Regarding the findings of infertility of varroa in brood and possible mechanisms of resistance in bees:

Finding infertile female mites in scans of brood cells says very little about the actual resistance mechanism.  Papers by Harbo and Harris and by Ibrahim and Spivak show that hygienic adult workers can preferentially remove pupae with reproductive mites and leave pupae with infertile mites.  The work by Denis Anderson suggests that there may be also effects of the brood itself on infertility.  Also, a number of experiments manipulating the age of sealed brood that a mite first encounters and the chemicals involved show that only the youngest brood stimulates egg formation and eventual egg laying by female mites.  So there are some cues that mites pick up from brood that could potentially be manipulated either as a treatment or in selecting resistant bees.  A group of us tried to select bees that reduced mite reproduction through their brood.  Initial results were promising, but in later generations the effect "dissipated".  There may be a very intense arms race between bees and mites to cloak and uncloak these signals.  Interestingly, fertility of varroa mites was low in Africanized bees decades ago and it appears to have been due to their brood, but it appears to have increased.  An explanation is that the population of mites changed towards a more reproductive form.  Not a simple system to work on.....

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