I treat for varroa w/ OA drip in broodless November/December.  OA eliminates varroa very effectively then w/o residues.  I checked some hives yesterday - not a single varroa in the drone cells [broken when boxes are separated] and I checked dozens of cells.

I know many sources recommend treating in the late summer for strong winter bees.  I believe treating in July /August gives the few remaining varroa a longer time into the following season to recover their numbers.  By treating late - if you have strong colonies! - you can go through the next summer and into the fall w/o treatments.  It also allows for selecting for strong/resistant colonies.

Waldemar   

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