Evolutionary pressure is all about simple arithmetic.
May I suggest studying "Randy's Varroa Model" to check out mite fitness
arithmetic?

The reproductive success (number of successful offspring, or fitness) of a
female mite is better than twice as high in a drone cell than in a worker
cell.  Thus, even if one could kill half the mites that entered drone brood
over the course of the season, it would still confer increased fitness upon
the mite to "prefer" drone brood.

More importantly, though, is to look at column BA, which calculates roughly
what percentage of mites are in the drone brood at any time period.  Based
upon my calcs, there is rarely even 20% of the mites in the drone brood,
substantiated by studies that measured mite reduction as a result of drone
brood trapping.

I my own operation, we run a drone frame in the the second brood chamber of
every hive.  We use it to rear lots of drones in the spring (yet upon
inspection, often find very few mites in the drone frame).  We sacrifice
the drone brood once when we split the hives after almonds (we split
everything at that time).  We add the drone frame back again once the
colonies get their second brood chamber, by which time in our dry
environment, there is little drone rearing.

Thus, lots of drones for spring queen mating, some mite control, virtually
no selective pressure on the mites to favor worker brood.

I understand that the situation would be different in areas in which
colonies rear drones all season long.

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

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