In a message dated 23/09/02 05:03:00 GMT Daylight Time,Peter Borst writes:

<< We used coumaphos in fall of 2000 & spring 2001. That fall mites were
 acceptable, so we went back to Apistan fall 2001 & spring 2002. It hasn't
 worked, so one wonders: 1) did the coumaphos not knock out resistant mites?
 2) did we pick up resistant mites from other hives? 3) did environmental
 conditions cause this rapid build-up? >>

Although Coumaphos is in a different chemical group to Apistan, the fact that
bees have already evolved resistance to one chemical may mean that mites have
been selected for that have the most rapid adaptability to any chemical that
is applied. I understand that the usual mechanism for resistance to Apistan
(out of several possible ways) is to select mites that are able to de-toxify
themselves better than their fellows. Put in human terms they are the ones
with the most efficient kidneys.  We know that mammalian kidneys can cope
with all manner of poisons. Why should not the arachnid equivalent be equally
clever?  Thus Apistan resistant mites will have a head start when it comes to
coping with Coumaphos.

Chris