Not sure where the OP is located, but measured low levels of mites in September can give you a false sense of security if you don't keep doing alcohol washes or sugar rolls right up until closeout.  

In my area, northern NY, I can get substantial surges of mites in late September through late October, even after treating earlier in August/September (and verifying efficacy afterward) .  High enough levels to require re-treatment at that time, even though I plan on cleaning things up  very thoroughly with a one-shot of OAV in December.  These crazy, long, and warm falls that we've had in the NE for the several years leave the bees warm enough to fly out of the yard, but with nothing to forage on, so I think there is an uptick in "species-level reallocation of resources", a.k.a., robbing.

My colonies are well-defended by robbing screens on every entrance by that time of year, but that doesn't prevent my own sweet bees from laying waste to the country-side, and coming home with more souvenirs than just stolen honey.

I'd suggest keeping right on assessing varroa levels until it's too cold to do so - that way you know that a well-treated hive stays well-treated right up until they aren't making any more winter bees and they aren't bringing new mites back to hive.  And if you do see a mite surge, then it's easy to lay on an OAV series to suppress them back down to a less-harmful level, while you wait for the mites to become almost entirely phoretic during the early winter brood pause.

Nancy

             ***********************************************
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