Factors contributing to the remarkable size variability of Varroa destructor in U.S. managed honey bee populations

Varroa destructor an ectoparasitic mite of the European honey bee (Apis mellifera), and the viruses they transmit, are the most important factors driving high rates of honey bee losses in the United States. In response to concern over the discovery of mites whose size (but not genetics), suggested they were V. jacobsoni, we conducted a survey of the varroa mite populations across the U.S. a total sample size of 9,065 collected in 2012-2013 and 2016-2018. We found that only 33 % in 20212-13 and 79 % in 2016-17 of mites collected in the surveyed years fell within the 95% confidence interval (CI) bounds of published V. destructor size. We also found a large shift in mite size over time: the mean body width increased 1,4 %, the body length increased 4,3 % from the years of 2012-13 to 2016-18. We explore several hypotheses to explain this shift, including shrinkage from long term storge, season of collection, colony disease state, and miticidal resistance.  Understanding the reason for size change may hold important clues for mite management in managed honey bee populations.