>Why this is so is interesting, but it goes against the notion of "improved bees.”

As has been pointed out any ‘improvements’ gained from careful breeding are quickly lost in the next generation when the bred queen’s daughter mates and the colony becomes hybridized with the local metapopulation.

When an area does not have adequate habitat and natural forage for swarms to succeed in the feral state there’s not enough opportunity for natural selection play out.  The feral lines discovered by Delaney, Szalanski, and others show that when given that opportunity the bee is able to adapt and overcome novel threats such as varroa.

I happen to live in in area that supports feral colonies and I am working with stock derived from feral cut outs.  This stock has been thriving off treatments for many years now.  My overwintering losses this year came in at 16.7%  The workability and productivity of these bees are comparable to the commercially bred bees that others in my area are working with.

I would submit that nature has ‘improved’ these bees in ways that may not be available to those of us trying to accomplish improvement, which should not be surprising given the limited tools at our disposal and in the context of the arbitrary stressors we impose such as the crowding of colonies and the taking of resources. 
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