Varroa mite (Varroa destructor) migration into honey bee (Apis mellifera) colonies affects mite population levels and control strategies

Varroa is a devastating pest of honey bees and responsible for significant colony losses each year. Varroa reproductive rates are relatively low so populations should grow slowly, but this is not the case. Based on measurements showing rapid growth in Varroa populations, we suspect that these populations might be increasing due to factors other than reproduction alone.  Varroa commonly move among colonies on foragers especially when they rob weak or failing hives. We measured migration levels of Varroa into colonies during summer and fall at two sites in Arizona to determine the effects on the mite population size in the colony. We found a population of mites on foragers that increased in the fall and had a significant positive relationship with the mite population size in the colony. Mite reproduction  in cells did not have a significant effect on Varroa population growth. The results suggest that migration can cause mite populations to reach levels that would not be achieved from reproduction alone.  The movement of mites among colonies needs to be considered when developing Varroa control programs.