Amitraz is the active ingredient in some of the most utilized miticides in the world such as Apivar and Apitraz. Popular due to its low cost, ease of use and seaming effectiveness, its history as a varroa control goes back almost 30 years. For years however, beekeepers have reported diminishing results, proclaiming that mites were becoming resistant to the synthetic chemical. Such reports had not been proven, as no study could qualify the claim without first eliminating the other possible contributing factors. In a first-time study, researchers have successfully demonstrated this claim.
The objectives of the 2020 study as outlined by Frank D. Rinkevich, were to
"establish baseline data on an amitraz-sensitive Varroa population,
identify the prevalence and intensity of amitraz resistance in commercial beekeeping operations with a history of amitraz use, and
verify that reduced Apivar® efficacy is correlated with amitraz resistance" (Rinkevich 2020).
The study examined varroa collected from commercial beekeeping operations in South Dakota, Louisiana, and New York, all of which had been treated with Amitraz for 3+ years in succession. Researchers examined lab population bioasssays and compared the results to the amitraz control efficiency rates found in the commercial apiary sourced varroa mites. Resistance ratios from in vitro amitraz bioassays were correlated with reduced Apivar efficacy. This result directly links cases of varroa mite control failure to amitraz resistance.
Rinkevich concluded that, "…amitraz can be an effective Varroa control product in many operations. However, apiaries across operations displayed a wide range of amitraz resistance from no resistance to high resistance that resulted in Varroa control failure" (Rinkevich 2020).
For experts in this field, such results are not so surprising. Resistance is not an uncommon phenomenon for many arthropod species, and can occur rapidly.
Amitraz now joins the list of other commonly used synthetic miticides tau-fluvalinate and coumophos that have proven resistant to varroa control.
With Amitraz also being linked to increased viral susceptibility and reduced cardiac function (O’Neal, Brewster Bloomquist, Anderson), the question can again be asked: how long should beekeepers keep relying on this type of varroa treatment?
Resources
Detection of amitraz resistance and reduced treatment efficacy in the Varroa Mite, Varroa destructor, within commercial beekeeping operations
Frank D. Rinkevich, January 17, 2020
https://doi.org/10.1371/journal.pone.0227264
Amitraz and its metabolite modulate honey bee cardiac function and tolerance to viral infection
Scott T O'Neal, Carlyle C Brewster, Jeffrey R Bloomquist, Troy D Anderson, 2017
https://pubmed.ncbi.nlm.nih.gov/28797906/