Treating newly split <i>Apis mellifera</i> honey bee colonies with organic miticides—an opportunity for Integrated Pest Management of <i>Varroa destructor</i> mites (Mesostigmata: Varroidae)

Parasitism from Varroa mites (Varroa destructor Anderson and Trueman, Mesostigmata: Varroidae) is a major driver of honey bee colony losses (Apis mellifera L., Hymenoptera: Apidae). While synthetic miticides are valuable for Varroa management, high reliance on these compounds has selected for miticide-resistant Varroa populations. To enable more sustainable Varroa management and provide options when synthetic miticides such as amitraz fail, effective Integrated Pest Management (IPM) approaches are urgently needed. Here, we show that organic miticides (oxalic acid, "OA"; and hops beta acids) can achieve high efficacy against Varroa when strategically combined with a widely used cultural control (starting new colonies, "splits," with queen cells). This common splitting practice acts as a cultural control by temporarily reducing the amount of brood (developing bees) in colonies. This forces Varroa mites out of the protected environment of brood cells and on to adult bees-where they can be more effectively targeted with miticides. Based on Varroa infestation rates of adult bees, we determined that the organic miticide treatments OA dribble (75.5% efficacy), 5× OA dribble (82.2%), and HopGuard (82.7%) were significantly more effective than no treatment and provided comparable efficacy to amitraz-based miticides. In contrast, OA vapor (44.3%) did not provide effective Varroa control. Based on observations of queen and colony success, colony strength measurements, and hive weights, none of the organic miticides showed signs of harming colonies. Therefore, this combination of cultural and chemical control provides an additional opportunity for beekeepers to implement IPM for more effective and sustainable Varroa management.