Assessing Phenotypic and Genotypic Resistance to Flumethrin in Varroa destructor Populations in Muğla, Türkiye

Beekeepers use a variety of methods to control <i>Varroa destructor</i> (varroa). Chemical control relies heavily on flumethrin, amitraz, coumaphos, and tau-fluvalinate products. However, increasing colony losses in recent years have been linked to the development of resistance in varroa mites to these insecticides. Varroa mites develop mutations in the voltage-gated sodium channel (VGSC) that confer resistance to pyrethroids such as flumethrin. Specifically, researchers have identified substitutions of the leucine amino acid at VGSC L925 with isoleucine, methionine, or valine. This study investigated phenotypic and genotypic resistance to flumethrin in varroa populations in Muğla, Türkiye. LD₅₀ values (lethal dose for 50% mortality) were quantified, and PCR and sequencing were used to analyze the VGSC L925 gene region. The PCR results confirmed mutations in the target gene region in all samples. Sequencing revealed that 95% of the population carried homozygous resistant alleles, while 5% were heterozygous. At the VGSC L925 locus, leucine was replaced by isoleucine (91%), methionine (6%), and valine (3%). Phenotypic assays showed an average LD₅₀ value of 49.1 µg (range: 31-61.8 µg). Comparison of LD₅₀ between resistant and susceptible populations was not possible because no susceptible individuals were identified. Despite the resistance, mortality increased with escalating doses, suggesting that current protocols may be temporarily mitigating infestations. However, urgent dose adjustments and alternative control strategies are critical to prevent imminent colony collapse.