All:

A bit of information, a bit of common sense.

1) Mites can survive severe cold -- I've had tracheal mites (some, not all)
revive after 2 weeks in a -20F freezer.

2) Mites in your beehive in the winter will take advantage of the heat
generated by the bees.

3) Wax moth larvae can be killed by storing combs/boxes in an unheated
building in the winter -- but that assumes below freezing temperatures for
some period of time.  Wax moth may be purged from hives in the winter, but
that assumes that the bees get rid of any larvae in the cluster area.
Again, unless you get "real" winter, I wouldn't count on it.

4) Cold winds will suck the heat out of an open hive.  You may be able to
have screened bottoms on hives with no wind barrier in Maryland, but unless
you have some form of wrap or wind barricade (and most of us can't count on
snow around the hive all winter), its an iffy choice for many northern
states and much of Canada.

For example, I think one might get away with unwrapped hives and screen
bottoms in West Yellowstone, MT -- since snow hits in the fall and is still
there in the spring.  But in Missoula, we almost always get a January thaw,
followed by winds from Canada in February, just as things get really cold
again.  Since it freezes up lots of houses, no reason to think that the
wind wouldn't also pull the heat out of an open-bottomed hive.  Remember,
the hives are ONLY WARM in the cluster.  The air between the cluster and
the walls of the box drop off to ambient or near ambient very fast.

Another example, I can park my truck at the University in the morning at
-20F and expect it to start at the end of the day -- if there's no wind.
That engine block holds a lot of heat, which slowly dissipates.

When we get one of our hollowing Hellgate Winds (named for Hellgate Canyon,
next to the campus), I have to go start the truck at noon.  On those days,
even after less than an hour, the truck is stone cold.

Jerry

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