What happens to honeybees in the winter?

It’s snowing, and I figure that’s good, for the bees: they slow down, go into cluster, don’t consume as much food, and the queen stops laying.  There’s no food here for bees from mid-October to February, so there’s no use in the bees looking for any.

Around this time of year, I’m often asked: what happens to the bees in winter, do they go to sleep?  No, they don’t but it’s a good guess; we are in an area where many animals go into hibernation in the winter.

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 centers itself in the vertical center 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.  (At the end of this piece, there’s a temperature / “what’s happening” chart).

Before I launch into this, I’ll be using are two sets of numbers (besides Fahrenheit, F, and Celsius, C).  There is the temperature of the bee cluster and the temperature outside the cluster.  The bees only heat the cluster; the rest of the hive is the same temperature as the bee yard.

The bees need to keep the cluster’s core between 93 and 96 Fahrenheit (around 35 Celsius). The very lowest the cluster’s center can drop to is 55F (13C). Lower than this and you have a dead hive.  Different sources offer different exact temperatures, but they are all in the same ballpark. The numbers here are drawn from internet searches, and from “The Biology of The Honey Bee”, by Mark L. Winston (a fabulous book and fun to read).

Around here winter temperatures can drop to 20 F (-6 C). That is warm compared to some beekeeping areas, so how do the bees manage to keep the heart of their cluster between 40F (4C) and 95F (35C)?

When temperatures outside the hive drop to about 64 F (18 C) the bees start form their cluster (that ball of bees).  An outer shell of bees 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, 55F (8C), the outer shell has a minimum temperature: 46F (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 beeyard temperature.) If the outer shell drops below 46F (8C) the bees will fall off the cluster, and die.

Here’s what happens when the temperatures in the bee yard start to drop:

Bees will begin to cluster when the temperature is about 65F (18C).

When the thermometer drops to 57F (18C) that 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 41F (5C) the cluster can still be quite large, and can cover 8 to ten frames.

As temperatures plummet  from around 41F (5C) to 23F (-5C) the cluster begins to contract.  When the outer temperatures hit 23F (-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.  This generates heat rather like human isometric exercises (or like me when I get in my pick-up on a freezing day and the heater has not yet kicked in).

If it keeps getting colder, the cluster will keep contracting. At 28F (-2C) the cluster may cover 6-8 frames. 7F (-14C) it can get down to 4-6 frames. At -15F (-26 C) they tend to jam themselves onto two frames.

This contraction explains why in a 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.

I have heard that beekeepers have suggested drilling holes into the middle of frames to allow bees to move more easily around and through their clusters.  This makes sense, but I’ve yet to meet anyone who has tried it.

There are times when I need to over winter a nucleus hive.  I start these throughout the year so that I always have a viable queen in case one fails in a stronger hive, or, if they do well; I’ve a new hive in the making.  This means I often have nucs as winter approaches.  The number of bees in a nuc is not sustainable in our winters here.

My solution has been to load the nucs with honey and put them on top of a strong hive.  They are separated from the lower hive by a cover/bottom board piece (one could use a simple piece of wood).  This gives them total separation from the lower hive, with the benefits of subfloor heating.  Something I wish I had here right now.  Yes, it’s still snowing, and I don’t “do” cold.

Here’s an easy to read beehive winter temperature chart:

Optimum Core Temperature : 95F (35C)

Minimum inner cluster : 55F (13C)

Minimum cluster shell : 46F (8C)

Bee Yard Temperatures and Bee Results:

64F  18C            bees begin to cluster

57F  14 C           cluster exterior becomes compact shell.  The bees are motionless in the outer                                shell, but still moving  around inside cluster.  This shell can be many beelayers                              thick

41F    5 C            Cluster can cover 8-10 frames

23F   -5 C            Outer shell of bees starts moving their muscles to generate heat

28F   -2C            Cluster can cover 8 -9 frames

7F    -14 C            Cluster can cover 5-6 frames

15F  -26 C            Cluster may only cover 2-3 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.

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