Temperature affects assessment of amitraz resistance in Varroa destructor

Authors: Rinkevich, Frank

Submitted to: Bee Culture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2023
Publication Date: N/A
Citation: N/A
DOI: https://doi.org/10.55406/ABRC.23

Interpretive Summary: Measuring amitraz resistance in the major honey bee parasite, Varroa desctructor, is an important issue in commercial beekeeping in the U.S. However, it is important to understand the environmental conditions under which this test needs to be performed so the results can be interpreted correctly. Assays with control cages showed that Varroa fell of honey bees at temperatures >30°C at a high rate. At temperatures <20°C, Apivar was less effective at removing Varroa compared to higher temperatures. Bioassays with technical grade amitraz showed that amitraz became less toxic at low temperatures. The reduced amitraz toxicity is likely due to reduced metabolic activation at low temperatures as amitraz needs to be hydrolyzed to DMPF to become toxic. These results show that the amitraz resistance test needs to be performed at 20-30°C to yield accurate results.

Technical Abstract: Amitraz resistance in Varroa destructor is a phenomenon that reduces the efficacy of amitraz to control Varroa. Surveys have shown an increasing trend of amitraz resistance in many beekeeping operations. Amitraz resistance in Varroa is measured using the Apivar efficacy test, which is a consistent predictor of treatment success or failure at the colony level. To ensure the reliability of the test, we investigated how temperature affects the outcomes and interpretation of the Apivar efficacy test. The Apivar efficacy was run at a range of temperatures from 10-35°C in cages with Apivar and control cages. At temperatures higher than 30°C, the proportion of Varroa that fell off the bees under control conditions increased with temperature. In cages with Apivar, the Apivar efficacy was reduced and much more variable with lower temperatures. This result was followed with Varroa bioassays with technical amitraz at the same temperature ranges. Amitraz became less toxic at lower temperatures, thus corroborating the reduced Apivar efficacy at lower temperatures. These results show that the Apivar efficacy test should be performed between 20-30°C in order to provide consistent results. Considerations on past and future amitraz resistance monitoring efforts are discussed.