Hi.
Just my 5 cents of observation. I have used  Apistan before 1999. No
appreciable drop off onto the bottom boards. Anywhere between below 10  to
plus 50 in 14 colonies.( Spring 1999, Fall 1998)
Modified bottom boards to screened ones  in july 1999.
Applied Coumaphose  in Fall ( End August ,Beginning Sept.) 1999 and counted
2000 to 5000 mites on bottomboard. Daily for 2 weeks.Then deminishing to
zero drop off. No evidence of varroa  spring 2000.
Theory : The space between combs is too narow to allow bees to get maximum
contact to miticide. Therefore, either  ( In 10 frame Hive bodies) the strip
was placed in the spot where the frames were spread apart to allow bees to
walk AROUND the strip and just not being in contact with  the edges. Or as
with 9 frame bodies there is enough space for bees to get sufficient
contact.July is usually known as conducive to varroa increase, so I was
informed.
This summer I treat with FGMO fogging method. In Fall I will check with
Apistan and Coumaphose as well as formic acid to compare the drop off.
Interesting side line info from Dr Tom Sanford Apis letter this month's
relative to pesticide contamination of honey research results.
JDF