IIRC, what it comes down to.... is that to become resistant they would have to EAT it, or it would have to penetrate somewhere, and if they did that it would kill them. As I understand it........ its been a "few" years since I read about the research....
The OA gets into the mite through the soft lobes on its feet, that apparently remain moist, and it kills them... IF, enough of it also got into the bees system, it will also kill them. Since this does not happen, OR, does not happen to a majority of the bees, (With sublimation) is why they do not become immune... If it gets in, they die, that makes it hard to build immunity.
I DID ask a lot of questions about why it doesnt kill brood etc... I was told.. because it doesnt....
The research at that time was being done over in Europe, where OA had been in use for over 20 years, and showed that it was still just as effective as it was 20 years earlier...
20 years.... I can't honestly say mites will NEVER become immune to it, but if it lasts as long over here I won't live long enough to deal with it.... However, using OA DRIP method, DOES kill bees, they clean it off themselves, and ingest it... I do not know if the mites do the same thing?
The last research I saw on OA as it was being tested over here, they were using it in a hive EVERY WEEK for months, with no noticeable mortality to the bees, but they had close to zero mites in those hives... I have been using it since well before it was legal to use in commercial apiaries, after I read the research from Europe and Asia. I will conceded that I cant say they CANT become immune to it, but like I said, from what I read, using the sublimation method it is very unlikely.
Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state. Randy Oliver was one of the people testing it back then, and he adamantly opposed the DRIP method, and swore by the vapor method... Because with Drip they ingested it and it did cause some mortality.
If there are people who want to know more I will try to take the time to FIND all of that research and re post it, instead of just running off my faulty memory.
