"His comment  "mite treatments that kill a high percentage of mites,  lead to
the mites building up restiance faster than a treatments that kills a
smaller percentage.""


Yep, that is the way it works, contrary to many peoples opinion.    The reason has to do with genetics.  You start with some native population.  Within that native population some mite will have slightly more tolerance for a given treatment than the vast majority of mites as a result of some minor genetic difference between mites.  Lets say this mite that has some tolerance amounts to 1% of the population.  So, you treat somehow and only kill 70% of the mites.  You now are breeding the next generation of mites from a population of 29 mites with no special tolerance for the treatment and 1 tolerant mite.   Not all mites reproduce successfully.  By the time you treat again that tolerant mite may have died without reproducing and you are treating a population of normal mites.  Even if the mite does reproduce there is little chance it will end up on the same bee larva with another tolerant mite allowing the daughters to be crosses with different tolerant
genes from two parents that could result in a new mite that is highly tolerant.

Well, it is fine to delay development of resistance by killing only a small % of the population.  But, killing only 70% of the mites will not break the virus epidemic and the hive will die.  So, you either have to kill more mites and risk development of resistance or let the hive die.  Personally I am killing the mites.  What would have happened if you killed all 99 susceptible mites and only left the one resistant mite?  The very next generation every single mite is going to be resistant to some extent.  It will take some time for the mite population to build up to the problem level.  But, after some time the population will build up.  You treat and no longer get a good kill.  Maybe you have to double the dose to get a good kill.  By doubling the dose you are again selecting for the few mites that tolerate the higher dose and those will reproduce and get you in time.  You can not double the dose every other year or pretty soon you are killing your bees.

Resistance nearly always results from some very low background level ability of the organism to deal with the poison in the native population.  To take a well understood case consider DDT resistance of flies.  Long before DDT was used flies had the genetics to allow carbon chlorine bonds to be degraded slowly.  This biochemical pathway did not exist in mosquitoes.  By exposing both flies and mosquitoes to DDT flies were selected with better and better genetics to degrade carbon chlorine bonds and rapidly became highly resistant to DDT.  Mosquitoes had no starting point to develop resistance with the result they did not ever become resistant.  Rarely a chemical comes along and resistance simply does not seem to ever develop.  For instance the fungicide chlorothalonil has been in use since the late 1960s and no evidence for resistant fungi has turned up the last I knew.  For some fungicides resistance has turned up within ten years of first commercial use.

Less commonly a gene that confers resistance may move across species.  Mother nature has been moving genetic material by non sexual routes from one species to another for billions of years.  So, we should expect it to happen with resistance genes sooner or later.  Once moved selection can pretty rapidly improve the ability of the plant, or animal to degrade the pesticide and render it useless.

Probably the most effective way to slow development of resistance is to alternate use of two or more different pesticides which work by totally different chemical pathways.  For mites perhaps alternating between MAQS, oxalic acid and amitraz would slow development of resistance for all three products.

Dick

" Any discovery made by the human mind can be explained in its essentials to the curious learner."  Professor Benjamin Schumacher talking about teaching quantum mechanics to non scientists.   "For every complex problem there is a solution which is simple, neat and wrong."  H. L. Mencken

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