Managing amitraz resistant <i>Varroa destructor</i> mites in honey bee ( <i>Apis mellifera</i> ) colonies: effects of winter treatment with extended-release oxalic acid

The parasitic Varroa destructor (Varroa) mite is the foremost cause of honey bee (Apis mellifera) colony losses worldwide. To combat Varroa in the United States, beekeepers frequently use the synthetic acaricide amitraz. However, continued reliance on amitraz-based treatments has led to the emergence of amitraz resistance in Varroa mite populations. This experiment aimed to determine whether a treatment with a non-amitraz acaricide during winter could provide better efficacy against amitraz resistant mites than an amitraz based treatment. In coordination with a commercial beekeeper with known amitraz resistant Varroa populations in the southeastern United States, we tested two extended-release oxalic acid treatments – a commercial formulation (Aluen CAP®) and a homemade formulation (oxalic acid and glycerin in absorbent pads – OA Pads) – as alternatives to amitraz applied as Apivar® during early winter (November). The OA Pads treatment had significantly higher Varroa infestation rates on Day 67 (mid-January) compared to Aluen CAP® and Apivar®. In February, three months after treatments were applied, the OA Pads treatment again had higher Varroa infestation rates compared to Apivar®, indicating that homemade extended-release oxalic acid may be an inferior treatment in early winter. Meanwhile, Apivar® was effective despite amitraz’s average resistance levels of 38.5% in Varroa populations. For colony strength, Aluen CAP® had a negative effect on adult bee population throughout the study and had numerically lower survival by early spring (February) compared to the OA Pads and Apivar® treatments, signaling a potential negative impact of this treatment when used over winter.