Oxalic acid vaporization: effectiveness against <i>Varroa destructor</i> (Mesostigmata: Varroidae) and safety for <i>Apis mellifera</i> (Hymenoptera: Apidae)

Oxalic acid (OA) is a promising tool for controlling varroa mites (Varroa destructor Anderson and Trueman), a devastating pest of the European honey bee (Apis mellifera L.). Applying OA as a vapor inside honey bee colonies has been gaining popularity among beekeepers. We conducted a 2-year field study to evaluate the effectiveness of OA in controlling Varroa and assess the potential negative impacts of OA on honey bee larval development. Different doses of OA (1, 2, and 4 g in 2021; 3 and 4 g in 2022) were applied once a week over 3 wks in August at Oregon State University's apiary in Corvallis, OR. Each experimental group included 8 honey bee colonies, with control groups receiving no OA. The Varroa levels in colonies treated with 1 g OA (the current label dose in the US) and 2 g OA were not significantly different from each other and the control group. In both years, 4 g OA effectively suppressed the Varroa population growth but also appeared detrimental to the honey bee larval development. Our findings suggest that while the legally permissible dose of 1 g OA per brood chamber in the United States is insufficient for effective Varroa control, higher doses (3 and 4 g) can significantly suppress Varroa population growth but may also increase larval mortality, highlighting a trade-off between Varroa control and colony health.