They have been using OA across the pond for better than twenty years, and the mites have not gained any resistance to it... AS I UNDERSTAND IT.... It is because the OA is not ingested, it kills through crystallization and penetrating the mites SOFT PARTS, feet joints etc... Back when I was trying to decide if OA was the way to go, I looked up a LOT of the research that was then being done. Some of the resdearchers wre vaping OA every three days, some very week, for months, and seeing no side effects to the bees themselves, while still keepiong their hives clear of mites. At that time, it was unclear the effects of vaping with uncapped honey, the concern was adding OA to the open cells... THey did state that honey had OA IN IT naturally... I still try nbot to vaporize when they are making honey. Early in the spring, Late summer, and then november.
This is where I posted some of the stuff I had found at that time.
http://www.outyard.net/treatments.html The inability of mites to become resistant to it, the lack of side effects on my bees, and the ease of application. made me decide to try it. I have never looked back. I still see a lot of people advocating using an insecticide, to kill a little insect, on a bigger insect, and I often wonder how they justify it. I had issues using some of the prescribed methods. I have seen my bees bearding heavily after applying some of those insecticides, they did NOT want to go back in the hive. I see some people that stil use Liquid OA to treat. I tried it, it works, mostly, but it also shortens the life of the bees dramatically, because they clean themselves after getting drenched, and ingest it. I decided I didn;t like that, and went the Vapor route. I will test4ify formally that it works. It is easy to do, quick and painless, and the bees don't seem to mind. The hardest part of it, is paying for a decent vaporizor. I own two Varox vaporizers, they both still work like the day I bought them. It was VERY worth the money. I can;t ven begin to guess how much I have saved, despite the initial cost of the vaporizors. Amazon sells OA, I always buy the 99.6% pure or better if I can find it. Brushy mountain is now out of business I guess? Sad, they did a LOT of work getting OA legalized...