Broodnest In The Cold

Physics in winter cluster
Source: Randy Oliver

Not many beekeepers pull brood frames out of their hives in the middle of winter. Whilst this is not surprising, given the risks, it does mean that we tend to imagine what’s going on inside the hive. And some people have terrific imaginations. Every beekeeper has theories about what the bees are up to when hidden away in the cold dark winter months. In fact, like many topics, there are disagreements about what is happening, why, and what to do about it.

Curious Walrus

As a curious walrus, I have searched for any actual science on winter bees. I also own an infra-red camera to see where my bees are without opening up the hive. I care about (a) preventing colony starvation and (b) preventing death or severe damage through varroosis. I’m interested in more, but if my bees are healthy and well-fed, my chances of having bees in the spring are good. Over-wintering in cool climates is yet another honey bee adaptation that is mindblowing the more you look into it.

Oxalic Acid

If there is one nailed-on way to kill varroa mites, it’s treating the bees with oxalic acid when they have no sealed brood. Frankly, treating them with any approved medication for mites when they are broodless is excellent. It’s just that oxalic acid, especially administered by sublimation, is proven to be both potent and safe for bees. I don’t think there is such a thing as an organic beekeeper, given the area that bees forage. Still, if you were trying to be one, you could use oxalic acid free from the guilt and shame that you may associate with using synthetic miticides such as amitraz. What’s more, you could feel content that the chances of mites becoming resistant to oxalic acid are remote. I rotate treatments between organic acids, thymol and amitraz, and feel no guilt; killing mites is what makes me happy.

No Brood?

The times when bees naturally have little or no brood are rare. It can happen after swarming if the new queen takes a while to get mated and lay eggs. For the same reason, if the queen dies unexpectedly – for example, the beekeeper squashed her or shook her outside the hive – there can be a period with no brood. There are also environmental factors that can cause queens to stop laying. In particular, a lack of pollen coming into the hive can put the breaks on her majesty. Baby bees (larvae) need milk made using pollen, amongst other things. Sometimes, the bees will eat the larvae because they don’t have food to keep them alive in a shortage. It’s a shame to waste all those nutrients. 

Many people find their hives broodless at the end of the summer. It’s almost as if the queen enjoys a holiday before going on a laying spree to make the winter bees. It seems to coincide with the time when drones are no longer welcome; they get booted out and not allowed back in. There is also winter; many people say that bees don’t have brood during the winter. It’s not always true, but people say it.

Cluster

One scientist that I trust when it comes to bees is Randy Oliver. He’s a very clever chap and formerly made his living as a commercial bee farmer in California. He has read the research papers on the brood nest in winter (there aren’t that many) and summarised it on his website. I have already written about heater bees and how the winter cluster expands and contracts, but there is more.

Bees form a cluster with an outer ‘shell’ made up of workers linked tightly together with their heads inwards and backsides outwards. The cluster can expand and contract depending on conditions, allowing bees some control over moisture, carbon dioxide, temperature and so forth. In Southwick, EE (1982) Metabolic energy of intact honey bee colonies. Comp. Biochem. Physiol 71A: 277-281, it was stated that “the response of the bee cluster resembles that of the ptarmigan, a very well insulated bird (with down feathers even on its feet) inhabiting arctic and alpine regions of western North America.” 

The metabolic rate of clustered bees is lowest at ambient temperatures of around 3-7 degrees C. It increases if it gets freezing outside because they need to generate more heat which requires the consumption of more honey. The metabolic rate increases as ambient temperature climbs and peaks at around 22 degrees C. It so happens that where I keep my bees, the winter conditions are ideal for low consumption of stores (mid-November to mid-March). 

Average temperature chart for Knutsford
Climate near me (Source: Weatherspark)

Randy Oliver

From Randy Oliver: A bee must never allow itself to get too cold—it will go into a ‘chill coma’ at around 10 degrees C, after which it loses the ability to reactivate its flight muscles for heating and will eventually die. Thus, 10 degrees C is the lowest permissible temperature for any bee in the cluster. And that is precisely the thoracic temperature of the outermost bees in the cluster, which form a tightly packed insulating shell of workers, roughly 1-3 inches thick. As described by Southwick, these outermost bees typically face inward (or upward if drops of condensed water are dripping on their backs), “packed thorax-to-thorax with a multitude of interlacing thoracic hairs.”

This head-in orientation of the shell bees takes advantage of their anatomy—a bee’s circulatory system pumps hemolymph (and any heat) towards the head. And due to a fancy countercurrent exchange arrangement of the aorta in the petiole (waist), virtually no heat makes it to the abdomen. Thus, a shell of tightly-packed bees (including those in empty cells) acts as a combination of insulation (due to the downy “hairs” that cover their bodies), a barrier to convective air circulation (thus trapping heat and humidity in the cluster), and as a “heat pump” that continually pushes heat back toward the centre of the cluster.

Heat and Humidity

A large cluster will find it easier to maintain a warm core than a small one, and a loosening up of the shell bees allows them to vent out excess heat. Given the delicate balance in a clustered colony of bees in winter, it’s best to leave them alone. Moving or shaking them can cause the cluster to fall or break up, causing considerable stress. 

Roughly 0.6 lbs of water are released during the digestion of a pound of honey. Most of it is ‘metabolic water’ from the breakdown of sugar into CO2 and water. That means that a non-flying colony consuming a typical third of a pound of honey per day produces roughly a half cup of water vapour per day. Mobus shows a table of metabolic water production and loss from bees’ bodies at different temperatures. The table indicates that the warm bees in the cluster core would lose water, whereas the cool bees in the insulating shell gain it. We’d expect metabolic water from the core to condense on the cool comb (perhaps diluting the honey in opened cells) or the bodies of the bees in the shell. It gets passed back to the bees in the core, perhaps with the honey passed from the shell of bees on the cluster’s periphery to the hungry bees in the core.

Data table of glucose and water consumption be bees
Table Showing Water Production in the Hive

I Got COVID

In other news, I caught COVID-19 three weeks ago and have been a miserable, grumpy walrus. I still have symptoms, but they are fading at last, thankfully. It’s a strange one; sometimes, you feel like you are getting better, then the following day, you are wiped out. The brain-fog element of the virus is most disturbing, but hopefully, I’m through the worst of that now. 

2 thoughts on “Broodnest In The Cold

  1. As always clear information and useable. Thank you hope you are feeling better soon and this is all behind us. Continue to stay well.

  2. Living in central florida and seldom removing more honey than I can bottle in a day keeps my bees filled with honey combs. I think it’s important to leave enough honey for the bees to winter or live through the next flow. We should never need to feed bees that have been in a hive box with comb unless they have just been installed.

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