Beehive Deadouts and Thermoregulation. How a colony of bees keeps itself warm, and what that asks of us.

It’s the end of May here in the foothills of Alberta and it’s rainy and cold. I’m looking outside and worrying about my bees. Then I think about how bees cluster to stay warm and I relax a bit. Honeybee thermoregulation is a fascinating story that’s worth reading about on a rainy day, especially in the context of how to maintain healthy colonies year-round in a northern climate.

I sell nucleus colonies to beekeepers, and I’ve had numerous conversations recently with beekeepers who lost hives this past winter. None of them can explain why. The hives did well most of the winter and then died. Why? I’ve experienced this with my own hives, and I decided to do a deeper dive into the causes of deadouts and whether they relate to the bees’ ability to regulate the temperature of the hive. It turns out that most winter losses are caused by heavy varroa loads and poor queens. The bees’ ability to stay warm is fascinating, but it is not the cause of hive death.

The fact that bees cluster together to stay warm is a marvel of nature. How can a colony of insects, without a leader, without a plan, without any individual bee that understands what the colony is doing, build itself into a living organ that holds heat against a Canadian winter for five months at a stretch?

The house made of bodies

When the air outside the hive falls below about fourteen degrees Celsius, the bees begin to gather. As the temperature drops further, the gathering tightens. By the deepest part of winter, the cluster is a dense, roughly spherical mass of bees clinging to one another across several frames of comb. It has a structure, and the structure has a logic, though no single bee knows what the logic is.

The outer layer is called the mantle. It is made of the oldest workers, packed so tightly against each other that researchers have compared their combined insulation to fiberglass. They face inward. Their heads point toward the heart of the cluster, their abdomens toward the cold. This is not arbitrary. A bee’s hemolymph pumps heat away from her thoracic flight muscles toward her head, which means her abdomen is her coldest surface. The mantle bees are turning their coldest surfaces toward the dark, the way you would turn the back of a coat against the wind.

Inside the mantle, the core bees are warmer, and they are working. They have decoupled their wings from their flight muscles, and they are vibrating those muscles in place. The wings stay still. The muscles burn. This is called shivering thermogenesis, and a single bee performing it can raise her body temperature to forty-four degrees Celsius, roughly nine degrees above her normal range. Multiplied by thousands of bees, the heat is enormous. The differential between the core of a winter cluster and the air outside the hive can exceed sixty degrees Celsius, and it can be maintained for months, on nothing but stored honey and the willingness of small bodies to keep working.

And the cluster moves. It breathes. Mantle bees who grow too cold push inward toward the warmth. Core bees who have warmed up drift outward toward the cold. No one directs this rotation. There is no foreman keeping track of whose turn it is. The bees simply respond to the temperature around them and the temperature in their own bodies, and the rotation emerges from the responses. The geometry is calculated by bodies. The mathematics is performed by instinct. No one single bee solves the equation. Every bee solves it.

The heater bees and what they tell their daughters

For a long time, scientists thought the warmth at the heart of a hive came from the brood. The developing pupae were assumed to generate heat through their own metabolism, and the workers were assumed to cluster around them for that warmth. We now know this was backwards. The pupae do not heat the colony. The colony heats the pupae. And it does so through a specialized role we did not understand until recently.

They are called heater bees. They can be of almost any age, and they can leave the role and return to other work as the colony requires. When a heater bee is at her post, she crawls into an empty cell among the brood, presses her thorax against the wax, and begins to vibrate her flight muscles. The heat she generates radiates outward through the comb. A single heater bee, positioned in a single empty cell, can warm as many as seventy adjoining brood cells. The colony keeps small archipelagos of empty cells scattered through the brood nest for exactly this purpose. The architecture of the comb is designed for the heating system.

Heater bees burn honey at ten to twenty times the rate of a resting bee. They cannot leave their cells to feed without losing temperature. So other workers come to them. They deliver honey mouth-to-mouth, refueling the heaters where they stand, the way a pit crew refuels a car without taking it off the track. The colony has invented a logistics system for its own heating infrastructure. No one designed it. It is simply there, in every hive, in every winter, everywhere honeybees live.

Scientists have recently discovered that the temperature at which a pupa develops shapes who she will become as an adult. A pupa raised at thirty-five degrees Celsius is more likely to emerge as a forager, oriented outward, drawn to flowers and flight. A pupa raised even a degree or two cooler is more likely to emerge as a house bee, oriented inward, drawn to the comb and the queen. The heater bees, in choosing which cells to warm and how warmly, are shaping the future workforce of the colony. They are deciding, without deciding, what kind of sister will emerge from each cell. The colony is not only keeping itself alive through the winter. It is composing the version of itself that will meet the spring.

What the cluster asks of the keeper

I have been keeping bees long enough to have lost colonies in winter, and I have lost them in most of the ways a colony can be lost. I have opened hives in March and found a small cluster of dead bees, heads pressed into empty cells, having starved within inches of full honey frames they could not reach because the cluster was too small to span the gap. I have opened hives and found wet comb, blackened with mold. I have opened hives and found nothing at all, the bees gone, the queen gone, only the smell of wax remaining.

For a long time I told myself most of these were failures of thermoregulation. The cold got in. The moisture stayed in. The cluster could not hold. And sometimes that was true. But the longer I keep bees and the more I read, the more I have had to admit what the research has been saying for years. The cluster is rarely the thing that fails first. A healthy colony, going into winter with a strong queen and a low mite load, can survive almost anything Alberta throws at it. A compromised colony will die in February no matter how well I built the box. Surveys of beekeepers consistently put poor queens at the top of the list of reported causes. The research consistently puts varroa and its viral payload at the top of the actual causes, with queen failure as the most common visible symptom.

This complicates the lesson I thought I was learning from the cluster, and it makes the lesson more honest. The bees can build the most extraordinary thermal organ in the insect world. They cannot dispatch a parasite that arrived on the back of a forager in October. They cannot replace a queen whose ovaries are damaged by viral infection. They cannot vote on whether their genetics are suited to a Canadian winter or whether their mother was a Hawaiian queen flown north in a package. Those are our problems to solve, and we solve them not in February but in July and August and September, in the months when nothing looks wrong.

So when I say the cluster asks something of us, I do not mean only what it asks of our carpentry. I mean what it asks of our attention. Are the mites under threshold by mid-August. Is the queen still laying tightly into October. Are the winter bees, the long-lived generation that will carry the colony to spring, developing in a hive that is not already saturated with virus. Is the hive itself built and sited so that the cluster, when it forms, has a fighting chance. All of these are parameters in the same equation. None of them are the bees’ responsibility. All of them are ours.

In Alberta, the average overwintering loss runs near thirty-eight percent. In bad years it climbs past fifty. Each dead colony costs a beekeeper up to three hundred and fifty dollars to replace, and that is only the money. What it costs the bees is something else. A colony that dies in February has spent four months performing the most extraordinary act of cooperation in the insect world, burning honey, rotating bodies, feeding heater bees mouth-to-mouth in the dark, and it has done all of that on a starting hand we dealt them in September.

I think about the heater bees vibrating in their empty cells, refueled by sisters who come and go in the dark, shaping the foragers of next summer one degree at a time. The colony is doing everything it can. There is so much that we can do as beekeepers. Treat the mites before they break the queen. Replace the queen before she breaks the colony. Build the box so the cluster has somewhere worth doing its work. Stop treating wintering as something the bees will figure out and start treating it as something we are doing together, beginning in summer.

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