When colonies are widely separated as in the Arnot  forest
>the mites have nowhere else to go so it is in their interest  not to
exterminate
>the host.

The key point that I glean from the Arnot and Gottland studies is the 
influence of a long, cold winter.
Horizontal transmission is greatly decreased since colonies that die from 
mite stress during the winter are not immediately robbed, and the mites 
within are killed by the cold.  Therefore, there is a strong selective 
pressure for mites NOT to kill the colony.

In warmer climes, this selective pressure would not apply, since collapsing 
colonies would be robbed while their mites were still alive, and the mites 
would actually benefit by killing the colony, as long as there were other 
colonies around to be infested.  Without human intervention, colony density 
in warm climes would likely decrease to the point that mite avirulence would 
become a benefit to the mite.  But that is unlikely to happen, as beekeepers 
will keep restocking the bee population.

Randy Oliver 

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