>The replacement of low-fecundity Japanese race Varroa with a
high-fecundity Korean race in Brazil demonstrates the selection pressure
for greater virulence in Varroa.

All excellent points John.  So long as we keep restocking apiaries with new
colonies of susceptible bees, we maintain an artificially high host
density, which will continue to give the selective advantage to the most
virulent mites.

And what we must keep in mind is the added factor of DWV.  It is not the
mites alone that cause collapse of colonies--it is an epidemic of one or
more viruses (typ DWV or one of the paralytic viruses).  So long as we
maintain a high host density (plenty of well-stocked apiaries), it is to
the mite's advantage to utilize viruses to kill the host colonies in fall,
so as to precipitate robbing, and thus disperse those (successful) mites to
other colonies.

Thus, even a few virulent mites (carrying virulent strains of virus) may
successfully overwinter, and then out compete any less virulent mites come
next spring.

We are dealing in managed bee populations with an artificially propped up
population of bees, and a continually evolving coevolution between the
bees, varroa, and varroa-transmitted viruses.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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