Key Points:

• Preliminary overseas work suggests that a vapouriser temperature around 230°C is ideal.
• Sealing hives for at least 10 minutes after treating with oxalic acid vapour improves mite control.
• Figuring out the treatment time for your vapouriser model is essential for efficiency and efficacy.

Why Oxalic Acid Vapourisation is gaining attention

With synthetic miticide resistance spreading in Varroa mites across Australia, beekeepers are increasingly looking to non-synthetic treatments as part of their Varroa management plans. Oxalic acid (OA) vapourisation is one option attracting more attention. However, non-synthetic does not lower risk. Any treatment has the potential to cause harm if used poorly. This article does not cover risk mitigation for the beekeeper, nearby people, or the bees. The applicator is responsible for assessing those risks and managing them appropriately before using OA vapourisation.

The following points are based on information discussed during a 2025 visit to the University of Florida, including discussion with Dr Cameron Jack and Marie Yanchak.

Understanding how Oxalic Acid Vapourisation works

First, some background. Oxalic acid is a solid at ambient temperature. Vapourisation is simply a way of delivering it into the hive. Initially, OA vapour appears to trigger grooming behaviour in bees, which may help disturb mites. Importantly, as the vapour cools, it turns back into a solid and deposits OA crystals throughout the hive. These crystals provide ongoing Varroa management after the application itself is complete.

Vapouriser temperature matters

Temperature is one of the most important factors. If the vapourisation temperature is too low, the OA may not fully vapourise. This can mean the hive receives less than the intended dose, or that treatment takes much longer than expected. If the temperature is too high, OA can break down into formic acid and other products. This again means less OA reaches the colony and may also increase the risk of negative impacts on bee health. Initial work from Jack and Yanchak suggests a vapouriser temperature around 230°C may be the best balance. Different vapourisers vary greatly in how they measure and control temperature, and some do not do either.

Determining the correct treatment time

Treatment time also needs to be measured. The aim should be to continue applying until all OA crystals have vapourised. The practical way to determine this is to measure the time required for complete vapourisation for your specific vapouriser model, temperature setting and OA dose. That measured time then becomes your treatment time.

The importance of sealing the hive

Sealing the hive is another critical step often skipped, especially in commercial operations under time pressure. But the more OA vapour that stays in the hive long enough to cool and recrystallise, the more of the intended dose remains in the hive rather than escaping. One useful approach when treating each hive for the first time is to apply half the dose, check for leaks, seal them, then apply the second half. On later treatments, the full dose can usually be applied with less risk of leakage. Hives should be sealed for at least 10 minutes post-treatment, or as directed by product label.

Airflow and vapour delivery

Finally, consider how the vapour is moved into the hive. Some vapourisers rely on gas expansion in a sealed chamber, while others use a blower fan. The choice of vapouriser is personal, but the total volume of air and vapour pushed into the hive matters. Too much volume may displace air from the hive and carry OA vapour out with it. The goal is to get as much of the planned OA dose into the hive as vapour and keep it there long enough for crystals to re-form.