While you – the beekeeper – likely read this from a warm, winter abode, science writer Dave Black nestles into the cluster of your honeybee colonies to help us understand how we study their use of energy…
By Dave Black
For a long time no-one really appreciated the sophistication of honey bee’s control over the energy they use, what we now think of as ‘thermoregulation’. Individual bees were ‘cold-blooded’ poikilotherms, meaning they could not regulate body temperature. If they were social bees they could rely on a collective ‘colony-level’ response to temperature change. Things have changed…
Re-thinking the ‘cluster’
In the 1960s this simple view began to unravel. Scientists like Harald Esch were measuring the body temperatures of honey bees and describing quite clearly metabolically induced temperature changes in lots of different circumstances, but the knowledge was taking time to disperse. Perhaps that was because, well, “Über die Körpertemperaturen und den Wärmehaushalt von Apis mellifica”[i] took a bit of translating. I remember my father, a geological scientist at the time, was expected to be able to read German, but...
In 1971 the United States Department of Agriculture published a ‘Technical Bulletin’ (No.1429) by Charles Owen based on 1,200,000 temperature readings from thermocouples installed in a full-sized Langstroth beehive – a proper ‘colony-level’ study. Still work relevant today, it reflected a view that we might need to revise.
Owen’s study began by describing a winter cluster like this:
“The bees form a cluster, clinging tightly together on the combs in the hive. The outer bees form an insulating shell that prevents excessive loss of heat. Within the cluster the warmth permits normal cluster activity such as rearing the young and consuming food. However, the precise nature of the cluster, its temperature, size, movement in response to external temperature, and ability to survive extreme cold for extended periods have not been investigated in detail.”[ii]
The cluster was described by that study, but not explained, but nevertheless it informed the conventional view of an over-wintering colony.
A box full of venomous insects
Since then, people like E.E. Southwick, James Simpson, and Anton Stabentheiner have gradually been filling the gaps. Anyway, by now we have much better information about how individual bees can control their body temperature, some information about the hive environment, but almost nothing about natural honey bee nests. Still, the idea of a warm winter cluster covered by an insulating ‘mantle’ of ‘bees persisted.
Derek Mitchell is a mechanical engineer (with a MSc in microelectronics) studying at the School of Mechanical Engineering at the University of Leeds[iii]. By sheer chance he got interested in beekeeping. As an engineer he thinks of a hive as a building, one that hasn’t changed much, and one where the occupant hasn’t had much input into the design. Hives are designed for beekeepers, portable, easily examined, simple and cheap to make. The air-conditioning experts inside haven’t had much of a say. He puts it this way:
“It is clear that heat transfer inside and to the outside of the nest is integral to the life and success of honeybees. But almost nothing is known about it. This is partly because the expertise is associated usually with non-biological concerns and partly because it is difficult to study or measure heat flow in a box full of venomous insects and even more difficult when those insects are deep inside a massive tree.”[iv]
Air-Conditioning engineers
In taking a slightly more holistic perspective, just as a spider needs its web, a cavity-dwelling bee needs its enclosure. Biologists talk about ‘extended’ organisms (an ‘extended phenotype’) in that the organism isn’t complete without these physical (and cognitive) extensions to its life. It doesn’t really make sense to study them in their absence. Honey bees are air conditioning engineers and sugar refinery workers and they are not that without a suitable dwelling; the nest architecture is part of what makes them who/what they are. They harness the nest structure to flourish in tropical environments, arid desserts, and severe winters. They precisely control several temperatures in different parts of their nest, and engineer close to 80% relative humidity near brood, but only 50%RH where they desiccate nectar for honey.
Thermofluid Engineers
Studying heat transfer, thermal energy, and the behaviour of liquids, gases and vapour, is a branch of science and engineering blending thermodynamics and fluid mechanics known as ‘thermofluids’, and Mitchell has produced several challenging papers on the topic as it applies to honey bees in recent years. For example, ‘Honey bee cluster – not insulation but stressful heat sink’ (2023)[v]; ‘Are man-made hives valid thermal surrogates for natural honey bee nests’ (2024)[vi], and culminating in January of this year with his PhD submission, ‘Thermofluid Engineering of the honey bee nest’ (2024)[vii]. As he puts it, “Our core hypothesis is that the nest enclosure is an intrinsic part of the honey bee colony, which uses nest properties to manipulate the thermofluids within, which have in turn shaped the honey bee. This has not been understood in either academia or agriculture and has led to adverse consequences in both the study and husbandry of this important pollinator.”
It’s not just that we haven’t really understood what overwintering behaviour and cluster formation are all about. When it comes to this aspect of bee husbandry, have we even been studying the right thing? Mitchell again says, “...new research into honey bees now needs to: first, carefully validate whether the conclusions they arrive at are innate honey bee behaviours that would occur in their natural nests or are anthropogenic i.e. artefacts of the conditions in man-made environments; second, have a clear understanding of heat transfer and the heat transfer implications of their experimental treatments and controls...”
Thermodynamic economy
Nor is it just about hive materials and design. Thermodynamics is fundamental. It makes no more sense to ignore principles of thermofluids than it would to deny gravity, or believe in a different kind of chemistry. It affects everything. Thermal efficiency has a role in determining how far bees forage, what they forage for, even on colony size. Foraging efficiency is key to a colony’s very survival. Brood nest temperature has been implicated in pupal mortality, brain development, short-term memory, pesticide tolerance, the behavioural performance of adults, and the age structure of the colony. Varroa seldom reproduce successfully under high humidity. Temperature control is also a factor in chalkbrood proliferation.
There is a useful interview with Mitchell and Dr Jamie Ellis with Amy Vu available in a podcast episode, from the series ‘Two Bees in a Pod’ produced by the Honey Bee Research and Extension Lab at the University of Florida[viii]. It provides a helpful synopsis of his argument. For one thing, considering the ‘physical’ thermofluid properties, a wooden hive is nothing like a tree cavity, and it’s possible the behaviour we see in a wooden box may only be a way of coping with the profoundly different thermodynamic circumstances they find themselves in.
Second, he points out that a dependency on foraged sugars, and the concentration of these by evaporation, makes the thermal energy efficiency of the whole process highly significant for a honey bee colony. A matter of life or death in fact. To an engineer or physicist, a honey bee is in the business of collecting and storing energy which they will then use later, and like businesses, the economy matters.
Computational Fluid Dynamics
Mitchell brings a new empirical perspective and rigour to studying the honey bee and its nest. Much of his work is in measuring their physical properties as carefully as possible and using mathematics to solve the complex equations that describe the physics of moving fluids and gases. The resulting data is then compared with the real world.
It’s a field of study known as Computational Fluid Dynamics (CFD). Thanks to burgeoning computer power these models reliably predict fluid flows and the associated physical properties, such as velocity, pressure, viscosity, density, and temperature. CFD is a commonly applied design tool in aerodynamics, weather forecasting, marine engineering and so on – think F1 car design, SpaceX, or America’s Cup yachting if you like.
A New Langstroth?
Beehive design has remained unchanged for a considerable period of time. Moveable frames date back to Wiltshire’s John Aubrey in 1683, although in a more modern sense to Augustus Munn (1834) and L.L. Langstroth in 1852[ix]. The later’s particular innovation was to extend the use of the bee-space to the horizontal plane which allowed boxes to be tiered and separated, but which also created spaces above the brood nest that increase thermal transmission. Practices such as the use of queen excluders and the supering of honey boxes plausibly constrain the colony’s natural thermoregulatory behaviour, increasing the energy expenditure of the colony and exacerbating colony stressors.
Could the Langstroth hive be improved or superseded to allow the bees to optimise the colony’s thermal gradient?[x] I’m not convinced Mitchell is too optimistic about that, and I am even less convinced New Zealand would be leading the way on any change.
Dave Black is a commercial-beekeeper-turned-hobbyist, now retired. He is a regular science writer providing commentary on “what the books don't tell you”, via his Substack Beyond Bee Books, to which you can subscribe here.
References
[i] Harald Esch, Z. Vergl. Physiol. 43, 305–335 (1960) 10.1007/BF00298066
[ii]Owens, C.D., 1971. The Thermology of Wintering Honey Bee Colonies (Technical Bulletin No. 1429), USDA Technical Bulletins. USDA ARS.
[v]Mitchell, D., 2023. Honeybee cluster—not insulation but stressful heat sink. J. R. Soc. Interface. 20, 20230488. https://doi.org/10.1098/rsif.2023.0488
[vi]Mitchell, D., 2024. Are Man-made Hives Valid Thermal Surrogates for Natural Honey Bee Nests (Apis Mellifera)? SSRN Journal. https://doi.org/10.2139/ssrn.4713810
[vii]Mitchell, D.M., 2024. Thermofluid Engineering of the Honey bee (Apis Mellifera) nest. https://etheses.whiterose.ac.uk/34266/1/thesis-28-01-2024.pdf
[ix]Eve Crane, 1999. World History of Beekeeping and Honey Hunting, pp 405-424. Routledge, ISBN 0-415-92467-7
[x]Cook, D., Blackler, A., McGree, J., Hauxwell, C., 2021. Thermal Impacts of Apicultural Practice and Products on the Honey Bee Colony. Journal of Economic Entomology 114, 538–546. https://doi.org/10.1093/jee/toab023