>  It seems the cluster regulates the internal temp by increased oxygen
> uptake. Obviously if the bees trachae are blocked with mites they cannot
> absorb enough oxygen to keep the cluster warm under our low temp
> conditions. This accounts for excessive winter kill due to T-mites in
> the North.
 
Not to discount Nasr's work which is pretty significant, there has to be
more to this than meets the eye.  We winter *outdoors* in an area where
temperatures reach minus forty -- sometimes  for weeks on end -- often
with wind.  We do not find that our (untreated) bees die then.  The losses
seem to come usually at the very *end* of a very long winter.  In such
cases, after 5 to 6 months of confinement, the bees simply age and die
without replacing themselves adequately.
 
I remember that beekeepers here (Alberta) were reasonably happy with their
wintering results in late February 1996, but by the end of March the ABA
was declaring a disaster.  This was because of an earlier than usual fall,
and a lack of good early spring flying weather.  It affected both indoor
and outdoor wintering and it did not seem to matter much what the mite
load was, if any.  Nonetheless, I doubt there is arguement that tracheal
mites do shorten bees lives.  And, FWIW, It occurs to me that tracheal
mites also tend to add selection pressure that tends to favour breeding
prolific bees that may have shorter individual lives.  (That will affect
wintering in severe years).
 
>  We have been in the Ontario Breeding for 4 years, and when we
> first started to test and select for T-mite resistance we were finding up
> to 250 mites per bee. Now by selection and breeding we have reduced that
> to no mites or figures like .25 mites in 50% of bees tested. Well below
> the economic threshhold. In other words, at these figures there is no
> need for treatment or loss to T-mites.
 
Does breeding for resistance work?  Perhaps.  It *seems* reasonable that
it can, but effectiveness can only be *proven* if there are adequate
controls to show that other effects are not responsible for the decline
in infestations.  I doubt in most cases if there are.
 
Although we were breeding from our own stock a few years back, recently we
have buying queens from everywhere and still do not see high counts.
Maybe the mites have been selected, not the bees. Our results are very
similar to what David reports above -- with very little breeing for mite
resistance, I assume, in the Aus and NZ bees.  I hear that the Hawaiian
ones do get some injection of genes from mite selected bees.  Maybe one of
our Aus or NZ breeders can indicate whether they are injecting mite
resistance into their stock in advance of mite appearance on their turf.
 
After writing this, I have to state that there is now a *tiny* doubt in my
mind, since two of our yards made from splits -- which have imported
queens, I'm not sure from where -- have 10% infestations, the highest in
our outfit.  Nonetheless, these two same yards were the highest reading
yards previously and I have thus far assumed that it is due to
microclimactic factors.  But I'm going to check on this.
 
At any rate, I wonder, in the cases of both Vince and David, what controls
have been used to verify that the reason for mite decline is actually
related to the breeding?  Mites seem to have their cycles and the levels
David reports seems consistent with what we are obtaining buying queens
from wherever they are available and cheap.
 
Allen