>
> >Bees tolerate varroa . Varroa develops resistance to chemicals.


Technically, you are correct Bob, since bees will always have mites.  But
bees can be bred for mite resistance.

As usual, nothing is simple.  What we really need are virus-resistant bees,
since it is actually the viruses that kill the colony, not varroa.  Varroa
is a vector of viruses and stressor of bees.

But the easiest way at present to control virus transmission is to control
mite levels.  Mite-resistant bees do just that.

And here is where it gets more complex--there are a number of ways for bees
to resist mite parasitism.
Rigby (free download
http://www.lifesci.ucsb.edu/eemb/labs/kuris/pubs/Rigby_etal_02_TRIP.pdf)
suggests some alterations in the host:
These are alterations in the:
(i) host immune system;
(ii) host biochemistry;
(iii) host behavior;
(iv) host life-history; and
(v) host structure

Bees can use any or all.  Some take a more active and energy-costly form,
such as grooming or VSH.
Others, such as restriction of drone rearing, take less energy.
Increased trapping of the foundress mite in the cocoon would be another
type of active resistance.

But recent research has found that natural selection may favor more
"passive" sorts of resistance, such as the tweaking of the bee pheromones
or other chemical signals that the mites use as kairomones to help them to
find the right sort of larva, or the right age of bee to hitchhike on, or
the signal for the foundress to ovulate, or to produce a male.  These sorts
of resistance can lead to a much more varroa tolerant bee--allowing the
colony to "tolerate" the mites at a much lower iinfestation level.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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