<<<<<<<>  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.>>>>>>>
 
Whilst I do not in any way dispute the claims that bee breeders have
produced resistance to tracheal mites by selective breeding I am very
puzzled as to how this may come about.  It is said that the bees are
damaged by bacteria entering the blood stream by  way of the bites.  I can
understand bees becoming resistant to such bacteria but not in a few years.
 This in no way would lessen the number of mites appearing, which appears 
to be the case.  The mites enter the tracheae by the main spiracles and  
only when the bee is young.  Any physical change to keep the mites out
would need hundreds of years of evolution. Have we already got bees whose 
spiracles restrict entry?   Fewer mites means restricted breeding.  Have
the bees suddenly become distasteful to the mites?  Have they in some way 
upset the breeding cycle?  Is it not possible that the mites go through
periods of vigour and depression?   All very puzzling.  Lots of
suggestions, please.
 
In a recent letter to the list I said that acarine was no longer a problem.
 I had two replies from local beekeepers.  One told me that in a recent
test carried out on 120 colonies only two showed slight traces of mites.  
The other had found a colony with around 100% infestation "tracheae
absolutely black with mites" and was treating with grease patties.  So it 
seems to come and go.
Although it is snowing and bitterly cold as I write this we are normally
blessed with mild winters.  We usually get short  breaks when the
temperature rises to allow the bees to make cleansing flights and breeding
can start in late January.  Although acarine is said to shorten  the lives
of wintering bees this does not have much effect in a short winter.  If you
live in an area where the winters are long, hard and unbroken and breeding
is delayed. the situation changes and you can well have the old bees dying
off faster than the young are produced.  Thus you have an apparently
healthy colony in midwinter collapsing in March or April.  In such a
situation I would accept that autumn testing and treatment would be
necessary.      Sid P.