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
****************************************************
* General Information About BEE-L is available at: *
* http://www.honeybeeworld.com/bee-l/default.htm *
****************************************************