How does a cicada, a butterfly, or a yellowjacket colony prepare for the winter? Generally, by dying! Immature cicadas and butterflies survive in the soil or leaf litter, and newly mated yellowjacket queens hide under bark in autumn and wait to freeze solid until spring. For most adult insects, winter means death. But honey bee colonies manage to live through the cold months by staying in a large group for warmth, using the honey they collected ahead of time as their winter provisions. Forming a winter cluster is an amazing trick that honey bees use to survive as a whole family until flowers reappear in spring.
The winter cluster
In his book "Honeybee Democracy", Professor Tom Seeley describes the cluster clearly:
...each colony contracts in winter into a tight, well-insulated cluster of bees about the size of a basketball. The cluster's surface temperature is maintained above 10°C (50°F), which is a few degrees above a worker bee's chill-coma threshold, and so is warm enough to keep the outermost bees alive.
He explains how the bees survive the winter by alternately contracting (or “shivering”) their flight muscles to generate heat and calculates how much heat the whole cluster could produce if they shivered in unison. But he continues:
At any moment, however, only a small portion of the clustered bees will be shivering with maximum intensity, so the total heat output by the approximately two kilograms (four pounds) of bees in a winter cluster isn't 1,000 watts, but is only about 40 watts, a rate of heat production like that of a small incandescent light bulb. In a snug cavity, sheltered from heat-robbing winds, a colony with this level of heat output will survive the winter quite nicely. The importance of inhabiting a protective cavity is demonstrated by the sad fate of the occasional colony that fails to find shelter and nests in the open; almost certainly, it will perish when winter's cold arrives.
A thermal imaging camera lets you see the warm basketball-sized bee cluster through the walls of their hive.
Movement of the cluster
The honey is stored above the bees as they collect it from spring through fall, and as it accumulates, the bulk of the colony moves downward, lower and lower towards the bottom of their combs. Then, during the cold weather, they empty their honey pantry from the bottom, slowly eating their way upwards. Picture a thick dome of honey that fills the top of the hive, and extends down along the outer frames towards the bottom of the hive. Under that dome of honey, a sphere-shaped cluster of bees will form to eat the honey and conserve their body heat.
The cluster of bees can be egg-shaped, or round like a ball. It contracts when temperatures become colder, and loosens and spreads out when temperatures rise a little during the winter. In warmer weather, or very well insulated colonies, the cluster may be so loose that it doesn't really resemble a cluster anymore. Some beekeepers try to prevent their bees from having to form a cluster by insulating the hive or moving it to a warmer location, while other beekeepers consider clustering natural and not harmful for the bees, since they have been doing this for a lot longer than humans have been keeping them.
Imagine the colony working its way down the combs in the summer as they fill the top of the nest with honey, and then eating their way upwards when there's no forage available. It's a bit like a yo-yo that takes a whole year to complete one down-up cycle.
Movement of individual bees
But don't think of the cluster as a mere blob of bees. The bees in this expanding and contracting cluster are organized into layers, with the outer insulating shell of bees being the coldest. Bee clusters can take on complex shapes depending on how the bees cling to each other, and whether they're trying to stretch to reach two different areas of honey. And when the cluster moves from one side of a box to the other, none of the bees can teleport through the solid wax combs. Instead, each individual bee must leave the cluster, run around the edges of a comb, and then rejoin their sisters on the other side.
Who determines if you get to be a lucky bee on the inside of the warm cluster, or an unlucky bee on the outside? Is there some kind of lottery in the fall? Or does the queen assign jobs by royal decree? Of course not! It should come as no surprise that honey bees are more egalitarian than humans. U.K. beekeeper Chris Slade shared a story online a few years ago that caught our attention because it gives valuable insights into what the bees are actually doing:
Many years ago a chap in Weymouth, Dorset, UK kept bees in a skep [an open-bottomed woven hive] placed over a glass ceiling in his boat room. He sprayed the winter cluster with paint. The marked bees went gradually up the outside of the cluster then went into the top, later emerging again from the bottom to repeat the process.
Those painted British bees revealed what's happening in many of our hives. The bees cycle across and through the cluster, much like the liquid in a blender moving up the sides and over into the top/center, and from there down to the bottom again. Thus, cold bees from the cluster's insulating shell trade places with bees from the warm center continuously.
Dwindling clusters
Dead bees on the snow shouldn't cause alarm, since it means that healthy undertaker bees have been cleaning these dead workers out of the hive and dumping them on the snow.
But not every bee makes it through the winter. Each day, some bees can't go on any longer and fall, dead, to the bottom of the hive. As long as the queen survives, and there are enough workers to keep warm and regrow the colony in the spring, the loss of a few bees is not a disaster. However, uncontrolled varroa mites and viruses weaken too many of the winter bees, leading to too many deaths before spring flowers reappear. This can spell doom for a colony. If a colony dies before January 1st, our first suspicion is that their parasite and disease levels weren't controlled well in the fall.
During a long winter, the pile of bee corpses on the bottom board can get quite deep. On warmer days undertaker bees leave the cluster to collect and remove dead bees from the hive. This can sometimes alarm new beekeepers, who think a snowy field covered in dead bees means trouble in the hive, when it's actually a sign of a healthy colony cleaning up their unlucky casualties. The main thing for beekeepers to monitor is if too many dead bees have built up at the bottom entrance. On warm days, undertakers have plenty of work to do, and the rest of the bees need to take brief so-called cleansing flights to empty their intestines of the waste they've been holding in for the last few months. If the pile of dead bees is preventing them from leaving, a good beekeeper should poke a stick (or even a dedicated hive entrance cleaning hook and scraper) into the entrance to pull out dead bees and create an entrance.
Clusters usually enter spring smaller than they went into the fall, even though brand new bees start joining the cluster when the colony starts raising brood in the later winter and early spring.
Brood rearing changes things dramatically
One of the biggest dangers for an overwintering colony is committing to raising more brood than their honey supply can support. Bees on the outside of a winter cluster can do just fine around 45°F, with the bees inside the cluster being much warmer, but still sometimes as low as 82°F. Not much heat is needed to keep a small cluster alive. That all changes when brood must be raised, since developing bees need to be kept at temperatures around 90-95°F. If a small cluster of brood is being raised in the center of the cluster, the energy costs aren't dramatically increased, but if the colony tries to raise four frames of brood in the middle of a cold February, they're going to need a huge amount of extra honey to keep all of that comb area warm.
Another concern is the location of the year's first brood. Worker brood takes 21 days from egg to adult bee. That means if the cluster has to move upwards to find more stored honey, they may have to abandon brood in progress on lower combs. A colony's foragers can't zip out to the local flower patch for a bit more nectar when there's a blanket of snow on the ground, so the colony has to carefully balance the need for early rounds of brood in spring with the available honey and pollen that will be needed to grow and heat that brood. This is why the most common time for a colony to starve is very late winter or early spring.
Feeding bees in winter
Any time the cluster gets to the top of their hive, it makes sense to ask why. If they have run out of food, putting a few sugar bricks on top of the frames could be the difference between life and death. Winter patties, sugar bricks, or fondantshould be ready to feed your colonies if they run out of food before winter ends.
In this dramatic example, a pile of winter patties was placed on a colony with very little stored honey in the fall, and surrounded with a spare hive box. In spring, the cluster had eaten their way up through much of the patties and were happily preparing for spring. We don't recommend this as a standard beekeeping practice, but we were fascinated to see the spherical cluster licking a spherical hole out of the stack of patties.
What else can we do to help our winter clusters?
Varroa mites are the constant enemies of bees and beekeepers. It's a good idea to give colonies a mite treatment in winter, when they have little to no capped brood for the mites to hide in. If you are dribbling or vaporizing oxalic acid into the hives, aim for days above 38 degrees F. At and above that temperature, the cluster expands and becomes “looser”, allowing more of the oxalic acid to penetrate the whole cluster instead of just sticking to the layer of bees on the outside.
Otherwise, a healthy winter cluster of bees may not need much from you. The bees have very precise systems to keep themselves safe and warm all winter. Good rooftop insulation helps to contain their heat and keeps condensing moisture from dripping down on their heads. (For more on insulation and condensation, check out this article about condensing hives.) Occasional winter visits to the apiary let you check on the hives, and pull out excess dead bees that may be piling up near the entrance. Check the snow or ground in front of your hives to make sure that corpse removal and cleansing flights (look for yellow or orange streaks of bee poop) are carrying on as they should during warmer days. And just like our bees, we just need to dream of spring flowers and wait for warmer weather.
