As a beekeeper we can follow the basic advice which is to ensure that each hive has enough stores to last it through the winter. But how much is enough? The rule of thumb for a two box hive is 20-30kg but where does this figure come from? Having spent several seasons keeping bees I am of the opinion that this is from experience and of course what better rule to follow than the one that has worked best in the past.
We are also advised to heft the hives from time to time over the winter to make sure that there is plenty of weight in the hive that indicates that the bees still have honey. If we feel that the hive is getting light then we need to add some sugar fondant especially if it is winter time and into early spring.
This rule of thumb works in most cases, but I wanted to know more about the bee biology and what the bees actually do during this time of cold.
Honeybees are cold blooded as individuals, but the colony is warm blooded. An individual bee needs to maintain a temperature of at least about 10 degrees C; if it drops below this it cannot build its temperature back up again, so to keep itself warm as the temperature drops (outside the hive to -5 deg C) and the temperature of the bee drops to 8 deg C it will activate its thorax muscles, the ones it uses to fly, but in this case it activates them to generate heat.
The colony on the other hand needs to generate a heat of about 35 deg C in order to raise its young and hence is warm blooded. This is handy in the sense that the colony can get a good start in rearing its young in spring.
Back to what happens in winter, as the temperature drops below 8 deg C outside the hive the bees don’t go out, they form a cluster. Indeed they form a cluster when the outside temperature drops to 15 deg C, but at this temperature bees will still venture outside to forage. At 8 deg C all the bees are in the hive in a cluster. The centre of the cluster will be at around 35 deg C, but this will be a small volume, and at the outside of the cluster the temperature will be around 10-15 deg C. With the cluster in this condition it is the ‘tightness’ of the cluster or the closeness of the bees to each other that maintains the temperature of the cluster. As the temperature gets lower the bees will move closer together to tighten the cluster. When the temperature outside reaches -5 deg C then the bees on the outside of the cluster will start to activate their wing muscles as explained earlier. If they don’t do this then they are in danger of dropping below 8 deg C and then falling off the cluster and dying. The implication of the outer bees activating their wing muscles is that they need more fuel than if they are just clinging to the cluster.
The centre of the cluster will drop below 35 deg C during times of no brood and if it drops below 13 deg C the colony will die.
The cluster will move slowly as the winter progresses and the movement tends to be upwards into an upper box, however, it is conceivable that the cluster will move laterally towards stores on the outer edge, but this may not be that easy as the combs have to be traversed.
With the idea that the bees form a cluster for most of the winter and if we start by assuming that the temperature (in the UK) won’t drop below 5 deg C for many nights then the bees will survive on smaller amounts of stores than if the ambient temperature outside the hive is well below freezing for long periods. As a quick estimate if a bee weighs 1oomg maybe it needs 10mg a day to survive (sounds a lot really but I’ll start with that). If the cluster is 5000 strong ( an over-wintering nucleus) then the cluster needs 50g of stores a day which is 1.5kg a month and for a winter period of 4-5 months 6-7.5kg are needed. (10000 bees would need 15kg etc).
Tom Seeley suggests that the cluster creates a power output in the region of 40 Watts. (40 joules per second. 4,194 joules in 1 calorie. 1 gram sugar creates 3.87 calories. 1 gram sugar can create 4194 x 3.87 joules = 16,230.78 joules. At 40 joules per second one gram will last 16,230.78/40 seconds = 405 seconds or about 7 minutes. That is 213 grams per day or 6kg per month). By my calculations that would be a colony of 20,000 bees over-wintering.
But how much stores does one bee need remaining pretty much motionless in the cluster? Does it really need 10mg a day? I have read that a honeybee in a swarm has between 30-40mg of honey in its abdomen. The swarm can hang around for four days or more before moving to a new home and then this honey is used to start wax comb building, so I wonder how much a bee needs in the winter cluster. I will stay with the idea of 10mg a day unless I find out otherwise. It fits with my hypothetical calculation and with Seeley’s proposed 40 Watt cluster.
Another reason for my interest in actually what happens in winter is that I am overwintering nucs this winter and wanted to make sure that they would survive. My thoughts are that of course they must have plenty of stores but that extra insulation will help to keep the cluster warmer than without such that the bees don’t use more stores than they might otherwise. As mentioned earlier bees form a tighter cluster as it gets colder which means the opposite happens if it is warmer and hence the bees will find stores further out in the hive.
Mike Palmer of New England is an advocate of overwintering nucs, he uses Langstroth nucs and keeps two together with stores in top boxes. I have heard beekeepers say that they keep nucs on top of normal sized hives to help keep them warm. I am also aware of at least one large commercial beekeeper who overwinters nucs in polystyrene hives and has very low losses.
There is what I will call a ‘warmer’ window for a ‘full’ winter cluster which is from 8 deg C to -5 deg C (outside ambient temperature) where the temperature regulation of the cluster is managed by the bees closing more tightly together or moving apart (exposes them to outer stores) If the ambient temperature drops below -5 deg C the bees have to activate their wing muscles and use more fuel. There may be some debate about when the bees start to activate their wing muscles to keep warm, but I will keep with this idea for now.
There may be the situation whereby the ambient temperature remains low for a prolonged spell such that the cluster remains tight and needs to move up as its easiest way of reaching more stores and hence leaving behind the outer stores down below. Normally this is managed in the full hive with the outer stores in the lower box being used for rearing young in spring.
My thoughts are that with a nucleus the prolonged spell of cold can be the biggest issue for starvation, but if the hive is well insulated; a polystyrene hive used, then the cluster doesn’t spend too many days tightly packed together.
This all remains to be seen over this winter (2014/15 mid Wales) and having plastic observation sheets is very helpful in watching the movement of the cluster as the winter progresses. Fondant at hand just in case its needed.
The following diagram is for reference taken from Honeybee Democracy Thomas D. Seeley:
The Biology of the Honey Bee, Mark L. Winston
Honeybee Democracy Thomas D. Seeley
Mike Palmer overwintering nucs video from the Honey Show 2013