Paul, I started my OAV treatments on Aug.31 after all honey supers were off. 
5 treatments 5 days apart, so 20 day span.
On Sept.20 4 hives were at almost zero mite count, on sticky boards, in fact 2 were zero.
The largest, broodiest hive was still at 0.5%, but decided to not continue treatment. 
Based on Randy Oliver's Colony Demography chart, I will do one last OAV shot on all hives around Oct.24th. That should be 
around the point of minimum capped brood here in the north. I am also hoping we still get a day or two above +5 C.
I totally agree brood is the key, and I think up here the bees 'brood down' earlier. I'm hoping I didn't wait too long but the low numbers of mites in 4 of the hives leaves me optimistic. 
As an aside, 2 hives were packages and were treated 5 days after hiving in May. No mites for months...
Brian
115W, 53N, El.850M

             ***********************************************
The BEE-L mailing list is powered by L-Soft's renowned
LISTSERV(R) list management software.  For more information, go to:
http://www.lsoft.com/LISTSERV-powered.html