13 November 2021

Oxalic acid treatments against varroa mites are both efficient against the parasite as well safe for the bee colonies. Efficiency and safety – those are the most important criteria for a bee health product. Or for any drug in veterinary of human use, for that matter. Oxalic acid was used in Asia and Eastern Europe since the mid-1980s. But it was only in the 1990s when researchers from Italy, Switzerland, and Germany began to develop the treatment in a more systematic way. This is important: efficacy and safety depend on correct use. You have to find the right dose, the right time, and the right way for applying the treatment.

How to apply oxalic acid treatments

The most common way of applying oxalic acid is by trickling. For this, the oxalic acid solution is trickled with a syringe on the top bars of the frames at a dose of 5 ml per occupied frame. For instance, if your colonies occupies 5 frames, the right dose is 25 ml. If it’s on 8 frames you trickle 40 ml. This is important: the right dose maintains the balance between efficacy (kills the mites) and safety (doesn’t harm the bees). A stronger colony needs a higher dose, such as your doctor may give you different doses of medicine depending on your weight. The procedure, therefore, is:

  1. Check the strength of the colony.
  2. Multiply the number of occupied frames with 5 ml.
  3. Fill the right amount of the oxalic acid solution into a syringe.
  4. Slowly trickle it over the top of the combs. Do this transversally to  the combs, so that the drops stay on the top bars of the frame.
  5. Close the colony and repeat with the next one.
You apply the oxalic acid solutions transversally to the frames, so that you see the droplets. The bees then distribute the solution.

Many beekeepers also use sublimation – often called “vaporizing” or “fumigating” oxalic acid. The correct term, however, is sublimation: solid oxalic acid is transferred directly into a gaseous state. Nerdy details apart, in this case the dose isn’t adjusted as accurately as with the trickling. The only product that is registered for this form of application in the EU, Api-Bioxal, indicates a dose of 2 g per colony.

The most important thing for treating by sublimation is a device that heats up the oxalic acid crystals slowly and continuously. If this happens too quickly, the oxalic acid molecules degrade and aren’t efficient anymore.

Good and bad applications

If I had to chose an application method, I would chose trickling. It’s quick and easy. If the handling with a syringe bothers you, there are also applicators which are easier to handle. Sublimating oxalic acid is as efficient and safe for the bees, too. However, the user safety isn’t as high as with trickling. During sublimation, very small particles of oxalic acid disperse in the air. If you breath them in, they get deep into your lungs. That’s definitely something to avoid. So, if you prefer to sublimate, take the appropriate security measures: a face mask as indicated on the label of the product you’re using is mandatory.

With applicators like this, you can “load” oxalic acid for several colonies. It’s also easier to read the scale and to distribute the solution in a regular manner.

There are also some ways of doing oxalic acid treatments I definitely wouldn’t recommend. The first one, spraying, for a very simple reason: It’s time-consuming and the right dose is difficult to obtain. When spraying oxalic acid, you take each comb out and spray the solution directly on the bees. To obtain the right dose, you have to try out how many times you have to spray to obtain a certain amount of the solution. So it takes some effort to get it right. The only product that is registered for this use in the EU, Oxuvar 5.7%, describes this very well on the label. However, it remains very time-consuming.

When to treat with oxalic acid

Now that you know how to treat, the next step is knowing the right moment. Oxalic acid acts only on the mites on the adult bees. Therefore, it’s a treatment for colonies without brood. This can be a natural break in winter or during drought periods in summer. But it’s also possible to produce an artificial break by caging the queen. Note that these aren’t the cages you use when you send queens by mail. They’re larger cages with a grid like the queen excluders. By this, the workers still tend the queen, but she doesn’t move around on the combs and isn’t able to lay eggs. She stays in the cage for 25 days, to make sure every brood cell, even the drone brood, hatched. Then you release the queen and treat by trickling.

Cages like this keep the queen from laying. The artificial brood interruption helps with oxalic acid treatments when brood is naturally present.

Caging the queen, in my opinion, will get more important in the future. In many regions, honey bee colonies now also breed in winter due to the global warming. The presence of brood reduces the efficacy of oxalic acid because it doesn’t act on the mites in the brood cells. Varroa mites reproduce in sealed brood cells. In consequence, with brood there remain more mites in the colony than without brood. Beekeepers may feel secure because they see mites falling after the treatment. But it’s much more important what remains in the colony. Later, in the active season, the infestation will double every month. That’s why you want to reduce the infestation during winter as much as possible. In the future, to maintain the high efficacy of oxalic acid, producing artificial brood breaks may become necessary even in winter.

Sweet and sour – misunderstandings about oxalic acid treatments

The first available recipes or products for treating against varroa with oxalic acid by trickling always indicated that the solution must be prepared with sugar (sucrose, to be exact, the same sugar you use in cakes or your coffee). Beekeepers often think that it’s for making it more attractive for the bees. Bees like it sweet and oxalic acid is sour – so obviously you have to sweeten it up. Others think that it’s to make the solution sticky so that it stays on the bees for longer. Both thoughts may seem plausible. But they’re wrong.

The real reason is the sugar helps the oxalic acid to stay sour. Let me explain: Acids maintain their acidity only when they’re liquid. Oxalic acid as a powder (i.e. its solid form) isn’t “sour” in a chemical sense. That’s why you have to make a solution before trickling. Because the important part in in “oxalic acid treatments” is the acid. It’s the acidity that kills the mites. OK, you may say, but why the sugar? Easy: The sugar helps the oxalic acid solution to stay liquid.

Imagine a pot of sugar that you leave without a cover on your kitchen counter. After a while it gets clumpy, doesn’t it? That’s because sugar is “hygroscopic”, meaning that it attracts and holds water. In the oxalic acid solution it “holds” the water, the drops don’t dry as fast. In studies comparing the efficacy, solutions without sugar killed only about 50% of the mites. With sugar, the oxalic acid treatments reached consistently over 90% efficacy. Sugar, therefore, is a “helping agent” to increase the efficacy.

An issue and a solution

There’s a caveat, though. Oxalic acid solutions with sugar don’t remain stable for long time. After a few days, the solution gets brownish. This is a sign that the sugar degraded in presence of the acid. The concentration of HMF, a substance that results when sugar degrades in an acid environment, increases. This substance is toxic to bees, though it’s not clear if it can harm bees during the short exposure like during a treatment. However, better safe than sorry: Only prepare the amount of solution you need and use it up within two days after preparation. It should be stored cool and dark to slow down the degradation process.

This issue has a solution. As I already explained, the sugar is a helping agent keeping the solution liquid for a longer time. So, every other substance that has this hygroscopic properties could be used. The solution was using glycerol. It’s a common substance in the food industry or in cosmetic products like soaps. And it’s also able to hold water. Studies by Antonio Nanetti from the CREA in Bologna and his group showed that it has the same effect on the efficacy of oxalic acid treatments like sugar. The bees tolerate it well. I still remember when I came to his office one day and he showed me two bottles with the remaining solutions of his experiments: the solution with sugar was brown, while the solution with glycerol was still perfectly clear, indicating the higher stability.  

Why you should know the science behind oxalic acid treatments

If you got this far: congratulations! It’s for a reason I told you all this science stuff: I want you to make good and informed decisions. Oxalic acid is the most efficient treatment we have against the varroa mite. Also the most flexible one. Beekeepers worldwide use it, in hot or cold regions alike. However, with it’s reputation comes also a lot of myths. Like the “sugar because it becomes sticky”. Myths about varroa treatments or any other sanitary issues do harm. They oversimplify, create false expectations and paint the world in black and white. Hence inviting to DIY “products” or treatments done wrong. Which harms the bees, the beekeeper by losing his colonies, and the environment by spreading honey bee diseases. Varroa treatments are medicinal products. And therefore need to be used correctly. The only way to encounter these risks is knowledge.

This post is part of my mission to improve honey bee health. I’m deeply convinced that science-backed good practices are the only way to do so. For this, bee science and its applications needs to be accessible. I do so by this blog, articles in magazines, talks, and courses. If you’re interested in going deeper into the subject, contact me by the form on the top of this page.


  • Thank you Claudia,

    for these interesting facts about oxalic acid treatments for bee colonies. I didn’t know until now that you should do it across the combs so that the drops stay on the top frames. I have always done it parallel to the frames in the spaces exclusively on the bees, as I have learned in the past.

    Your comments on sugar are also very interesting for me.

    I still know the applicator you recommended from the treatment with BAYER Perizin in the 80s. Unfortunately, I no longer have this applicator. Can you give me a tip where I can buy this applicator? Thank you very much!

    With kind regards

      • It’s described in the label of the products. Don’t use just oxalic acid powder, as the concentration is important! In the registered products you get the right amount and the exact description.

  • Sehr geehrte Frau Dr. Garrido,

    mich interessieren Ihre weiteren Ausführungen zu der Varroa und ihrer Bekämpfung.
    Ich wäre Ihnen dankbar, wenn Sie mir einige übermitteln würden.

    Mit freundlichen Grüßen

    Franz Kasper

    • Sehr geehrter Herr Kasper,
      Ich arbeite gerade an einem kleinen ebook, dass ich hier anbieten werde. Solange bitte ich Sie um Geduld. Zur Varroa habe ich hier aber schon viel geschrieben, Sie finden oben auf der Seite ein Suchfeld – einfach Varroa eingeben.
      Viele Grüße,
      Claudia Garrido

  • What is the proper ratio for mixing oxalic acid ? The article says everything except the recipe.

    • For the simple reason that I don’t want beekeepers to mix up something themselves. They should use registered products. You don’t mix up your medicine yourself, don’t you? Varroa treatments are medicine.

  • Hello, how would you recommend dribbling the solution over a top bar beehive? I can’t do transverse as the bees don’t have access to the top of the bars.

    • Top bar hives are difficult – just because it’s nearly impossible to treat them. Trickling isn’t possible in this case, you could go for sublimation. Or for strips with synthetical compounds.
      That’s why I usually don’t recommend top bar hives. As soon as a colony is in human hands, it’s not “natural” anymore and you have to take care that they stay healthy.

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