Arising amitraz and pyrethroids resistance mutations in the ectoparasitic Varroa destructor mite in Canada

The ectoparasitic mite Varroa destructor remains a great threat for the beekeeping industry, for example contributing to excessive winter colony loss in Canada. For decades, beekeepers have sequentially used the registered synthetic varroacides tau-fluvalinate, coumaphos, amitraz, and flumethrin, leading to the risk of resistance evolution in the mites. In addition to the widespread resistance to coumaphos and pyrethroids, a decline in amitraz efficacy has recently been reported in numerous beekeeping regions in Canada. The goals of this study were to assess the evolution of resistance to amitraz in Canadian mite populations and to evaluate the presence and incidence of mutations previously associated with resistance to amitraz and pyrethroids in V. destructor. Our bioassay results confirmed the presence of amitraz-resistant mites in the population of Alberta. These phenotypic results were complemented by targeted genotyping of the octopamine receptor gene Octβ₂R which revealed the presence of the mutation Y215H in 90% of tested apiaries with local allele frequencies ranging from 5 to 95%. The phenotypic resistance showed a significant correlation with the presence of this mutation across apiaries. In parallel, the L925I and L925M mutations in the voltage-gated sodium channel were identified in 100% of the tested apiaries with frequencies ranging from 33 to 97%, suggesting that resistance to pyrethroids remains widespread. These results support the notion that the practice of relying on a single treatment for a prolonged period can increase rates of resistance to current varroacides. Our findings suggest the need for large-scale resistance monitoring via genotyping to provide timely information to beekeepers and regulators. This will enable them to make an effective management plan, including rotation of available treatments to suppress or at least delay the evolution of resistance in V. destructor populations.