Timing of oxalic acid extender pads is critical for Varroa mite control

The parasitic mite, Varroa destructor, is the primary management concern for beekeepers worldwide since they vector disease (mainly viral pathogens), resulting in diminished colony health, productivity, and longevity. Beekeepers most frequently use synthetic acaricides to minimize the impact of Varroa, but the mites have developed pesticide resistance, making IPM alternatives paramount. Established application methods of dribbling or vaporizing an organic pesticide (oxalic acid) can reduce mite levels but have limited efficacy outside of broodless periods, making oxalic acid less reliable as a pulse or knock-down treatment. Delivering oxalic acid through an extended-release system has proven effective, but results remain equivocal. We tested this alternative method both in the early season (during honey production) and late season (when colonies are naturally decreasing their colony populations) in the southeastern US. We found that extended-release formulations of oxalic acid were more effective in the early season, but the same treatment was not as effective later in the season. Thus, this approach of Varroa control may best be used as a preventative (not therapeutic) method, thereby enabling beekeepers in the southeast US to incorporate it into their IPM Varroa management plans. Our findings will help beekeepers rotate with or minimize the use of synthetic acaricides, promote sustainable beekeeping for large and small operations, and maintain honey bee health with respect to Varroa parasitism.