The nights are drawing in, the weather is getting colder and the days are shorter. So, what happens to honey bees during winter? We are often asked “what happens to the bees in winter, do they go to sleep?” No they don’t!
In colder weather and snowy months the bees do slow down, they go into cluster in the brood box and they don’t consume as much food. The queen is in the middle of all these bees. She’s waited upon and fed and groomed, just like normal, but her egg laying may have slowed considerably or stopped. This can begin from mid-October to February, depending on the weather and if their source of food is running low or stops all together.
What honeybees do is even more fascinating than hibernation. A strong, healthy hive will form a cluster. This is basically a ball of bees that covers a number of frames and usually centres itself in the vertical centre of the hive. The size of that ball will depend on how many bees are in the hive and what the temperatures are outside the hive. There is the temperature of the bee cluster and the temperature outside the cluster. The bees only heat the cluster.
The bees need to keep the cluster’s core around 35C. The very lowest the cluster’s centre can drop to is 13C. Lower than this and you will have a dead hive. Temperatures obviously vary throughout the winter months due to global warming and honeybees have been known to fly in the winter months. My bees flew out on Christmas day one year despite the snow! The temperature was warm outside and they came out to fly. Sometimes bees will clean up any debris from inside and clear up the dead bees and fly off with them and drop the dead bee mid-flight. I have often seen this happen in my own garden.
So how do the bees manage to keep the heart of their cluster around 35C?
When temperatures outside the hive drop to about 18C the bees start to form their cluster. An outer shell of bee’s line up side-by-side, facing into the cluster to create a thermal barrier. At this point, the body heat of the outer layer of bees supplies enough warmth to maintain the bees inside the cluster. This outer layer of bees can be one bee-layer thick, or can build up to a number of bee-layers in thickness. The bees on the inside can still walk around over the comb and sometimes eat a little honey. Periodically bees switch places, with ones from the inner core taking up a position on the outer core to allow the outer bees to go “inside”.
Just like there’s a minimum temperature for the heart of the cluster, 13C, the outer shell has a minimum temperature of about 8C. If the outer shell drops below this temperature, the bees can no longer move their flight muscles (Remember this is their temperature, not the temperature outside.) If the outer shell drops below 8C the bees will fall off the cluster, and die.
When the thermometer drops to about 18C the cluster becomes a compact shell of motionless bees. At this point the body heat of the bees lined up side-by-side on the outer shell generates enough heat to keep the colony warm. Remember, this layer can be many bee-layers deep. At 5C the cluster can still be quite large, and can cover 8 to 10 frames.
As temperatures plummet from around 5C to -5C the cluster begins to contract. When the outer temperatures hit -5C, the outer shell of bees will start to flex their thorax muscles. These are attached to the wings, but the wings don’t move, just the muscles. In really cold winter, or when over-wintering hives with small populations, the bees can die surrounded by honey. They simply cannot leave their cluster to go get the honey. As supplies dwindle where the cluster has formed, the bees will move up. They seldom move to the sides. After all, going up between the frames is far less work than moving the ball of bees around all the frames.
Want to read more about it? Check out The Biology of the Honey Bee, Mark L. Winston. You can download it, buy it, or find it at your local library.