The parasitic mite, Varroa destructor, is the most important and ubiquitous threat to honey bees worldwide. Varroa and the viruses they transmit account for a significant majority of colony losses. Synthetic miticides such as amitraz, tau-fluvalinate, and coumaphos are the primary tool beekeepers use to control Varroa due to their ease of use, reasonable costs, and efficacy. However, overuse and overreliance on miticides can lead to resistance. While widespread treatment failure with tau-fluvalinate and coumaphos were documented shortly after their introduction, amitraz resistance has only recently been cause for concern with many anecdotal but unconfirmed cases. Beginning in 2019, two methods were used to document amitraz resistance in Varroa in US beekeeping operations. The glass vial bioassay used technical grade amitraz to determine the amitraz LC50 while the Apivar® efficacy test evaluated resistance to a formulated product in a cage test. There was a significant negative correlation between the amitraz LC50 and the Apivar® efficacy test, demonstrating that reduced miticide treatment success was due to resistance to technical grade amitraz. Resistance monitoring in 2020 and 2021 was performed using the Apivar® efficacy test through a combination of on-site visits and remote sampling in coordination with beekeepers, apiary inspectors, and university colleagues. Over 3 years of monitoring, we have sampled 674 colonies across 62 apiaries from 44 beekeeping operations. Approximately 25% of all colonies and apiaries sampled had levels of Apivar® efficacy that are associated with treatment control failure at the colony level (i.e., &lt;70%). We collected Varroa that were phenotyped with the Apivar® efficacy test for genomic analyses. A total of 498 individual Varroa genomes were sequenced to perform a Genome Wide Association Study (GWAS) to identify loci associated with amitraz resistance and population genetics analyses to identify the origin and evolution of amitraz resistance. Mutations in the beta2-octopamine receptor were strongly associated with amitraz resistance. Population substructure analyses are still ongoing but preliminary evidence shows multiple origins of amitraz resistance. These molecular markers are undergoing development with collaborators to develop a high-throughput diagnostic pipeline to provide beekeepers with fast, reliable, low-cost, and actionable information on which miticide is most effective based on Varroa genotype.

Amitraz resistance in varroa destructor: Insights from monitoring and genomics

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.