Research: Bees gather together when it is cold, know why they do this

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(Derek Mitchell, PhD candidate in Mechanical Engineering, University of Leeds)

LEEDS – Bees have been unnecessarily wintering in man-made hives for more than a century, new research shows, commercial hive designs are based on bad science. For 119 years, the notion that the way bees congregate provides them with a kind of evolutionary adaptation has been at the core of the beekeeping process, hive design, and bee studies. More recently, California beekeepers have been putting bee colonies into cold storage during the summer because they think it’s good for their health. But studies show that clustering is a rather than benign response to falling temperatures. This is dangerous behavior. In light of these findings, beekeeping practices done intentionally or by poor hive design may be considered poor care or even cruelty.

Bee colonies (Apis mellifera) do not like cold. In the wild they overwinter in tree cavities, allowing at least some numbers to remain above 18 °C in a variety of climates, including winters as low as -40 °C. But popular understanding of their behavior in winter is dominated by observations of their behavior in thin (19 mm) wooden hives. These man-made hives have very different thermal properties than the natural habitat of thick-walled (150 mm) tree hollows.

go through winter

During cold days, colonies of bees form dense disks, called clusters, within these thin-walled hives. The center (core) of these disks is less dense and hotter (up to 18 degrees Celsius). This is where the bees generate most of the heat by eating and metabolizing the sugar from the honey. The cool outer layers (mantle) produce very little heat because bees have a very low body temperature. If the temperature falls much below 10 degrees Celsius, the bees will die.

Since 1914, beekeeping texts and academic papers have stated that the casing “insulates” the interior of the hive. This means that beekeepers considered clustering to be natural or even necessary. This belief was used in the 1930s to justify keeping bees in thin-walled hives even in -30 °C climates. This led to the practice in Canada in the late 1960s of keeping bees in cold storage (4°C) to keep them together during the winter.

In the 2020s, beekeepers are refrigerating bees in the summer to facilitate chemical treatment of parasites. This is happening across the US – for example in Idaho, Washington, and Southern California. Apart from the harsh winter, if beekeepers want to treat a mite infestation, they usually have to locate the queen and isolate her. But cold storage means beekeepers can skip this laborious task, making their commercial pollination services more profitable.

Struggling for warmth

However, my study found that the clusters act like mantle heatsinks, reducing insulation. Clustering is not like wrapping yourself in a thick blanket to keep warm, but more like a desperate struggle to get closer to the “fire” or die. The only benefit is that the mantle helps keep the outer bees alive.

As the temperature outside the hive drops, the bees around the hive go into hypothermic shutdown and stop producing heat. As the bees try to stay above 10 degrees Celsius, the mantle contracts.

As the mantled bees come closer to each other, thermal conductivity between them increases and insulation decreases.

Heat will always try to move from a hotter area to a cooler area. The rate of heat flow from the core bees to the mantle bees increases, leaving the bees outside the mantle 10 degrees cooler.

Think of a down jacket – it’s the air gap between the feathers that helps keep the wearer warm. Bee ruffs are similar in action to what happens in a down jacket, allowing thermal conductivity to eventually increase to the dense solid of the feathers, much like a leather jacket. In contrast, when penguins are huddling in the Antarctic winter, They all keep their bodies warm at the same temperature, and so there is little or no heat transfer between penguins. Unlike bees in the mantle, there are no penguins in hypothermic shutdown.

Academics and beekeepers have overlooked the role played by the invisible air gap between the hive and the colony. The thin wooden walls of commercial hives act as little more than a boundary between the air gap and the outside world. This means that for the hive walls to be effective, they must be insulated to a considerable degree, such as 30 mm polystyrene.

This misunderstanding of the complex relationship between the colony enclosure, thermofluids (heat, radiation, water vapor, air) and bee behavior and physiology is a result of people not recognizing the hive as an extended phenotype of the bee. Other examples of extended phenotypes Includes a spider’s web and a beaver’s house.

There are almost no ethical standards for insects. But there is increasing evidence that insects feel pain. A 2022 study found that bumblebees react to potentially harmful stimuli in a way that is similar to pain responses in humans. We urgently need to change beekeeping practices to reduce the frequency and duration of clustering.

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