I do not know why apistan is left on for as little or as long as it is,
but would guess that it is on long enough for two complete brood cycles.
This way it interrupts the life cycle of varroa by killing the newly
emerged mites as well as those alive when the strip is applied and does
it over the two cycles. I don't know the half life of apistan in strip
form, but it seems that-in cold climates- if you left the strips in the
lower hive body over winter and removed them when you rotated hive
bodies in the spring, you would do no harm and probably some good. The
bees would only be in contact with the strips during the fall and early
winter and would migrate up and away from them as the winter wore on. By
early spring, they would be in the upper body and away from the strips.
The strips would do the most good, because they would be present during
the time the queen stops laying which further harms the varroas ability
to survive since there is no brood and all are exposed. And they would
not be in contact with the bees much more than a month or two more than
the instructions.
Problem with what I postulated is that you would have to put the strips
on late in the year to keep close to the lable instructions. You really
want to knock down the varroa in late summer so the bees have had a few
brood cycles to have healthy, varroa free bees go into the winter.
Anyway, its a thought.
Bill Truesdell
Bath, ME