Beecrofter writes:

(snip)

If you have been having a drought with no nectar coming in brood production
slows or stops and the bees can't outproduce the mites. This happened last
year to a lot of us in the northeast.  It gets hard to time treatment by
just
a calendar.

This sounds like good sense to me, and may explain my observation.  Thanks.

Walter Weller
Wakefield, Louisiana