We have finished our round of winter Varroa treatments. To treat our bees for Varroa, we have been using Oxalic acid vapour. Oxalic acid is an effective treatment with an efficacy of over 95%. This means that a single treatment can kill 95% of the mite population in the hive. Unfortunately, it is only effective on phoretic mites and has no effect on mites within the brood. Because of this, treatments are usually done during broodless periods and the winter is one of the best opportunities.

A frame with signs of Varroosis and a bee with a phoretic varroa mite on its back

We also use Oxalic acid as an autumn treatment. As brood is present within the hive during the autumn period, we administer four treatments, each one a week apart. As the Oxalic crystals can stay present in the hive for more than a week, these repeat treatments mean that we cover a whole brood cycle. This is important as the acid has no effect on mites in the brood and this sustained treatment period means that mites will be exposed to the acid as they emerge.

How does it work

The acid is dispersed through the hive by heating it in a vaporiser where it will re-crystallise across the inside of the hive. It will then be spread further by the bees as they move around the hive. Any mites that come into contact with the acid. It is thought that the acid causes damage to the mites feet and mouth parts which means that they can no longer grip or feed. How ever it works, it has a massive effect on the mite population with evidence suggesting that the varroa mite will not develop any resistance.

Api-bioxal, an Oxalic acid based treatment for varroa mites on honey bees

Oxalic acid vapour is considered to be less harmful to bees during administration than trickle treatments. This is because it has no harmful effects on the bees and can be administered without opening the hive. It does, however, present a greater risk to the beekeeper and the appropriate safety equipment must be worn.

Oxalic vaporisers

There are many Oxalic vaporisers on the market, at a range of costs. Many are reliant on a battery pack that is used to heat a dish with a measured amount of acid in. The long wand on these allow the beekeeper to keep a good distance. However, the battery packs make these units heavy and difficult to lug around.

Oxalic acid fogger used for admistering varroa treatment to bees

Foggers have also become common as a means of delivering Oxalic acid. (A fogger works by heating an Oxalic acid solution (such as oxalic acid and distilled water) and forcing it into the hive as a mist. The effectiveness is similar to that of vaporising and gas powered foggers are lighter and easier to use than electric vaporisers.

We administer Oxalic acid with a gas powered vaporiser. This involves heating a chamber with a gas torch and then adding a measured amount of acid in a treatment cup. As the acid is vaporised it is forced out of the outlet into the hive. We use a vaporiser called Gas-Vap which is an inexpensive and light weight unit which we have found very effective. We will be writing a review on it shortly.

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