroa population is roughly fortyfold, so a well-aimed treatment in the broodless period, taking off more than 95 per cent of the mites, can practically cancel out a whole year of growth. The same product used when brood is present is money thrown away and a missed opportunity.

What products we have at our disposal

Let us start with something often forgotten: in Poland oxalic acid used on bees is formally a veterinary medicine, so the legal route is a registered product, not technical-grade acid bought from a chemical shop. There are several products based on oxalic acid, and they differ in form and in which methods they are approved for.

The first is Api-Bioxal in its liquid version (distributed in Poland by Biowet Puławy, marketing authorisation holder Chemicals Laif, authorisation number 3092/21). It is a ready-made solution with glycerol, 62 mg of oxalic acid dihydrate per millilitre, equivalent to 44.2 mg of anhydrous acid. It is applied only by trickling, undiluted, at a dose of 5 ml per seam and a maximum of 50 ml per colony, at most twice a year. The honey withdrawal period is zero days, but it must not be used over stores of honey destined for consumption or during a nectar flow. There is also an older, powder Api-Bioxal 35 g sachet, dissolved in syrup for trickling, which the maker also allows for sublimation at a dose of 2.3 g per hive.

The second is Varroxal by Andermatt BioVet (registered in Poland). It is a powder containing 0.71 g of anhydrous acid per gram, equivalent to 1 g of dihydrate per gram. Its advantage is that the leaflet approves all three methods of application: sublimation, trickling and spraying, in broodless colonies only. The doses are set out very concretely: for sublimation about 2 g of dihydrate per colony, for winter trickling a 4 per cent solution (eight level 1 g spoonfuls in 200 ml of 1:1 syrup for four colonies), and for spraying a 3 per cent water solution, 2 to 4 ml per bee-covered comb side. The leaflet even gives temperature ranges: sublimation at 2 to 10 degrees, trickling above minus 15 degrees, spraying above 8 degrees.

The third is Oxybee (Dany Bienenwohl), registered across the whole EU, based on oxalic acid dihydrate with added essential oils and glycerol, intended for trickling, also with a zero-day withdrawal.

The fourth is interesting because it is a combination: VarroMed (BeeVital) contains 44 mg of oxalic acid dihydrate and 5 mg of formic acid per millilitre. This matters, because formic acid is the only one of the popular agents that can partly penetrate the capping, so VarroMed is used across a longer part of the season, with the dose depending on colony size. It is not, however, the classic one-off winter treatment this post is about, but a separate philosophy of treatment.

For the record: dissolving technical-grade oxalic acid yourself and treating colonies with it is off-label and formally outside the law, even if the recipe for a 3.5 per cent solution has been circulating on forums for decades. If we want to act within the rules and keep our honey legally marketable, we reach for one of the products above.

Trickling (the dribble method)

This is the most popular, cheapest and fastest way, for years the backbone of the winter treatment in Poland. The oxalic acid solution is trickled straight onto the bees, into the seams between the frames, and the insects spread it around the whole cluster as they contact one another and lick up the sweet liquid.

The concentration is sacred here. The classic level, proven in studies, is about 3.5 per cent oxalic acid dihydrate in warm 1:1 sugar syrup, and this is what the recipes of the registered products also come down to. The sugar is no accident, because it is what makes the bees lick up the solution and spread it better among themselves. The dose is easy to remember: 5 ml of solution per seam occupied by bees, a maximum of 50 ml per colony. In winter the cluster rarely covers ten seams, so in practice it usually comes to 30 to 50 ml. You trickle only the seams where bees are actually sitting, because an empty seam is wasted acid and needless irritation of the nest. It is worth having the solution lukewarm, around body temperature. You choose calm weather, at a temperature from zero to a few degrees above, when the bees are drawn into a tight cluster, because on a warmer day the cluster loosens and it is harder to hit where it is needed. The treatment itself takes literally a moment, with no dismantling of the nest.

Trickling in the broodless period is effective, but in the Sussex study it worked only at a suitably high dose, of the order of 2.25 g of pure acid per colony. It has two drawbacks that must be spoken about plainly. First, it burdens the bees more than sublimation does: in the same study trickling and spraying increased the drop of dead bees, which sublimation did not. Second, and this is the most frequently broken rule, trickling is a one-off treatment. Acid applied onto the bees irritates their gut and shortens the life of the workers, and repeating it within a single generation of bees genuinely weakens the colony. The product leaflets warn outright that overdosing raises mortality, worsens spring survival and, in extreme cases, leads to queen loss and weaker spring development. A triple dose in trials clearly raised bee mortality. So you trickle once, well, and in the broodless period, not three times just in case.

Sublimation (vaporisation)

The second basic method, gaining more and more followers, and in the light of the studies often more effective and gentler. Instead of a solution you apply dry acid, which is heated in a vaporiser and sublimes, that is passes straight from a solid into vapour. The vapour condenses throughout the hive as fine crystals, settling on the bees, combs and walls, and in this way reaches the varroa.

The figures from the LASI study are a strong argument here. At a dose of 2.25 g per hive, in broodless colonies in winter, sublimation gave 97.6 per cent efficacy (a trial on 89 hives in 2013). Just as importantly, sublimation did not increase bee mortality, whereas trickling and spraying did. This translated into colony survival to spring: in the sublimation group 95 per cent of colonies overwintered, with trickling 83 per cent, with spraying only 63 per cent, and in the untreated control 80 per cent. Colonies after sublimation also came out stronger in spring, with more frames of brood, probably because the most mites were taken off them at the least cost.

The procedure in brief: I measure out the acid, plug the entrance, deliver the vapour into the hive and leave it closed for at least a few minutes, so the vapour condenses on the bees before it is aired out. The dose depends on the product and the hive volume. With the registered powder it is usually 2 to 2.3 g of dihydrate per colony, though some current foreign recommendations go higher, up to about 4 g per box. Always take the dose from the specific product's leaflet, not from a forum.

It is worth knowing here that the sublimation itself can be done with very different kit, and this is where the most solutions multiply, from cheap to professional. The most popular are passive insert vaporisers (the Varrox type), that is a metal pan on a wand, powered usually from a 12 V battery. You put the acid in it, slide it through the entrance under the bees, seal the hive and wait for the element to vaporise the acid, then a little longer for the vapour to settle. It is cheap and simple, but slow, because between hives you have to wait for the pan to cool, so with a large apiary it turns into a long evening. The second group are active, pressurised, so-called instant vaporisers (the Sublimox or ProVap type), in which the acid vaporises rapidly in a heated, closed vessel and the ready vapour is injected through a nozzle via the entrance or a hole above the floor, without sliding an element under the cluster. They are much faster, a few or a dozen seconds per hive, but also more expensive. Some beekeepers deliver the vapour from above, through a spacer eke, some through the entrance. The mechanism is the same; only the convenience, speed and price differ.

Sublimation can be repeated if needed, which cannot be said of trickling, and it works at lower temperatures too. One must, however, state plainly what it does not do: the vapour does not penetrate the capping, so with brood present efficacy drops just as it does with trickling. Sublimation when brood is present is not a miraculous way around broodlessness.

And now the thing that must not be made light of, because it is the only method on this list that can do serious harm to the beekeeper. Oxalic acid vapour is strongly corrosive to the airways and eyes. For sublimation I always put on a proper half-mask with a filter for acid vapours or an FFP3 mask, protective goggles and gloves, stand upwind and never breathe in the plume. This treatment is not done in a closed room or shed. A moment's inattention and a lung burn is real. For trickling an FFP2 mask is enough, but with sublimation the bar is clearly higher.

Spraying

The third method, rather marginal in overwintering colonies, but worth knowing. Oxalic acid as an about 3 per cent water solution, this time without sugar, is sprayed with a hand sprayer directly onto the bees sitting on removed combs, roughly 2 to 4 ml per one bee-covered side of a comb.

The problem is that spraying requires taking out frames and spraying the bees on both sides, and in winter that means breaking up the cluster and chilling the nest, which is exactly what we do not want to do in November and December. On top of that, in the Sussex study it was spraying that burdened colonies most: it gave the highest drop of dead bees (at the 2.25 g dose even about a tenfold rise in the daily count of dead bees) and the worst survival to spring. That is why spraying works not in overwintering colonies but where we have the bees out of the nest anyway and by definition without brood: in packages, in freshly hived swarms and in nucs made up on broodlessness. In such a situation it is fast and effective. For the winter treatment in full colonies, though, it is not the first choice.

Extended-release methods: sponges, towels and strips

Finally a whole family of methods, the loudest talked about today, because this is where the most experimentation is happening. The common idea is this: instead of a one-off strike of acid, you soak an absorbent material in a hot mixture of oxalic acid and glycerin, place it in the nest and leave it there for several weeks. The glycerin makes the acid release slowly, and the bees spread it around the nest as they contact the carrier. The theoretical advantage is tempting: action spread over time also covers the mites coming out from under the capping as the brood emerges, so the method at least partly bypasses the broodless limitation and suits summer use.

The father of this approach in the form half the world now copies is the American Randy Oliver (scientificbeekeeping.com). His recipe is very concrete: oxalic acid and glycerin are mixed 1:1 by weight, heated (but not above about 70 degrees, because the mixture starts to foam) and the carrier is soaked in it. In the towel version he used workshop paper towels cut in half, two halves per colony, which gave about 18 g of acid per hive, laid on the top bars between the boxes. He also tested Swedish cellulose sponge cloths, which held far more acid. In his own field test of 2019 (222 colonies, with no formal control group, so it is not peer-reviewed research) the median varroa reduction was about 88 per cent, and recalculated by the Henderson-Tilton method it came out at even about 98 per cent against the comparison colonies, and that with colonies rearing brood. The method is in any case designed for summer use, even with honey supers on the hive. Full action takes about 40 days and the treatment may be repeated once a season. It is from these trials that the glycerin strips popular in Poland grew.

And here is the crux, which must be said plainly. First, efficacy in different studies varies a great deal, from decent to disappointing, and it depends on the amount of acid, the type of material, the humidity and how eagerly the bees chew the carrier. In Poland the method has been studied, among others, by INHORT. Second, and more important, there is the legal question. Self-soaked sponges, towels or strips are not a registered medicine in Poland or in the EU, so treating colonies with them is action in a grey zone, and honey from such colonies should formally not go to market. Oliver himself, for that matter, stressed for years that his method is experimental and advised against using it until it was registered. And here is the good news for the future: in the United States it gained a registered equivalent in 2025, the strip product VarroxSan, which works for about 42 days, that is two brood cycles. This shows where the method is heading, but until we have a registered strip product in Poland, I treat it as an interesting direction to watch, not a ready, legal alternative to trickling or sublimation.

Which method to choose

For most beekeepers, especially at the start, trickling is the simplest, cheapest and perfectly sufficient: once, in the broodless period, 5 ml per occupied seam and done. If, however, you look at the figures from the studies alone, sublimation comes out better: higher efficacy, less burden on the bees and better overwintering. It is the choice for those who have the kit, more colonies and who will take their own safety seriously, because the vapour really is dangerous. Spraying I leave for packages, swarms and nucs. The extended-release methods, that is sponges, towels and strips, I watch with curiosity, because they tempt with action even when brood is present, but I do not recommend them as the mainstay while they remain unregistered in Poland. The common denominator of the classic winter treatment is one, and no kit changes it: oxalic acid, whether trickled, sublimated or sprayed, works really well only when it hits the broodless period.

Curiosities and controversies

The name comes from wood sorrel. Oxalic acid was first isolated from wood sorrel (Oxalis), and it is from this that its international name derives. The Polish word szczawiowy in turn refers to the related, common sorrel. It is the same compound we taste as the sour tang of sorrel or rhubarb.

We trickle onto the bees something that is in the honey anyway. Oxalic acid is a natural component of honey and of many plants, from rhubarb and spinach to sorrel and beet greens. This is one of the reasons the honey withdrawal period for the registered products is zero days: a correctly done treatment does not raise the acid level significantly above what is naturally present in honey. This does not mean, of course, that you can trickle supers during a flow, because at working concentrations we do not use it on honey for consumption anyway.

We do not fully know how it kills the mite. Even though oxalic acid has been in use for decades, the exact mechanism of its action on varroa is still a subject of research. A good example that in beekeeping effective practice can run ahead of full theoretical understanding.

One good treatment cancels a year of varroa growth. From the Sussex team's calculations it follows that at the roughly fortyfold annual growth of the mite population a treatment of 97 per cent efficacy in the broodless period practically zeroes a year of growth, while a treatment at the 75 per cent level delays it by only some two-fifths of a year. A difference of a few per cent in efficacy translates into a completely different start to the next season.

Sublimation beats trickling not by force but by gentleness. The most interesting conclusion from the comparisons is that the advantage of sublimation lies not mainly in a higher mite-kill, but in the fact that it harms the bees far less. That smaller burden then translates into better overwintering and stronger spring development.

Formic acid is the only one that goes under the capping. This is precisely why combination products like VarroMed exist: the oxalic acid deals with the varroa on the bees, and the added formic acid, as the only one, can partly reach the mites hidden in the brood. This is, however, a different story from the classic one-off winter treatment.

The Polish glycerin strips have an American father. The soaked strips and sponges popular on the Vistula are in a straight line the method of Randy Oliver from California, who refined it over successive field trials. In the USA it gained a registered successor in 2025, the strip product VarroxSan. A good example of how an apiary experiment from a garage can, years later, become a medicine with a label.

There are as many vaporisers as beekeepers. Sublimation alone is a whole industry of kit: from a cheap pan on a wand for a few dozen zloty, slid in through the entrance, to pressurised instant vaporisers for serious money that do a hive in a dozen seconds. The method is the same; only the speed and the thickness of the wallet differ.

The most common mistakes

Treating when brood is present. The most common and most costly mistake. The acid does not penetrate the capping, so with brood present, however little, a large part of the varroa survives, whatever the method. You wait for real broodlessness, usually after the first proper cold spells.

Repeating the trickle. Trickling is a one-off treatment. A second or third dose at a short interval does not finish off the varroa any better, but it genuinely weakens the bees and ruins the wintering. If you want a method that can be repeated, it is sublimation, not trickling.

Trickling onto nothing. Trickling seams with no bees in them is wasted acid and needless irritation of the nest. You dose only where the cluster is sitting.

Sublimation without respiratory protection. Oxalic acid vapour is corrosive. No filter mask or FFP3, no goggles and no working upwind is a straight road to a lung burn. This is the only stage at which oxalic acid is more dangerous to the beekeeper than to the bees.

Technical-grade acid from a chemical shop. Cheaper, but outside the law. On bees it is a veterinary medicine, so legally you use a registered product, not acid bought loose by weight.

The wrong timing on a warmer day. Trickling on a warm day, when the cluster is loose, lowers efficacy, because it is harder to hit the bees with the solution. A calm, cold day, when the bees are drawn in, is better.

Sources and further reading

• Al Toufailia H., Scandian L., Ratnieks F.L.W. (2015). Towards integrated control of varroa: effect of variation in hosting of mite reproduction and of oxalic acid dose, method and timing. Journal of Apicultural Research. The LASI team's study at the University of Sussex, the basis for the data on efficacy (97.6 per cent with sublimation at 2.25 g), colony survival and the share of varroa in the brood over the year.

• Rademacher E., Harz M. (2006). Oxalic acid for the control of varroosis in honey bee colonies, a review. Apidologie. A review of the knowledge on using oxalic acid, its mode of action and safety.

• Summaries of product characteristics for the veterinary medicines Api-Bioxal, Varroxal, Oxybee and VarroMed (the medicinal products register, ezdrowie.gov.pl, and the EU veterinary medicines database). The source of the data on composition, doses and approved methods of application.

• Oliver R., Extended-Release Oxalic Acid Progress Reports, scientificbeekeeping.com (2017 to 2019). The source of the recipe and data on the extended-release method (1:1 acid to glycerin ratio, dose on towels and sponges, efficacy in the 2019 trial, experimental status).

• Materials of the Institute of Horticulture (INHORT) on studies of products for varroa control, including oxalic acid and glycerin strips.

• Resources on the types of oxalic acid vaporisers (passive insert and pressurised instant vaporisers) and on the VarroxSan strip product registered in the USA in 2025.

• The posts on varroa treatment and on a warm autumn on the Beehive Hub blog, which this text complements with the practical winter treatment in the broodless period.