Amitraz resistance in French Varroa destructor populations: geographic distribution, association with N87S, and changes in resistance phenotype after amitraz withdrawal

Varroa destructor remains a major threat to honey bee health, and amitraz is widely used for its control in France. However, increasing treatment failures suggest emerging resistance. This study investigated the spatiotemporal distribution of phenotypic amitraz resistance in French V. destructor populations, its association with candidate resistance mutations, and its dynamics following amitraz withdrawal in a longitudinally monitored experimental apiary. Phenotypic resistance was assessed using an in vitro bioassay based on corrected mortality at Lethal Concentration 90. From 2018 to 2025, 184 apiaries across France were classified as susceptible, moderately resistant, or strongly resistant. TaqMan assays targeting mutations (N87, Y215, F290) in the β-adrenergic-like octopamine receptor were applied to selected populations (2019–2023). Resistance dynamics were also monitored in the experimental apiary subjected to amitraz treatments (2018–2022) followed by alternative treatments (2023–2025). Susceptible populations predominated in early years, whereas resistant populations became increasingly frequent over time. Among the targeted mutations, only N87S was detected, with frequencies rising across resistance classes (0.03, 0.21, and 0.39 in susceptible, moderately resistant, and strongly resistant populations, respectively). In the experimental apiary, resistance increased under repeated amitraz use and partially declined after withdrawal, without returning to full susceptibility. Overall, these results highlight the widespread nature of amitraz resistance in French Varroa destructor populations and suggest that its dynamics could partly reversible following changes in treatment pressure.