I've had bees for six years now. My hive count has steady increased to 12 production hives and about 12 nucs. Production hives looked great during the flow and produced 120 pounds of honey per hive.
I treat with OAV only with a ProVap 110, solid bottom boards and seal the entrance with a cloth for 10 minutes. I use 1 gram of OAV per brood chamber. I always treat during the winter broodless period.
I did 3 OAV treatments at 7 day intervals in July as I always do. One month ago all hives looked great. Solid brood patterns and lots of bees. I didn't do an alcohol count before starting but began treating with OAV again two weeks ago, about the same time I do each year. I'm in the middle of 5 OAV treatments, 5 days apart. I've inspected the hives over the treatment period and have been alarmed at the loss of population, suddenly horrible spotty brood, and more and more deformed wings. I did an alcohol count yesterday of a hive after three OAV treatments. Mite counts are still at 30 per 300 bees. I'm starting to think some of my production hives are a lost cause at this point. After my 4th OAV treatment last night, I installed a screened bottom board on a hive and got about a 400 mite fall overnight for one hive. Very discouraging. I'll keep hammering them with OAV though.
For whatever reason it seems to me that OAV is tougher and tougher every year to control mites. I know the Europeans say no resistance developed over 30+ years. What's everyone else's thoughts on this?