Temperature Dynamics of the Winter Cluster
It is that time of the year – winter is nearly here!
Granted, this is Texas…winter weather is apt to come and go randomly through much of the season. Despite its unpredictable nature, our bees prepare in much the same way as they would in a more northern climate. They have been hard at work finalizing their winter honey stores, removing the dead weight of drones and aged workers, and raising the generation of bees that will hopefully carry them through the winter.
Honey bees are unique in that they overwinter as a colony. Bumble bees, leafcutters, and the myriad of other native bees hibernate as mated females or dormant pupae and must start out the year alone, rebuilding their numbers over the course of the spring and summer. Hibernation is not in the program for honey bees. Instead, an entire colony dramatically changes its physiology and behavior to hunker down and ride out the winter.
Like most insects, honey bees do not maintain a constant body temperature. They are able to produce heat, however.
Even normal activity generates very small amounts of heat, and with colony populations often numbering in the 10,000’s, this can be significant by itself. Bees can also produce extra heat when they need it most. Their trick is using the strongest muscles in their bodies, the large paired flight muscles, to “shiver” their wings. Instead of producing the alternating up- and down-strokes needed for flight, they contract these muscles against one another to heat their thorax with little to no wing movement. Shivering for on-demand heat is a powerful tool; however, it is an energy-intensive activity and only sustainable for short bursts. This means that they would run out of fuel rapidly in winter if each bee had to warm themselves individually, or if large numbers worked to warm the entire hive cavity.
Extremely small winter cluster that died during a late season freeze
This is where the winter cluster comes in. If you have opened your hives when the temperature is somewhere south of 60° F, you have likely seen your bees clustered together in a spherical mass, clearly upset at the intrusion but briefly unable to fly. This clustering behavior begins when the temperature inside the hive drops to around 57° F. As you might expect, the usual Texas winter often has our bees bouncing back and forth, breaking cluster on the warm days and reforming most nights and when cold weather comes in.
What looks like just a ball of bees is actually an active and organized winter cluster. Layers of closely packed worker bees form an insulating “mantle” or “shell” on the outside of the cluster. They tend to orient with their heads pointed toward the center and their main function is to insulate and regulate airflow in and out of the cluster. Mantle bees may remain cold to the point that their movement is significantly slowed (around 50° F), though some are actively producing heat as needed.
Inward from the mantle is a much more active and loosely packed core. Activity of all these bees, supplemented by relatively few shivering workers maintains warm, stable temperatures near the center of the cluster. Additionally, packing into a sphere minimizes the cluster’s surface area to volume ratio ensuring the least amount heat is lost. The insulating outer mantle and heat-generating workers allow the center of the cluster to function fairly normally. Queens slow or cease egg laying activity as fall turns to winter, but colonies may maintain a small patch of brood on and off for much of the winter in our typically pollen-rich southern climate. Any brood and the queen will remain near the center of the winter cluster and be cared for accordingly. The cluster responds quickly to temperature changes. If the temperature inside the cluster drops, more bees shiver their wing muscles and insulating mantle bees pack tighter to reduce heat loss.
Video Credit: Clifton Kern - "This is a time-laps from inside the feeding chamber of the beehive. It was started around 9 PM and stopped around 10 AM the next morning. the minimum temp was around 4* F that night."
As it warms, the cluster loosens and allows heat to escape. This leads to an incredibly stable temperature in the middle of the winter cluster, with broodless colonies maintaining approximately 80° F or more and those with brood keeping it in the mid-90’s despite whatever the winter throws at them!
Bees cannot stray far from the edge of the cluster as long as the temperatures inside the rest of the hive remain cold. Individual bees lose heat quickly outside of the cluster. This means that their source of winter fuel, sugar – in the form of honey, uncapped nectar, or whatever supplemental sugar we provide – must be within reach if the weather stays cold for long. This is relevant to colonies of all sizes, but it especially important for those with small populations (more on that below).
Healthy, medium-sized loose winter cluster in January
The location of a winter cluster slowly drifts over time as they consume stored honey. Ideally, colonies in Langstroth-style hives go into winter with the population located low and in the center.
Wasted space such as undrawn supers are removed so that a relatively small brood nest area is surrounded to the sides and above with food. This setup gives clustered bees easy access to honey and explains why we find most bees occupying upper hive bodies by early spring.
Photo Credit: Randy Oliver - scientificbeekeeping.com
A relatively large winter population has the advantage of more bees to insulate and produce heat, whereas a very small cluster is one poorly timed cold snap away from death. Personally, in central Texas, I am pretty confident when I see a tightly packed winter cluster about the size of a basketball or larger.
That said, I have seen much smaller clusters, down to approximately softball-size, survive the winter here. I attribute those tiny survivors to a combination of setting them up just right…and luck. These very low population hives will keep you wondering about them all winter and struggle to build up in spring, which is why most beekeepers recommend combining them with another hive in the fall. On the other end of the spectrum, an extremely large colony is unlikely to die just because the temperature drops. A word of warning though – extra-large colonies can consume honey like it’s going out of style! Make sure they don’t starve in February or March.
The genius of the winter cluster means that our honey bees are already equipped to withstand a much harsher winter than any we should ever encounter here in Texas! Successfully overwintered colonies have the workforce to capitalize on early resources and build up in time to take full advantage of the main spring flows.