Most migration of varroa into colonies occurs in the late summer and fall. This was determined by measuring mite drop on sticky boards in colonies established with few or no mites. The mites on the sticky boards were assumed to have entered the hive on foragers from other colonies. When mites attach to foragers, they interfere with the bees’ homing ability and this could cause foragers to drift to other colonies and spread varroa.

In this study, we monitored varroa populations in colonies while determining the proportion of foragers entering and leaving hives with mites on their bodies. In our study, the proportion of adult bees with phoretic mites was not related to cell
infestation levels, but instead to the proportion of FWM [foragers with mites].

The transfer of mites to foragers is a shift in the mite behavior from attaching to nurse bees for reproduction to foragers for possible dispersal. The frequency of this behavioral shift seems to increase in the fall, and might occur for several reasons. There are indications that foragers carrying varroa have low returning rates to their own colonies. 

Viruses vectored by varroa that affect forager orientation causing them to drift could provide a mechanism for both the virus and the mite to disperse in the fall from colonies that are likely to die over the winter.

DWV and IAPV titers increase with the growth of the mite population throughout the season reaching their highest levels in the fall. Left untreated, these colonies collapse over the winter.

DeGrandi-Hoffman, Gloria, et al. "Population growth of Varroa destructor (Acari: Varroidae) in honey bee colonies is affected by the number of foragers with mites." Experimental and Applied Acarology 69.1 (2016): 21-34.
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