When you find circles of honey bees head-down in cells, does that mean they starved? Were head-down bees licking the bottom of the cells to get every last morsel of honey?
A reader raised these questions a few weeks ago. Since then, I’ve asked many established beekeepers for their thoughts and observations. To my surprise, I then found a post I wrote back in 2011 that answers my own queries. Forgetfulness is embarrassing, so I wrote this two-part series to refresh my own memory.
My most recent post, “Did your bees die of cold or starvation?” was the preamble to this one. Today I want to review some other details of the winter cluster.
The wizardry of warmth
As I said before, honey bees are wizards at keeping themselves warm. According to Jürgen Tautz in The Buzz About Bees, workers have the ability to “uncouple” their wings from their flight muscles. By rapidly quivering those muscles, the bees produce heat which is used to keep the brood and the clustering adults warm.
Because wing muscles are attached to the thorax, close-up thermal images of nurse bees reveal some with glowing thoraces. These hot bees are known as “heater bees.” Where capped brood is present, a single heater bee presses her thorax down on a wax capping, transferring her heat to the developing pupa, one-on-one.
Large areas of brood are punctuated by a certain number of empty cells, usually about 5-10%. In a clever adaptation, these cells are used by heater bees to warm even more of the surrounding brood. The bees climb into the cells head-first and quiver their muscles.
In the past, the bees in these cells were thought to be cleaning or resting, but with the aid of infrared photography, you can see that the bees are producing large amounts of heat. According to Tautz, other bees, sometimes called “retrievers,” search the hive and bring back honey to feed the exhausted heater bees.
Bees head-down are normal
When temperatures are unusually cold or the cluster is dangerously small, many more bees will go head-down into the brood cells. When brood is not present, the nest is kept at a lower temperature, and the head-down bees don’t work as hard. Additional layers of workers cover the head-down bees and provide insulation to trap the heat.
Head-down bees are not trying to find honey. The tight grouping of head-down bees covered by insulating bees is a heat-conserving and energy-efficient design that is continually fed by the retrievers that bring food (fuel) to the quivering bees. Retrievers are adept at what they do and can collect honey from distant areas when conditions are right. Healthy, well-fed, head-in-the-cell bees are a normal part of a winter cluster.
When things go wrong
But things can go amiss. As I mentioned earlier, you may have illness due to disease or parasites, malnutrition, queen failure, poor genetics, predation, or any number of mishaps. If the colony gets too small to generate enough heat, it will die regardless of food availability. Cold bees enter a state of torpor such that their muscles no longer work. When the bees are that cold, they simply cannot operate.
A beekeeper finding circles of head-down bees may claim his bees died of starvation. While that may or may not be true, it’s virtually impossible to separate the effects of cold and starvation. It’s more of a cold/starvation complex, where each condition exacerbates the other. Were they cold because they couldn’t get food? Or did they starve because they were too cold to move? As one beekeeper explained, the only thing you know for sure is that your bees died while clustered.
Looking for other clues
When you see a pattern of dead head-down bees, especially with food nearby, you know something went wrong in the colony. The head-down bees died while trying to keep themselves warm but were unsuccessful. At this point, it’s the beekeeper’s job to do the postmortem and figure out why the colony wasted away.
Start with DWV
Although certainly not the only cause of colony death, deformed wing virus is a major player and the one I most often suspect. Collapse from deformed wing virus, spread by varroa mites, has such a recognizable pattern that it’s hard to miss.
- The colony was one of your largest
- The remaining cluster, if any, is small like a grapefruit
- You may find a small circle of head-down bees, sometimes covered with a fistful of other bees
- Only a small amount of brood, often with punctured cappings, remains
- You find few dead bees anywhere else in the hive
- Partially-emerged bees, some with tongues protruding, are scattered in the brood area
- It appears your colony absconded
- Guanine deposits cling to the ceiling of empty brood cells
The reason matters
Trying to determine the cause of colony death is more than pointing a finger at starvation or cold. Unfortunately, beginners often see the head-down circle of bees and instantly conclude that their bees died in a cold snap, and this relieves them of all responsibility for parasite/pathogen management. But when we fail to pinpoint the real problem, we are bound to repeat the mistake. It’s important to ask yourself why the colony was so small or weak it couldn’t take care of itself.
When you see dead head-down bees in their cells, just remember that live head-down bees are a normal part of a winter cluster. They do not enter cells to lick the bottoms but to maximize their chances of keeping the colony warm. Why they didn’t succeed is for you to figure out.
Honey Bee Suite