General Information

A major challenge for beekeepers is the control of varroa mites. This tiny mite, measuring one to two millimetres in size, is the bee’s most dangerous enemy. They colonise and breed in the brood, hide between the bee’s plates and suck their body fluids, reducing their fat and spreading viral infections through their stings. Their presence is accompanied by deformed, diseased bees and without treatment the colony is doomed to die. The varroa mite is everywhere, whether you have one or hundreds of hives.

There are several methods of controlling the mite, one of the most rapidly spreading being oxalic acid sublimation or vaporisation. In this process, crystalline white oxalic acid, which resembles powdered sugar in consistency, is sublimed at high temperatures (when the solid substance immediately takes a gaseous state skipping the liquid state) and injected into the bee hives, where it precipitates on the frames and bees, killing the mites. The mechanism of how oxalic acid kills mites is not yet known. What has been demonstrated is that oxalic acid vaporisation is effective against varroa destructor mites and, if applied correctly, is not harmful to bees or honey. It evaporates from the honey in 2 weeks and the honey regains its natural oxalic acid content. Oxalic acid is a natural substance found in many plants, including sorrel, although it is not harmless, as the name suggests: it is an acid. For this reason, appropriate protective equipment should be worn when handling and especially during treatment, in special closed clothing, a mask with a suitable filter (a suitable respirator should be a full face mask with at least ABE P3 combined filter pads) and chemical-resistant gloves. Oxalic acid vapour entering the lungs can cause serious damage even in small quantities and is absorbed through the skin! Vaporizers based on different principles of operation can be purchased, ranging in price from a few bucks to over a thousand. They all have the same basic principle: the oxalic acid is heated to the right temperature and the vapor is injected into the hive. Beekeepers are divided on how to vaporize oxalic acid: at what temperature, in what quantity and how often it should be applied. In our current short description, we summarise as follows, based on our experience:

  • the amount of oxalic acid (oxalic acid 2-hydrate) used ranges from 1 to 3 grams per hive, although 1 gram is only enough for really small hives and 3 grams are needed for large hives. In general, it is worth vaporizing around 2 grams of oxalic acid per average hive.
  • Oxalic acid starts to sublime at 157℃, but at this temperature sublimation is slow, making the treatment lengthy and there is a high chance of precipitation inside the vaporizer. In practice, the recommended temperature range is 200-230℃.
  • There is indeed a wide divergence of opinion among beekeepers as to the frequency. It should be noted that the mite will remain in the covered hive for 11-12 days, and treatments until then are ineffective for them. Sublimation should be repeated both at intervals between spring sprays and in late summer and autumn, with each treatment cycle consisting of 5-7 scheduled treatments. It can also be used as a final treatment before winter, but the temperature must be above 10℃ to ensure that the overwintering cluster is in a loosened state. It is important that mite control and eradication is not just carried out routinely, but that the mite load of the colonies is measured by sampling and the bees are treated carefully according to the situation.