It’s not the cold, it’s the humidity. We hear people say this a lot. That’s one reason 20º Fahrenheit (-7º C) in the eastern US can feel a lot colder than -20º in Montana. Generally, it’s the humidity that makes a colony of bees suffer the most through winter, too. As bees eat honey they physically convert some of the honey’s calories into heat, keeping the center of their nest far above the ambient temperature. You might remember that when animals eat, sugars combine with oxygen and carbon dioxide and water are respired. Remember that honey is about 17% water. If the bees eat 50 pounds of honey, over 8 pounds of water is released. If that moisture doesn’t leave the bees’ nest, it rises, condenses above the bees, and drips back down on the cluster. That is deadly.
The reason so many successful beekeepers keep upper entrances open during the winter is to allow excess moisture to drift out and away from the hive. You have probably heard the recipe for a good bee location – among other things, southern exposure, sloping landscape, and good air flow are usually cited. These all help keep the air around the bee yard dry. But first the water has to exit the colony. In addition to upper vents, I have even seen wintered hives supplied with burlap sacks draped over the top bars and extended out beyond the cover, thus serving as a moisture-wick. The wee bit of exposed burlap tends to stay dry and a capillary effect sets in, drawing moisture out of the hive. I have not seen this used out here in the dry west, but upper vents are almost always supplied.
Does the cold kill bees? Of course it does. A small cluster of bees exposed to minus 20º for a few hours will die. This is one reason package shippers don’t like to send 2-pound packages off to northern states in March. Nor should you install a package in bitterly cold weather. But large clusters of bees inside insulated hives typically survive even the longest coldest winters. That’s why beekeeping can be successful in the far north. The bees hug tightly for months, then quickly build strong spring populations and gather honey 16 hours a day during the long summertime flower feasts.
It’s not the cold, it’s the humidity. Seems to be true for bees, too. A Swedish beekeeper once told me that in his country a beekeeper’s success is not measured by pounds of honey produced but by the number of colonies that survive the winter. And, he told me, “I always expect 100% of the colonies to be good in the spring.” I have never had such good results in the north, but I always felt that any losses greater than 15% meant I was doing something wrong. Perfect wintering may be almost impossible to achieve in a larger outfit, but anything over one hive in six dead probably means some part of your wintering system needs to be fixed.