It's recommended to treat with OA between Thanksgiving and Christmas. Most of the mites at that time are phoretic and not under capped brood where OA can't kill them (formic acid kills mites under cappings but is temperature dependent). It's cheap, easy and does a pretty good job. I always treat any bees during the spring and summer with OA that dont have any capped brood. Swarms, packages, splits, requeening, etc. One issue with waiting until you see evidence of mites is it's too late to do anything about them once you do. Do monthly mite counts during the spring and summer.