A while back there was some discussion on what might cause the
sudden collapse of strong hives from varroa when they appeared
varroa free a month or so before the collapse. It was suggested
that the mechanism is robbing by strong hives of varroa weakened
and near collapse hives. The varroa that are transferred to the
strong hive are also virus carriers which is the real cause of
the sudden collapse.
Another supposition which mirrors most parasite/host behavior is
the cyclic nature of varroa infestation. You have massive colony
kills and reduce or eliminate the varroa pool then get gradual
buildup to another collapse.

It appears both factors are at work in Maine this year. Stong,
honeyladen, apparantly healthy hives are collapsing from varroa.
It appears the feral pool is back since for the last two years
many who did not see bees in their gardens after the 95-96 winter
are reporting them.
My guess is that what we are experiencing in Maine is similar to
other northern States. Would like to hear if that is true. If so,
it could be a bad spring.
Bill Truesdell
bath, ME