When your bees are settled in for winter, with hives full of stores and the winter bees protected by earlier mite treatments, chances are that you figure you're good to go. You'd be right about that - you've done everything you can for the bees that are alive in the hives right now - but what have you done to protect next year's bees from mites?
Summer and fall varroa mite treatments are all about making sure that your winter bees are healthy and long-lived enough to see the colony safely through the winter.
Suppressing mite levels in the summer keeps the nurse bees in good health, so they can do their job of raising the winter bees without passing on viral diseases. Fall mite treatments are intended to keep the young winter bees safe from mite-vectored problems that arrive due to drifting and robbing at the end of the season. As long as there is still brood in the hive, the best you can hope for is keeping the mite numbers low enough to maintain a level of harm reduction.
But then, just on the cusp of winter, a special period begins: the natural brood pause, when in most areas the bees stop raising brood for a short time. Since varroa mites require bee brood to keep reproducing (and the mites are protected from the effects of most treatments while they are reproducing under the brood cappings) the early winter brood pause leaves them unusually vulnerable.
You can capitalize on this heightened vulnerability during the brood pause when all of the mites in the hive will be in the phoretic state. For a few weeks, the entire mite population in your hives is defenseless against a treatment that works quickly.
There is one kind of treatment that will kill more than 95% of all the phoretic mites in one shot: oxalic acid. A single oxalic acid treatment, when there is no brood in the hive, can essentially reset the mite levels back to nearly zero.
And it gets even better than that. In most areas of the US, this natural brood pause coincides with, and is followed by, some period of colder weather. Often, it's cold enough to confine the bees, if not to their hives, at least to their own yards. The bees will not have to deal with an influx of new mites coming in from untreated colonies. Once they are cleaned up, they stay cleaned up until foraging flights start again, which may be months later.
That's a tough question, because it varies from area to area. Oddly, in seems to happen slightly earlier in southern areas than in more northern areas. (The bees return to brooding more quickly in warmer areas, too.) It seems to vary from year to year, as well. Short of opening your hive and pulling frames to check (a risky idea so late in the season), the best bet is to ask around locally. As general rule, the period between Thanksgiving (in the south) and Christmas (in the north) is a good window to aim for. But to get the most benefit, if you can wait until it's cold enough to regularly keep your bees at home, you'll get increased protection from their temporary isolation. Of these two factors, put more weight on broodlessness than on isolation, because it's lack of brood that makes this treatment so effective.
This means that when brooding resumes in January and early February (depending on where you are), the new bees will be raised in a nearly mite-free environment, safe from the physical damage of parasitization and safe from the harm of the diseases that mites spread. It is the new bees of the next spring that will be raising your strong production colonies next year.
In some ways, a broodless period oxalic acid treatment may be the most important one of the entire year. It lays the foundation for improved mite control that carries into, and beyond, next spring, because it starts out the inexorable mite-reproduction curve from the lowest point possible. It also means you won't be carrying over a low level of mite problems from the year before. It's as close to a fresh start as you can get.
So, if you'd like your bees to be able to start the New Year like it's 1986, (the year before Varroa destructor first appeared in the US) consider giving them a one-shot treatment with oxalic acid sometime in the next month.