My understanding was that late-season robbing-out of weakened, and dying colonies was the vector for reinfestation, and with the warmer falls and winters, those of us in the mid-Atlantic states of the USA have seen our varroa counts go sharply up well past drone eviction time.

Speaking as an old drone, I think it is bad enough that drones either die when mating or die because it's fall, it's not fair to also blame them for varroa reinfestations.

The proximate cause seems to be more due to the "treatment free" beekeepers letting colonies collapse.  I have been slightly successful in convincing them to use entrance reducers in fall, which helps even a very weak colony defend an entrance well, and once the colony dies out completely, the robbers do not bring back varroa with the honey they rob, so problem solved. 

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