Effects of release pattern and room ventilation on survival of varroa mites and queens during indoor winter fumigation of honey bee colonies with formic acid

This study examined the effects of indoor fumigation with formic acid on survival of honey bee, Apis mellifera L. (Hymenoptera: Apidae), queens and varroa mites (Varroa destructor Anderson and Trueman (Acari: Varroidae)). A relationship between cumulative formic acid concentration and varroa mite mortality was established for colonies subjected to high-concentration fumigation while held indoors at 2–4 °C during winter. We also examined the effects of the formic acid release pattern and room ventilation rate on queen loss and treatment efficacy during fumigation. Two experiments were conducted in a wintering building. In both experiments, room air had higher formic acid concentrations than hive air. In experiment 1, 50% and 95% of mites were killed when exposed to in-hive concentration × time combinations of 49 ppm × days (CT50 product) and 111 ppm × days (CT95 product), respectively. No queen loss was observed under either the increasing-concentration or constant high concentration fumigation pattern. In experiment 2, 33% of queens were lost when minimum ventilation was used with room air concentrations of 57 ± 8 ppm (mean ± SE), whereas no queens were lost in controls or colonies exposed to room air concentrations of 27 ± 8 ppm with variable ventilation. Queen loss was associated with peak in-hive formic acid concentrations >20 ppm, but not with CT product, suggesting that queens are affected by acute rather than chronic exposure to formic acid. Formic acid fumigation significantly reduced the mean abundance of mites under both minimum- and variable-ventilation treatments.