Oxalic Acid
Lexikon
Oxalic acid is an approved active substance against the varroa mite and the usual active substance for the winter treatment, the so-called residual mite removal.
Oxalic acid is an approved active substance against the varroa mite and the usual active substance for the winter treatment, the so-called residual mite removal. Unlike formic acid, oxalic acid does not penetrate capped brood. It only reaches the mites sitting on the bees. From this follows the single most important rule: oxalic acid is trickled, sprayed, or vaporized only in a broodless colony; only a few preparations, such as strips for hanging in or the combination with formic acid, are also approved for colonies with little brood. In a colony with brood, most of the mites remain protected under the cell cappings, and the treatment largely fizzles out.
There are three ways to apply it: trickling into the occupied comb spaces between the frames, spraying onto the bees, and vaporizing, also called sublimation. The widespread claim that vaporizing is banned in Germany is outdated. Since September 2023, an oxalic acid preparation has for the first time been approved as a veterinary medicinal product in Germany that may explicitly also be used for vaporizing, though only in a broodless colony; as suitable vaporizers, the product information names the types Varrox and Varrox Eddy as examples. The Federal Office of Consumer Protection and Food Safety (Germany) points out that effectiveness depends strongly on the technique of the vaporizer, because oxalic acid decomposes instead of subliming if the temperature is too high.
Vaporizing produces fumes that are dangerous to humans. The approval documentation therefore requires long-sleeved clothing, acid-resistant gloves, safety goggles and a respirator mask, as well as attention to wind direction. Anyone who mixes their own solutions is acting outside the approved framework, and for vaporizers other than those named, the Federal Office has no data on effectiveness and tolerability.
A winter treatment is not a mandatory program but a decision based on infestation level. As a guideline, depending on the institute, a natural mite drop in late autumn of more than half a mite or more than one mite per day applies, above which residual mite removal is advisable. Unnecessary or repeated applications at this late point in the season harm the bees more than they help. Preparations differ regarding the withdrawal period: for the oxalic acid dihydrate solutions under the earlier standard approval, honey may only be obtained in the following spring; newer preparations state zero days and rule out use during the nectar flow or with a honey super in place. Every application of a veterinary medicinal product must be documented.
Sources (4)
- LAVES Niedersachsen: Was man über Varroxal 0,71 g/g Bienenstock-Pulver wissen sollte
- BVL: Hinweise zur Anwendung des Tierarzneimittels Varroxal
- LAVES Institut für Bienenkunde Celle: Leitfaden zur Varroa-Bekämpfung (PDF)
- Arbeitsgemeinschaft der Institute für Bienenforschung: Varroa unter Kontrolle (Broschüre, PDF)