I would not count on the elevation being the factor here.  I know of many
beekeepers here in Utah that lost as many as 50% of their hives or more
to Varroa.  Most bee yards here will be located in elevations ranging
from 4,000 ft to 5,000 ft.  I suspect that other intermountain beekeepers
have also seen Varroa kills at their elevations.
 
Maybe Jerry can let us know what he has seen in Montana.
 
Anyone in Colorado or Wyoming?
 
Maybe there is some other correlation.  Maybe there is none at all.
 
Dirk Howard
Hobbiest Beekeeper in Utah
Currently with 3 hives looking to expand.