Comparison of oxalic acid drip and HopGuard for pre-winter Varroa destructor control in honey bee (Apis mellifera) colonies

Varroa destructor is a primary cause of honey bee colony death in the US. Multiple chemical treatment options exist, but their effectiveness can vary due to factors including ambient temperature and reproductive cycles. Thus some treatment options may be preferable to others at specific times of the year. Here, we tested the efficacy of a late season oxalic acid drip treatment and an application of HopGuard strips, compared to an untreated control. Resulting changes in mite populations were measured immediately after treatment application, the following spring during almond pollination, and again upon return to Washington State. Immediately after treatment application, both oxalic acid and HopGuard treatments resulted in a greater increase in mite mortality compared to the untreated control. In almonds, both oxalic acid and HopGuard groups had lower mite loads than the untreated control and were still below the economic threshold of 3 mites per 100 bees. By March, mite loads had evened out between groups, indicating that spring may be an important time to reevaluate mite loads and apply another treatment. There was no difference in colony size or mortality between the groups at any time throughout the study.