Hello Peter,
I will offer an opinion.
Peter wrote:
> 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?

The coumaphos knocked the mites down to what your apparently considered
acceptable. I believe the percentage left were highly resistant to
fluvalinate.

> 2) did we pick up resistant mites from other hives?

What you describe has been happening all over the U.S. with those trying to
rotate back to fluvalinate. Fluvalinate resistant mites were still around
even when fluvalinate had not been used for three years. You could have
picked up some fluvalinate resistant mites through drifting bees  but unless
a large amount of robbing was going on I doubt no. 2 is the answer.

 3) did environmental > conditions cause this rapid build-up?

I do not think so. I believe the varroa left after the coumaphos treatment
were resistant to Apistan. Apistan has no effect in areas where fluvalinate
resistant mites are. In fact many studies show varroa increases the same as
no treatment at all.

Several of my friends waited up to three years and tried to return to
Apistan with the same result as Peter.

Bob