Experimental parameters affecting the outcomes of amitraz resistance testing in Varroa destructor

Abstract

Honey bee colony health is impaired by a large variety of biotic and abiotic factors. The severe impacts of Varroa destructor and viruses associated with it on honey bees are undeniable and universal. Beekeepers often utilize miticides to mitigate V. destructor populations, but resistance to many widely used synthetic miticides such as amitraz has developed. The Apivar® resistance test has proven to be a valuable tool to monitor for amitraz resistance in V. destructor. However, a more thorough understanding of the effects that changes in testing parameters have on the outcome of the Apivar® resistance is critical for an accurate and comparable interpretation of the results. In this project, the effects of temperature, product age, product exposure surface area, and container reuse on the Apivar® resistance test were studied. High temperature significantly increased control mortality while low temperatures significantly increased Apivar® resistance. The increased Apivar® resistance at low temperature was due to reduced amitraz toxicity at low temperature. There was no association of Apivar® age or manufacturing batch on the outcome of the Apivar® resistance test. The maintenance of Apivar® efficacy with age is likely because the amitraz breakdown product N‘-(2,4-dimethylphenyl)-N-methylformamidine (DPMF) is as toxic as amitraz. The rate and efficacy of the Apivar® resistance test were higher with a larger surface area of Apivar® strip. Reused testing containers or Apivar® strips from colony application yielded reduced efficacy and consistency. This research shows the standard conditions needed to produce comparable data from the Apivar® resistance test. Implications for amitraz resistance monitoring are discussed.