Bobs further comments and experiences help round out my thoughts. A return to efficacy, if it will occur, doesnt seem to happen in a few or maybe even 9 yrs. Makes sense. As fluvalinate selects for resistant mites, the percent of the genes(s) responsible will increase so that, according to the example of the georgia expert, 98% of mites will be resistant. So for the 2% still sensitive to increase quickly to say 95% sensitive (thus an effective fluvalinate tx) would suggest either tremendous luck on the part of gene segregation, or an extraordinarily competative advantage of sensitive over resistant mites. More likely, there is no competative advantage, and if the genetics are simple, and the mechanism all or none, by arithmetic the population would tend towards 50% of each after enough generations. Whatever that number of generations  would be, it will happen faster in climates where there are more brood cycles / yr  than where there are winters,  but still only get to  50% sensitive. But pls comment on my line of reasoning and simple assumptions. Also, do we know the genetics of resistance, what the biochemistry is, or if there may be a competative advantage of sensitive over resistant? I know for sure I dont. 

Bob's colleague's experience also points towards another level of care that we will have to accept is needed to make treatment choices. Just as a culture and senstivity is needed to guide antimicrobial selection, the only tool we have, I think, is the Petis test. Or, I suppose, mite numbers in a  representive sample of hives  before and after a test treatment, before all hives are treated. Mite drops 24 hrs after a tx are interesting but cant be interpreted meaningfully beyond a general impression. There could be a ton of mites dropped but still be only 50% of those present, or you could see 50 but that was 95 % of the population.  The first two methods mentioned  are  cumbersome to a degree.  Interpretation and  determining the number required to be truly a representative sample would be affected by how prevalent resistance is in an area.  It aint perfect but thats what we have and anything else is really just a wild guess. If you go that way you have to ask yourself how lucky are ya feelin'.   
Greg
Everton ON

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