Some wrap their hives for winter, some don’t. Bees can survive either way, but there may be a difference in honey consumed, brood reared and bees lost (Owens 1971). It’s been said that cold doesn’t kill honey bees, but they’re actually very vulnerable.
Below about 60°F, honey bees begin clustering together to trap the heat produced by their metabolism. When they cannot contract any tighter, bees at the less dense interior compensate for heat loss by working their wing muscles. The insulating outer layer is obviously the most vulnerable, so that is where the oldest bees in the colony can be found (Allen 1959). Old bees not only have a greater tolerance for cold, but they’re also the most expendable.
Just as foragers are lost every day in the line of duty, so are some bees on the outside of the cluster. This happens when their body (thorax) temperature falls below about 41°F. The bee enters a coma and drops to the hive floor, where she’ll die in a few hours unless some external source of heat warms her back up (Free 1960).
It’s important to understand that the cluster makes no attempt to heat the space around it (Owens 1971). Strong colonies begin rearing brood soon after the winter solstice, raising the temperature at the center of the cluster to 95°F. In a broodless winter cluster, however, the objective is not to heat the core, but to keep the outer layer of bees above 41°F. Core temps average around 65–70°F (Fahrenholz 1989). This is why the thermostat is set to 41°F when hives are overwintered indoors.
Most of us don’t have our hives inside, so providing some insulation can have the same effect as providing heat. In both cases, the metabolic rate of the cluster is lowered and energy is preserved. This is especially helpful to small colonies, because the smaller the cluster, the greater the proportion of bees that are exposed at the surface.
On the issue of insulating a hive, there appears to be an obvious benefit in slowing the metabolism of the winter cluster. True, a healthy colony of sufficient size can overwinter fine without further intervention. The cost comes in the form of bees and honey. Like so many beekeeping questions, there’s a lot of right answers.
Lastly, there’s the issue of moisture. Insulation only helps to bring the ambient temperature inside the hive closer to 41°F, which is still pretty cold air. An upper entrance allows the warmest and wettest air to escape. A quilt and a gentle tilt of the hive will help prevent a cold rain from falling on the cluster. Constantly drawing warm air away from your bees forces them to work that much harder to conserve heat. Everything is give and take. We must find the balance, and work to provide a familiar hive environment for our bees.
“In the absence of human intervention, the honeybee usually constructs its nest in a tree within a tall, narrow, thick-walled cavity high above the ground. The heat transfer rate is approximately four to seven times greater in the hives in common use, compared to a typical tree enclosure in winter configuration. This implies higher levels of humidity in the tree nest, increased survival of smaller colonies and lower Varroa destructor breeding success. Many honeybee behaviors previously thought to be intrinsic, may only be a coping mechanism for human intervention.”— Mitchell, D — Ratios of colony mass to thermal conductance of tree and man-made nest enclosures of Apis mellifera