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Insulation Pontification

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 60F, honey bees begin clustering together to trap the heat produced by their metabolism. The compact outer shell minimalizes the loss of heat. When they cannot contract any tighter, bees at the less dense interior compensate for heat loss by working their wing muscles (which may also serve to maintain the strength needed for foraging when the weather breaks?) 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.

We see this same strategic division of labor employed in warm weather. The oldest bees are tasked with foraging, while the younger generation remains shielded on the inside. And 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 41F. The bee enters a coma and drops to the hive floor, where she’ll die in a few hours, or maybe a day, 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). Some heat does escape, however, and parts of the hive may experience a very minor temperature increase. This is particularly true of the space directly above the cluster, but that heat is mostly absorbed by the honey stores or ventilated overboard to rid moisture. For the most part, internal hive temperatures just inches from the cluster remain only a few degrees higher than the outside air, doing nothing to help the immobilized worker at the bottom of the hive.

Strong colonies will begin rearing brood soon after the winter solstice, raising the temperature at the center of the cluster to 95F. In a broodless winter cluster, however, the objective is not to heat the core, but to keep the outer layer of bees above 41F. Core temps fluctuate in response to these efforts, averaging around 65-70F (L. Fahrenholz 1989). Expending just enough energy to prevent the loss of bees at the surface results in the cluster’s minimum metabolic rate and lowest food consumption (Free 1963). This is why the thermostat is set to 41F when hives are overwintered indoors. That’s just warm enough to prevent chill coma, but not warm enough for the bees to break cluster.

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. Losses accelerate as colony size further decreases, due to this decline in heating efficiency.

(Drifting off subject, it’s interesting that large colonies also experience significant losses, despite a superior capability to thermoregulate. This is because populous colonies rear less brood in the fall, resulting in an older population. Optimum colony size in November was found to be about the size of a 3 lb package, or 11,000 bees (Jeffree 1956). Using my formula from an earlier blog, such a colony would only need 3 or 4 frames of stores (11,000 x’s .0022 = 24 lbs). This may ensure the colony overwinters on their own honey, which is important for their health and immunity (Berenbaum 2012). The beekeeper still benefits from a nice harvest, and saves the time and money normally spent on fall feeding. Realistic? I don’t know. Something to think about.)

On the issue of insulating a hive, there appears to be an obvious benefit in slowing the metabolism of the winter cluster. True, as long as there's honey to burn, a healthy colony of sufficient size can overwinter fine without further intervention. The cost comes in the form of bees and honey. On the other hand, there’s also a cost associated with providing insulation, because of the time and materials needed. You decide what is best in your situation. Like so many beekeeping questions, there’s a lot of right answers.

Lastly, there’s the issue of moisture. As air temperature rises, so does its capacity to hold water. There is widespread fear of condensation and mold inside the hive. It’s a discussion for a future blog. Just know that insulation only helps to bring the ambient temperature inside the hive closer to 41F, 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 obviously forces them to work that much harder to conserve heat. This is to say nothing of the importance of humidity in the hatching of eggs and disruption of Varroa reproduction. 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. For example, clustering in a tree enclosure may be an optional, rare, heat conservation behavior for established colonies, rather than the compulsory, frequent, life-saving behavior that is in the hives in use today. The implied improved survival in hives with thermal properties of tree nests may help to solve some of the problems honeybees are currently facing in apiculture.”

- “Ratios of colony mass to thermal conductance of tree and man-made nest enclosures of Apis mellifera”, Mitchell, D

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