I don't remember seeing this before, but it reinforces the idea that mite populations spike in late summer or fall due to factors other than in-hive reproduction
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. Varroa populations are at their highest levels in the fall and
brood production is decreasing. There are fewer brood cells to infest so more mites are on
worker bees perhaps including foragers
Left untreated, these
colonies collapse over the winter. 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.
Population growth of Varroa destructor (Acari: Varroidae) in honey bee colonies is affected by the number of foragers with mites
Gloria DeGrandi-Hoffman, et al. Exp Appl Acarol (2016) 69:21–34
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