Varroa destructor, an ectoparasitic mite of the western honey bee (Apis mellifera), and the viruses it vectors are the most important factors driving high rates of honey bee losses in the United States. Unfortunately, mites developed resistance to some of the pesticides, which creates a burden on beekeepers to keep their colonies healthy. Despite this threat to honey bees, we still know little regarding some of V. destructor’s basic biology. In Chapter 1, I describe the method of a novel system to measure the width and length of the ventral side of V. destructor, which allowed me to assess the size variability of V. destructor in the United States. This chapter is an observational epidemiology study on mite size and its association with year, the time of year, mite density in host apiaries, the virus load of the host apiary, and pesticide exposure in host apiaries. I also conducted a series of experiments to determine whether the mite size findings were biological or due to an experimental artifact. I’ve found a seasonal variation in mite size that is possibly driven by external pressures, arguably acaricide exposure or the diet received by feeding on different developmental hosts. Chapter 2 of my dissertation is the continuum of Chapter 1. I tested the size of mites as a confounding factor in their tolerance to amitraz of mites collected from a field trial and a toxicological bioassay. The field trial and the toxicological assay result suggest that amitraz sufficiently kills smaller mites. Lastly, Chapter 3 investigates how tolerance to acaracide exposure, through feeding on different developmental hosts, affects mites' survival to pesticide exposure or stress. To achieve this, I placed mites to feed on adult or pupal honey bee hosts before exposing the mites to various pesticides. I found that the mites placed on pupa had the highest survival rate 20 hours after a 4-hour exposure to pesticides in toxicological bioassays. Then measured the activities, and two key detoxifying enzymes, Glutathione S-Transferase and Cytochrome P450, were significantly higher in mites that survived the assays. In addition, mites fed on adult bees had a higher activity level of acetylcholine esterase than mites placed on the pupa. From proteomic analysis, I found that mites placed on pupae prior to pesticide exposure had higher levels of stress-induced proteins (heat shock proteins). However, living mites had higher amounts of honey bee proteins, suggesting a more recent feeding event and perhaps a more beneficial nutritional state. Interestingly, surviving mites specifically contained significantly larger amounts of honey bee antioxidant proteins, suggesting the use by V. destructor of host proteins for their survival.These findings contribute to the literature on V. destructor size variability and provide new information on pesticide resistance. My findings highlight the need to factor in size and feeding state when conducting toxicology bioassays. It also provides new insight for future research on Varroa feeding.