Since I wrap every colony, every year, I should add something here.

I keep bees on the 45th...northern VT and NY. We have long cold winters,
with temps getting into the -20 to -30 F range. The bees have no cleansing
flights from mid-November to mid-March...and sometimes to mid-April. Most
will agree that it is not the cold that kills bees. I believe it is the
long period with no cleansing flight that does.

  Two winters ago, it was very cold...frostbite weather on the
slopes...just look at my poor nose. When March rolled in, the temps
approached flying weather, but the west/northwest wind blew and blew. This
prevented many yards from taking their first flights...some yards until
mid-April. Dysentery was a real problem. Winter losses were higher than normal.

Last winter was another severe one. Again...poor nose. March came around.,
and again it was too cold for flights. One afternoon, with the sun shining
strongly, the temperatures came close...probably 30 something F. I took a
drive past one yard, just to look. In a strip about 4' wide, right down the
rows of hives, the snow was bright yellow. The bees had had a good
cleansing flight. But, it wasn't what you would usually see. More often,
there is yellow snow throughout the entire yard. Bees fly far from the hive
to release their loads. Dead bees are scattered here, there, and
everywhere. This day was different. The bees had only flown a short way
from the hives...perhaps 5' or so. There were no dead bees on the
snow...anywhere.

I would say from this, that the black wrap in the bright March sun had
warmed the hive, and the air around it enough for the bees to take a short
flight. They didn't go far, and they all got back in, but they got the job
done. For the rest of the spring, dysentery wasn't a problem.

I guess where I keep bees, wrapping isn't necessary every year. I know
beekeepers who never wrap. But, I believe that there are years when it
helps the bees make it through the winter, so I wrap every colony, every year.

Mike