A most compelling case for summer insulation of the beehive
A warm mid summers evening in the apiary and the hives are humming as they evaporate the nectar down to below 18% water by volume. There’s plenty of ventilation on top of the hive, at the entrance and via the mesh floor to allow all of that excess water vapour to escape…
That’s the thinking beekeeper’s logic and is what we’re taught. We get honey, mostly with a water content below 18% and we can leave some for the bees too.
But what should be happening if the bees had their perfect environment; a cavity in a tree with a small opening only…let’s think about the difference between the tree cavity and a thin-walled wooden beehive with lots of ventilation.
I wrote about this in November 2021 where we can see that the tree cavity has a much better insulation, the tree wood might be 400mm thick having a much lower conductance than the 20mm cedar hive wall. The tree cavity has no ventilation, it is a fixed space that doesn’t get cracked open all the time.
I’d moved many of my hives from cedar to polystyrene, a mix of Paynes and Maisemore between 2020 and 2023. In November 2023 I explained that I’d had over 1500lbs honey in the 2023 season, double the amount of the year before and three times the year before that. Only a front entrance, the insert in place all the time under the mesh floor with the hives directly onto a concrete slab.
I think most beekeepers can see the benefit of a well-insulated hive in winter, but how does it work to the benefit of the bees in summer?
Let’s start with the brood nest which the bees keep at around 36C to raise the brood. The brood also needs to be ideally at around 75-80% humidity, to prevent desiccation of eggs and larvae (Ref 1, very few books explain this).
The bees will bring in nectar as the season progresses, storing it above the brood. The nectar can vary in its sugar content from 20-60% sugar depending on the flowers that it came from (heather nectar can have up to 60% sugar content). During the day the nectar is foraged by the older bees and then passed to house bees who store it in cells above the brood. Then in the evening the bees start to ripen the honey, by desiccation of the nectar. Desiccation is the drying of something, not exclusively by evaporation.
We need to think about the hive thermo-dynamics.
Oh no!
Don’t worry I’ll try and make it simple.
In the traditional thin-walled wooden hive with an open mesh floor, the humidity inside the hive will by and large follow the humidity outside and because the hive is so open and ventilated, the bees must work very hard to have any chance of changing it, typically 50-60% in a UK Spring and Summer. This means that eggs and larvae are susceptible to desiccation. (Becoming too dry). More on this later.
The bees manage the brood temperature by ensuring that they have enough bees to create the localised environment. Whereas in the tree cavity with only a small entrance the bees have much more control over temperature and humidity.
In the evening following foraging, the bees start to desiccate the nectar.
In the tree cavity the dilemma (for us humans to fathom) is that the air is already quite humid inside the hive making it difficult to increase the humidity by evaporation of nectar at the same temperature.
BUT, by increasing the temperature from 36 to 39C in the stored nectar region the relative humidity is reduced and can take up much more water vapour by desiccation. In fact, at 39C the air is dry enough to desiccate nectar down to 18% water. (Ref 1 fig13) Wow!!
It is interesting to note that a fully saturated atmosphere at 15C becomes only 50% saturated at 25C.
In the thin-walled wooden hive because of the low insulation, high conductance, the open mesh floor and ventilation, the internal humidity is pretty much tied to that of the outside humidity, in contrast to the tree cavity, and the poly hive with solid floor, where the bees can increase the temperature with a resulting reduction in humidity fairly easily.
The humming that I hear is the bees increasing the temperature above the brood creating a flow of air around the hive, the process of nectar desiccation including evaporation.
The contrast between the two is so large, that the bees in the tree cavity and low conductance, highly insulated poly hive with solid floor can use much less energy than those in the wooden hive to desiccate nectar (Ref 1)
In the tree cavity and poly hive, as the bees fan the nectar, the water vapour at 39C above the brood will roll across the top and down the sides until it reaches its dew point temperature, the temperature that it condenses back to liquid water. At this humidity the dew point is 5C lower at 34C. (Ref 1)
As a simple rule of thumb the dew temperature decreases by about 1C for every 5% decrease in RH (RH>50%)
The ‘drier’ slightly less warm air continues to circulate in the hive, from the bottom back up. The low conductance of the tree cavity/poly hive means that there is low heat loss and hence the bees need less energy to keep the nectar sugar metabolization (process) going. The nectar is desiccated in this way by dehumidification and evaporation. The bees reuse the warm water. (Ref1 fig1)
The bees in the tree cavity and poly hive can collect, ripen and store so much more honey. They can even forage on flowers with a lower sugar content as the nectar can be desiccated much more efficiently when these flowers might otherwise be ignored. These bees can travel further to forage for the same reason. As much less energy is needed to desiccate nectar, the bees live longer! (Ref 2)
A study has shown that by raising the external temperature of a wooden hive in an internal apiary, the honeybees start collecting from flowers of lower nectar concentration, yet collect the same amount of sucrose by increasing total number of flights. It follows that a well insulated hive would have a similar result. (Ref1)
There is research that shows that the fecundity of varroa is affected at a humidity of 80% (Ref 1) As a caveat here, I haven’t treated my bees for varroa for several years now and my colony losses are negligible. There’s also the consideration of general brood health being improved at 80% humidity compared to 50-60%.
In my post of November 2021, I show a photo of a wooden hive with an entrance hole that has been modified by the bees using propolis. There was no other ventilation in this hive. The hive was set up with a brood box and three supers in Spring and left alone until the autumn because of the Covid lockdowns, the hive being in an out apiary of mine. When I harvested the honey, the supers were the heaviest and driest that I’d experienced at the time, honey with water content of 16%. This apiary is at sea level with higher than average temperatures, but it shows that given the right conditions the bees can get exceptional results. Apart from its high conductance wooden walls it had some similarities to the tree cavity in that it was a FIXED chamber, never disturbed during the main season. This hive was the exception, not the rule. I’m finding that my poly hives are giving similar results in the past two years.
Think about what happens when you ‘crack’ open the beehive, inspect the bees and then put it back together again. The bees must get the brood back to 36C, the humidity back to 75-80% and repair the draughty cracks created. The ‘crack’ as you open the hive breaks the propolis seal that bees make inside the hive. In the tree cavity the bees cover the insides with a propolis layer; this along with low conductance of the tree itself, is instrumental in creating the environment within the hive
For this reason, I inspect a few times during April and May to ensure that they are healthy and the queen is laying well, and then I leave them alone until I harvest the honey. Most of my hives are now poly hives with the insert permanently in place under the mesh floor.
In November 2023, I explained that I took frames out of the top two supers on hives (all polystyrene) on a weekly basis during the 2023 hawthorn flow. I was effectively keeping these hives at the same cavity size rather than increasing it with new supers. As a result, the bees kept filling the empty comb that I put in such that I could harvest again in a short period. The main reason for me to do this was that I had run out of new supers to put on! I do think that adding new supers isn’t such a problem, but we need to remember how the bees desiccate the nectar and how difficult this might become further from the brood as more supers are put on and also the effect on overall insulation/conductance.
In the poly hives the bees draw foundation right out to the inner wall of the hive and fill them with honey that they have desiccated down to below 18C. In my wooden thin walled hives I invariably have to switch combs around moving inner combs out to the edge and vice versa to get them drawn and filled.
In 2023, many beekeepers locally had no honey. This was probably because of the cold wet July and August. It is quite probable that their bees had Hawthorn honey, but then consumed it during the cold wet summer months.
Experienced beekeepers often say that they have to use a dehumidifier to get the water content of their honey down to 18% or less as the bees haven’t been able to. I’ve done so myself in years gone by, but not in the past two years.
I hear more and more beekeepers say that they don’t keep bees for the honey. It is indeed so wonderful to support a colony of honey bees, watching them through the season as they forage and pollinate, it is of great therapy to me to do this too, but I love honey too.
I have had honeybee colonies create a brood pattern in a thin vertical direction up the hive, particularly when I don’t use a queen excluder. This may be how some bees cope with the high conductance thin-walled hive, using the outer comb as insulation.
Every year I put out ‘bait’ hives. These are to attract swarms. I have learnt from experience and from talking to older beekeepers that the most effective bait hive is a box about 40 litres with old comb, a small entrance and a solid floor. I have tried using hives with mesh floors, but with little success. 2023 was a swarmy year; I had seven swarms come into bait hives. A swarm does not like the openness that the mesh floor provides.
My experiences in beekeeping have convinced me that beehives should be well insulated with benefits all year round. I feel that beekeepers have struggled to grasp the internal fluid dynamics of the hive ever since the moveable frame hive was invented….it isn’t easy to grasp! By trying it I have found great results. There are always improvements to be made, maybe the poly hives could be double thickness, maybe there’s a better material that could be used.
In conclusion, thanks to Derek Mitchell’s research, ref 1 and 2 are two of his research papers, links below, I have a better understanding of how a honeybee colony is so adept at controlling heat and humidity when given the right conditions. I always thought that the bees had to evaporate all the water from the nectar, but I now realise that an increase in temperature above the brood nest to 39C makes the air dry enough to desiccate the nectar down to 18% water. Of course, it’s simple when you ‘see’ it. A lightbulb moment, but it’s not an obvious way of thinking.
The best that we as beekeepers can get currently is the poly hive. I would like to see manufacturers produce a solid floor option, but it’s not too difficult to manage with a mesh floor with the insert fitted. I hope that you can see how this is important for the honeybees in controlling the atmosphere within the hive.
The bees manage the hive fluid dynamics, something that is not easy to visualise especially when the gases involved are all invisible; water vapour, oxygen, nitrogen and carbon dioxide, the lightest one being water vapour. It’s not surprising that beekeepers using moveable frame hives want to add ventilation when they so often see condensed water at the top of the hive. I am a chartered electrical engineer, but it has taken a post graduate mechanical engineer Derek Mitchell for me to improve my understanding of the hive thermodynamics a process akin to a nuclear power station process.
Who knows better? The bees or the beekeeper? The bees have been around a lot longer than humans 😉
References:
Derek talking to beekeepers in Florida on the improtance of summer insulation for beehives
The Mountain 