" Is it that some miticides are more like the hammer and others less so?"

The short answer is a resounding yes.  The long answer is really long and depends on a discussion of biological pathways to degrade materials, the biology of absorption and internal transport, pre-existing genetics that can be modified as well as pure chance on things like horizontal gene transfer between species.  Let me just give one example that shows how unpredictable this topic is.  Flies rapidly became highly resistant to DDT after it was introduced.  It was only something like seven years of wide use and DDT was near useless against flies.  Yet after 75 years of continuous use of DDT to fight mosquitoes they have not developed any significant resistance.  They have developed some avoidance behavior but not resistance.

Both formic acid and oxalic acid have been used for a lot of years in some areas without varroa showing the slightest resistance.  Other organic acids are also known to kill varroa mites.  They all likely kill in the same way.  The use record says resistance is not very likely to develop soon.  That does not mean it is impossible.  No one will ever know if it is impossible unless resistance happens and then we will know it is possible.  If resistance to one acid happens likely that resistance will apply to all the other acids at the same time.

The best way to avoid development of resistance is to rotate the products used so at least three different biochemical modes of action are involved in killing the pest.  For example rotate the use of oxalic acid with amatraz and perhaps thymol (which may kill by the same biochemical pathways as the acids) or an OP.  The other best way is to never kill more than about 75% of the existing pest population with a single pesticide at any time.  This last one is not very helpful as only killing 75% of the pest population seldom is enough kill to be satisfactory.  That 75% can vary from 60% or likely lower up to 90% but there is no way to know the right % until resistance develops and you know a lot about the biochemistry of that particular resistance.  So, all you can say specific to varroa is trying for a very high kill rate, like we all try for, is likely to speed up resistance development.  And if you do not try for a high kill rate your bees will die.  I prefer it when my bees do not die.

Dick

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