John Burgess wrote:
To me it seems illogical that resistance to Apistan would have developed in

> only one colony, if you treated all your colonies in the same way.
> Isn't it more likely that the colony in question has robbed out a heavily
> infested feral colony which was on the point of collapse?

I don't know why this should be illogical.  Varroa mites, like all biological
organisms,  make up a population of widely varying individuals.  This is
evidenced by the observation that 98% (or some such number) of varroa are
killed by the standard Apistan treatment.  Why is it not 100%?  Because of this
natural variation, following the same bell-shaped curve used to describe the
size of leaves on a tree or the performance of students on an examination.

During the course of Apistan treatment, some individuals are very susceptible
and die off fast; some more and more slowly; and finally some haven't reached
their death by the time the Apistan must be removed.  So the last 2% represents
the high end tail of this bell -shaped curve.  Among these may be an even
smaller population that may never fall to this chemical, and in the presence of
Apistan they will reproduce and pass on this genetic characteristic.

This resistance will start randomly, occurring only occasionally at first until
such mites are spread to other colonies of bees.  Then one should expect these
resistant mites to have an enormous reproductive advantage over nonresistant
mites, until Apistan will be of no use.

Therefore I feel that it is opportune to try the neuroactive chemical
coumaphos.  It will hit these Apistan-resistant mites at a different
physiological site, hopefully controlling them before they can spread to other
colonies.  Then one can return to the safer Apistan.

Why would not robbing behavior explain the premature appearance of varroa?  It
is a possibility, but bees are not prone to rob during a honey flow, which has
been going now at differing rates for the six weeks since Apistan was removed
from the colony in question.  I have even inadvertently left a full frame of
honeycomb for a couple days in the yard without attracting any attention at
all, except from mice.

Ted Fischer
Dexter, Michigan USA