Late-season immigration requires constant vigilance here (north of Albany, NY) right up until close-out, even after test-verified good results from an earlier treatment. I keep robber screens on until the bees are no longer flying out of the apiary (I arrange the robber screen entrances so that they double as mouse-guards during the fall.)  But that doesn't keep my own bees from coming back to their hive with more than just looted honey.  

So I keep on doing sugar rolls until the temps are really too cold for safety, along with weekly sticky boards on each colony.  And I am quite willing to run multiple series of OAV, as needed, to respond to any surges detected.  With the brood winding down, OAV becomes more effective - but it provides no prospective protection from a later influx of new mites.   And, of course, I plan on doing the one-shot, clean-up late in December when my bees are finally as broodless as they will ever be during the year.  And then I keep monitoring all winter (weekly sticky boards only in my very cold climate). If I don't like the results on the boards after the one-shot, I will hit them once again, temperature permitting. (Which if I stay on my toes, even up here, I can usually find a suitable day.)  By this time of year I like to see nearly no mites at all, week after week, on my boards. And as soon as I can, perhaps in another three weeks, or so, I will start monthly sugar rolls again.

Effective treatment in late summer (usually with a formic acid product), followed by vigilance - and additional treatments, as indicated - right up until Christmas and then a well-timed OAV before the New Year usually leave me with nearly negligible mite levels from that point through early summer, mostly as a consequence of my very long period of flightless weather in winter and the broodless-period treatment. Only after Memorial Day does the mite build-up start ramping up once again.  

I could not manage this relatively light-duty treatment program without constant monitoring. 

Nancy

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