In a career that has taken him from mechanical engineering to space craft software, Derek Mitchell returned to earth when his wife took up beekeeping. Looking at the hives that beekeepers used, he was startled because it was at odds with what he knew about heat transfer and therefore what a suitable home for an overwintering colony of insects might look like. So, he set about studying how honey bees operate and where they choose to nest in the wild – often tree trunks. That information made the hive types he saw beekeepers use make even less engineering sense in terms of heat insulation. From initial back-of-the-envelope calculations, he moved into serious scientific study and published an academic paper. Not content with that, he published more papers, six of which now await assessment for the award of a PhD from the Leeds Institute of Thermofluids at the University of Leeds.

Derek knows he is treading in a very contentious area with his controversial findings that run counter to much conventional beekeeping wisdom and previous studies. He is, therefore, at pains to be accurate and to point to evidence and calculations backing up his claims. He thinks that so much of the published research about heat transfer in hives is at best misleading or misdirected, often because it is based on what is happening in beekeepers’ thin-walled boxes, ignoring where bees choose to nest in the wild and how they cope without human intervention.

I met Derek to discuss his ideas as he was preparing for his PhD viva and dealing with the impressive publicity his work has recently had across multiple outlets – from the scientific press to national newspapers. To the layman, fluid thermodynamics is not easy territory, so he tailors his explanations carefully. Crossing disciplines – engineering and biology – he has even met with diverging definitions of particular terms which has posed challenging conversations at times and difficulties in finding funders who have a wide remit. Nonetheless, his ideas have been gaining credence and producing a change in beekeeper behaviour. The bees carry on doing their own thing of course.

Derek compares clustering to struggling to get closer to the fire to avoid death from cold. Bees need to be at 25C to be at their best to generate heat; below 18C their heating ability declines rapidly and at 10C they are on the edge of life, he explains. Those bees on the outside are trying to get closer to the core to avoid hypothermia; they aren’t there as insulation.

To make things even worse, a bee-less gap forms between the cluster and the hive wall. As the temperature becomes colder, the bees cluster tighter which increases the conductivity between them, warming up those further from the core but ultimately causing even greater heat loss because the bee-less gap widens, and the convection currents increase carrying away even more heat between the cluster and the wall.

It’s a vicious spiral because, as the temperature of the bees at the core falls, they must exert more energy to generate heat. In a sense they are caught between the devil and the deep blue sea – stress by over exertion or stress through cold temperatures. And, if the situation is extreme, at some point colony collapse can occur.

This idea counters long-held views that the cluster provides its own insulation by retaining heat through the interlocking hairs of bees forming an insulating coat. Derek says that his model shows that before they cluster, there is low heat conduction and weak convection, but that when they are clustered, there is high convection, dissipating heat around the cluster and a doubling of heat conduction within.

So, why, asks Derek, do beekeepers persist with thin-walled hives? “If they were mammals or reptiles, it would be considered cruelty,” he says. That’s an important consideration given the rising interest in the sentience of insects and the need to treat them well.

As a general rule, he says, the greater the cavity diameter, the thicker the walls need to be to maintain a similar internal heat environment for the bees. For example, creating a tall and narrow hive by turning frames on their sides, simulating the alignment of combs in a tree trunk, is likely to give the bees better insulation.

Skeps, he says, have quite good thermal properties mainly because they taper towards the top. Testing them, he found that if clusters can move up into the narrowing cavity provided by most skeps, the bees are likely to benefit in terms of warmth. The skep material is also likely to be better than the same thickness of wood in slowing down the movement of air/heat outwards. The straw of the skep operates in a similar way to the down in a duvet – the heat moving outwards from the bees hits fibres and has to go around them, thereby slowing down the heat loss. Nonetheless, even this is unlikely to be as good as is likely to happen in a tree trunk.

Timbers differ in their insulating properties. Marine ply is much inferior to cedar in terms of its insulating qualities. The difference between cedar and pine is probably marginal (standardised calculations demonstrate this). Balsa wood might be very good – but the bees would probably gnaw their way through it. Fortunately perhaps for beekeepers’ pockets, mahogany would be very poor for bees because of its high conductivity.

With WB Carr’s thriftiness in creating WBCs, Derek thinks that, if he was alive today, Carr would be searching skips for PIR insulation board (ie, Kingspan/Celotex) to make the WBC lifts (outer walls) and inner walls. Is the inner more important than the outer? It’s the combination of both, says Derek.

This doubled-walled WBC example highlights another complication – where is the condensation happening and can the bees access it?

With a varroa screen in place, the tray needs to be as close to the bees as possible and/or be highly reflective to minimise heat loss. Detritus falling on a reflective tray will absorb heat and counter the effectiveness of the reflective material. Ideally, the varroa screen and the tray should be one unit, like a cassette, and capable of being inserted for varroa counts for a day or two but replaced at other times by a reflective surface which will reradiate any heat from the bees back to the nest. Putting your hand underneath an exposed mesh will convince even the most sceptical of the potential for heat loss.

While insulation in the roof is very important, Derek advises not to forget the end walls where lots of heat is also lost. The old beekeepers’ trick of putting insulation on the sides of National hives is spot on, says Derek. And there is a big clue where to put that insulation – infrared photos of hives in winter usually show the cluster at the front or at the back wall – “That’s the hot spot and that’s where the insulation should go. Long before infrared thermography, the old-timers knew about that.”

Polyhives, plastics aside, offer very good Insulation but, “as with wooden hives, make sure they have small handholds – some have very large handholds and, with this thinning of the hive walls, lots of heat can be lost.”

Dry new honey bee comb is transparent to infrared, but once there has been brood in it (leaving behind moults) or it has held honey (containing proteins and water), it becomes opaque and that can lead to some strange effects. “The heat transfer environment inside the hive can be mind-bending at times. Trying to heat clean dry wax in a microwave is fairly pointless – it may melt but that’s because it has been in contact with its dish which has absorbed the microwave radiation.” Derek is currently working on a paper that includes an analysis of comb spacing and arrangements, but for sound academic reasons cannot speak about that at the moment.

Fluctuating temperatures from cold to warm and back again in winter is more dangerous for bees than consistent cold or consistent warmth, says Derek. The transition from warm to cold requires the bees to expend a lot of energy regulating the nest temperature, so to do this repeatedly endangers the colony. Ironically, he says, it’s best to keep the bees on one side of the ‘transition zone’ or the other. However, keeping them on the cold side is highly stressful and degrades their immune system, so the use of cold storage, as sometimes happens in North America, is flawed and therefore has ethical issues.

Derek has an intriguing take on why a high percentage of swarms don’t make it through winter, even in trees. When bees move into a tree cavity, they must expend a lot of energy to heat up the walls of the nest area. Like a stone cottage left unoccupied and unheated for a long time, the heating required to make the interior comfortably warm can be substantial. Once warmed, however, the insulation can keep it cosy. So, if a vacated cavity is quickly reoccupied by a swarm, the initial heating required is relatively small because of the energy expended by previous occupants. But that’s quite a contrast to moving into a long-vacated cavity which requires complete reheating.

The best time to add insulation to a hive is in spring when the bees have begun foraging and have energy to spare to heat the hive and its insulation without detriment to the colony’s comfort. “At that time, they have the equivalent of money to burn! However, if it’s added in autumn or winter, they then have to direct precious energy to reorganising the nest. There’s a beekeeper in the Yukon who says his bees don’t cluster unless it goes below -40C! Why? Because he insulates the hives so well and there is no top ventilation which would quickly lose heat.”

Now that Derek’s ideas are finding an audience, will beekeeping suppliers rise to the challenge of providing better beehives?  Already, he is being contacted by polyhive manufacturers wanting to know how they can improve their style of hive. Will hive manufacturers move from the ‘leather coat’ insulation which can work in mild conditions to the high-insulation qualities of a down coat?

Photos: Derek Mitchell, Richard Rickitt, Stephen Fleming