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

## TL;DR

Varroa mites in Muğla, Türkiye, show high resistance to flumethrin, with most carrying gene mutations that reduce pesticide effectiveness, requiring new control strategies.

## Contribution

The study identifies widespread homozygous resistance alleles at the VGSC L925 locus in Varroa destructor populations in Muğla.

## Key findings

- 95% of the Varroa population carried homozygous resistant alleles at the VGSC L925 locus.
- LD50 values for flumethrin were significantly elevated, indicating strong phenotypic resistance.
- Resistance mutations included L925I (91%), L925M (6%), and L925V (3%).

## Abstract

Varroa destructor (varroa) is a major parasitic threat causing significant colony losses around the world. While flumethrin remains a widely used acaricide, its efficacy has declined substantially due to evolving pesticide resistance, exacerbating colony and yield losses. This study evaluates flumethrin resistance through LD50 analysis and mutational profiling of the voltage-gated sodium channel (VGSC) L925 locus. Our findings reveal dramatically elevated LD50 values compared to previous studies, indicating widespread resistance. Notably, 95% of the population carried homozygous resistant alleles (L925I/M/V), underscoring the urgent need for alternative control strategies.

Beekeepers use a variety of methods to control Varroa destructor (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.