The obvious problem with relying on trying to breed a less sucessful
varroa is that it will obviously be replaced in the fullness of time
with a more agressive variety -- especially in circumstances where
defenses against varroa are used to protect the bees.
 
There is not even the probability of being able to have the former
interbreed with the more sucessful variety to lower viability since a
given varroa group may not breed outside its own family very often,
and only then when populations get large (which is precisely what is not
supposed to happen with the lower viability strains).
 
Regards
 
Allen
 
W. Allen Dick, Beekeeper                                         VE6CFK
RR#1, Swalwell, Alberta  Canada T0M 1Y0
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Honey. Bees, & Art <http://www.internode.net/~allend/>