Evaluation of late-season Varroa destructor treatments and their impact on amitraz resistant mite populations.

Varroa destructor mites are a leading cause of honey bee (Apis mellifera) colony losses worldwide. The acaracide amitraz has been among the most preferred Varroa treatments for more than a decade because of its high effectiveness and convenience of application. As a result, over reliance on amitraz has led to Varroa resistance to amitraz. Here, we evaluated U.S. registered Varroa treatments with differing active ingredients on Varroa infestation rates and their impact on amitraz resistant Varroa to identify alternative treatment options for commercial beekeepers just prior to winter, a critical time of year to have low mite infestation. Treatment groups included an untreated Control, Apivar (active ingredient amitraz), FormicPro (a.i. formic acid), HopGuard 3 (a.i. hops β-acids) and Api-Bioxal vapor (a.i. oxalic acid, “OA” Vapor). Treatments were implemented in late September; colonies were assessed in late November and early February, 63 and 133 days later. We found that the Apivar, FormicPro and HopGuard treatment groups maintained Varroa infestation rates, whereas Varroa infestation rates significantly increased in the untreated Control and OA Vapor treatment groups. Genetic analysis showed that the frequency of the amitraz resistance genotype significantly increased from Day 0 to Day 63 after the application of Apivar, but this change was reversed by Day 133 after the overwintering period. Comparatively, the amitraz resistance genotype frequency was not impacted by any other treatment group. This indicates there is no cross-resistance of amitraz to other active ingredients tested, and that amitraz resistance may not have a large fitness cost.