It has been a while since I attended a course on "evolution of disease" by prof Sabelis but the ideas I had to memorize and model with are still on my mind. 
 
As long as we keep mixing bees and mites and pathogens we enable increased "horizontal" transfer, this is, transfer between hives. Esspecially when we aggregate hives in high densities. With an increased horizontal transfer there are 2 important effects: 1) possibility of pathogen "strains" competing for the host, with a selection pressure on increased exploitation speed leading to increased virulence of the pathogen. 2) continuous re-infection and dilution of tolerant host genetics. Bees and pathogens have less chance to evolve towards a sustainable host-pathogen relationship by hard selection, brought on by vertical transfer as the main way of transfer for parasites. 
Throw in treatment of pathogens as a factor and things get complex...as treatment allows susceptible bees to continue and gives clean new hosts al the time, ready for pathogens that are already selected for increased exploitation speed*.
 
This means that if there is a good chance on a stable host-parasite couple (tolerant bees, less virulent mites/virusses...I put my money on the last one first) it will be in an isolated area that has a relatively low density of colonies. Places like islands with primitive beekeepers or like Arnold forest. 
 
As an anecdote...I have seen the virulence/effects of the tracheal mites of bumblesbees change as the horizontal transfer increased when breeding bumblesbees in high densities compared to the mainly vertical transfer of mites in wild colonies. 
 
*the theory I describe here is not complete/correct. It is what I remember and think up...correct me to make it better.
 
Lennard 
 
  		 	   		  
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