So what is happening in the bee yard with this weather and temperature change? We discussed some of the behaviors in honeybees as the seasons cycle around the year in our post Seasons of the Honeybee.
Photo of some beehives in winter:
photo: Boris Romanov
Honeybees, like most insects, are exothermic or cold-blooded animals. This means that they cannot maintain a warm body temperature on their own. Exothermic animals must find heat fro their environment. For example, reptiles are exothermic as well, and they find warmth by resting on hot, sun-warmed rocks. Humans are endothermic, or warm-blooded. Our bodies naturally generate warmth from within.
Most insects hibernate, or go to sleep, when it gets cold. Honeybees do not. They are unique in the insect world for this. While an individual honeybee is exothermic, the collective colony is actually endothermic. When the temperature drops below 54-57F (12-14C), honeybees gather in a group forming a ball of bees called a cluster. The bees within the cluster all shiver their fight muscles. Honeybees are able to detach their flight muscles from their wings. This way, when they vibrate those muscles with their wings detached, their wings do not move. The muscles simply move against each other and this generates heat. When the entire colony of bees does this within the hive, they can generate enough heat and maintain the hive temperature at around 70F (21C).
Infrared photo of beehives showing heat distribution within the hive:
photo: Boris Romanov
The bees will cluster over their stored food. As the bees eat the honey from one section of comb they will move to another, full section of honeycomb. The entire cluster will move from one section of honeycomb to another as they eat their winter stores. The honey and pollen they have stored for the winter gives them the energy they need to constantly shiver their wing muscles and keep their colony warm and healthy.
The cluster of bees can adapt as the weather changes. As the temperature gets colder, the cluster will contract. The bees will pack as tightly together as possible, contracting the cluster and generating the maximum possible warmth. When the temperature warms some, the cluster will expand and grow larger. It is the same number of bees, but they allow more space between individuals and do not generate as much heat. By expanding and contracting the cluster, honeybees are able to maintain an almost constant temperature within the hive.
There is one more element that allows bees to survive the winter without hibernating. Towards the end of summer, the new bees that hatch are actually “winter bees”. They are physiologically different than summer bees. Where summer bees are lighter and smaller for long flights and foraging for food, winter bees are born larger and fattier. The fattier bodies allow them to survive the cold with less food. Their blood is also different. The winter bee has a different blood protein profile than summer bees. This also helps them to survive the colder temperatures.
During the cold, winter months, honeybees will stop producing brood. The eggs and brood require higher temperatures to develop and hatch. So in most climates, the queen stops laying eggs and brood production stops while the temperature is too cold outside. Also, the healthier and larger colonies have a better chance of winter survival. The larger the cluster they can form (basketball size), the more warmth they can generate. The smaller colonies can only form smaller clusters (softball sized) and therefore generate less heat and have more bee deaths over the winter.
Here is a photo of bees clustering:
photo: Guia and Mark Hoffman
For a more in-depth look at what happens in honeybee colonies over the winter, check out this fascinating article at Beesource:
The Thermology of Wintering Honeybee Colonies
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