Hi Sid,
 
Sid Pullinger wrote:
<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.
 
        Numerous experiments in NY and Ont., some formal, some amateur, have
shown very good results with Dr. Nasr's program.
 
> 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.
 
  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?
 
 All very puzzling.  Lots of
> suggestions, please.
>
 
        This is a good question because there seems to be much confusion over
this. You are correct in saying that "Any physical change to keep the
mites out
would need hundreds of years of evolution". This is not how artificial
selection works. A selection program does not seek to produce mutations.
It is meant to increase the frequency of genes that express the desired
characteristic. These genes, or characteristics, must already exist in
the population we select from. There is a great deal of variability in a
typical population of honey bees. When we first began to select, the
local population had already been exposed to t mite for several years.
We found that about one fourth of the preselected colonies were valuable
as breeders. In Ont. began selection before their bees were exposed for
very long. They got about one in ten. Evidently we got a benefit of some
natural selection.
        Regarding your comment on restriction of spiracle entry, we do not know
if this is the mechanism. The assay devised by Dr. Page and Dr. Gary did
not identify any mechanism of resistance but showed that resistance to t
mite is hereditary and could be selected using that assay. It would be
wonderful to know the exact mechanism we are dealing with but it is not
imperative.
 
 
> 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.
 
        This should not be surprising. As I've mentioned in other posts these
relationships are cyclical. Some interesting, related, reading is "chaos
theory". There are plenty of books. Its really fun to model this stuff
on a spreadsheet  and learn how touchy population dynamics can be.