A laboratory amitraz resistance assay for Varroa destructor may not be a reliable predictor of field-level amitraz efficacy

Amitraz is used by beekeepers to control Varroa destructor infestations within their colonies worldwide. Evidence for resistant mites continues to be reported and could be a major factor in widespread colony losses. Over 2 years, we investigated a diagnostic bioassay that tested our mites for susceptibility to amitraz. After diagnosing mites with the bioassay, we genotyped the mites searching for the Y215H mutation of the β2-octopamine receptor. We found that the diagnostic bioassay accurately predicted the resistant genotype 94.6% of the time, and it accurately predicted the susceptible genotype in 66.8% of mites. Additionally, we performed a standard treatment with the product Apivar ® (active ingredient amitraz 3.33%) on the source colonies of these mites. Colonies receiving the Apivar ® treatment ( n = 32) were categorized by the expected efficacy of amitraz on their mites, and a group of colonies was designated as untreated controls ( n = 12). Both high and low expected efficacy groups had significantly reduced mite infestations by the end of the treatment period (reduced by 98.2%) compared with control colonies, which did not have a significant change in mite numbers over the same period (reduced by 23.0%). The diagnostic bioassay was a strong predictor of the resistant genotype of V. destructor , but it was not an accurate predictor of field-level amitraz efficacy.