In response to Jim Moore:
 
It wouldn't be accurate to say that drones are the primary vectors of
Varroa mites in Apis mellifera.  Varroa reproduces in both the drone and
worker brood in the European bee, unlike in its original host, Apis cerana.
Furthermore, it is the workers that are more responsible for the spread
of the mite, both within and between hives, due to the workers' greater
numbers.
 
It can be argued that the fall is the better time to treat in cooler
areas,  simply because there is less brood of any type, thus less mite
reproduction. In addition, a greater proportion of the mites in the hive
at that time are on the adults and more susceptible to chemical treatment.
 
Regards,
Paul Gatien