Jim,
Why should winter shut-down benefit Varroa?  It would have to stop breeding as well.  We don't have a shut-down period in the winter, but still have problems with varroa, the problem becomes acute at the end of summer when brood production is at its lowest and Varroa piles into every available cell, if we don't treat at this time of the year we would probably loose the colonies, because they just simply run out of bees.
Roger White
Superbee Cyprus