>  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?
 
Apparently there is already quite a bit of diversity in many honeybee
characteristics and the necessary traits may already be widespread
 
The most obvious explanation for fairly fast development of apparent
resistance is the quick elimination of bees that are particularly
susceptible to TM from the breeding pool.  This takes place after a few
big losses -- or in any selection program, although in the latter there is
an attempt to select the 'best'.  Since this is subjective, it may result
in skewing the results in some unpredictable way.
 
Since most of the 'worst' bees are quickly eliminated, it then becomes a
problem of fixing the traits.  This takes longer.
 
Interestingly enough, we are trying to find resistance in our bees to
tracheal mites while trying to avoid resistance in varroa mites to our
controls.  Understanding the mechanisms of resistance are of great
importance these days.  Resistance is working both for us and against us.
The mechanisms are subtle and there are many factors to consider.  I am
not at all sure that I have much understanding of the topic.
 
>  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.
 
I have heard that fall is too late, since by then the bees that are
going to winter are already infested; treatment will kill the tracheal
mites inside them and reduce thre infestation of the next generation, but
the lives of the winter bees are already shortened.
 
Allen