Kerry Clark (of British Columbia)  mentions Formic Acid as one of his Varroa
treatments.  To the best of my knowledge, using Formic Acid as a Varroa
treatment is illegal in the U.S. (somebody please say if this has changed
recently).
 
As for myself, I now plan on using two Apisan applications/year.  I use 2
strips/colony installed in late February (local restart of brood rearing),
and two more strips go in in late  August.  The strips stay in for 6 weeks,
and are then discarded.  My local honeyflow starts in the third week of
April.
 
My Varroa control goals are more long-range, and are directed toward finding
the resistant bee.  To my fellow beekeepers:  Should you catch a wild
colony's swarm (or otherwise get a colony of wild origins), if at all
possible keep it as a seperate colony, do not requeen it, and do not treat it
for mites.   If such a colony lives for a few years, then you might just have
found the Varroa resistant bee strain that we have all been looking for.
 
W. G. Miller
Gaithersburg, MD
 
ps  What other North American insects are Varroa hosts besides honeybees?