>> When I first joined this List a decade or two ago 
> a high proportion of the correspondence was 
>> about Acarine, the tracheal mite, but nowadays 
>> it is very rarely mentioned.

> But why? This is a great correlation is not causation. 
> It might be resistance but it could as well be that 
> Varroa treatments also take care of tracheal.

Tracheal Mite resistance has been overtly bred into most commercially-bred
lines of bees.  The Buckfast bee was the first example, Steve Taber selected
for, and quickly got good resistance in bees he was selling, the NWCs have
been highly tracheal-resistant for years and years.

So, tracheal mite resistance is not optional any longer.  It is a product
defect if it is not present.

Open mating means that a small fraction of bees from any lines will likely
not show the trait at issue, but for the most part, "store-bought" queens
are resistant.  The locally-raised stuff are "selected" from much smaller
numbers of queens, so one takes one chances with those so-called
"locally-adapted" and "bred from feral survivor" queens.

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