Warm hives

Recently I went to a brilliant lecture organised by OBKA given by Derek & Elaine Mitchell, who have done some rigorous research into hive insulation and its effects. They have submitted their results for publication to the Journal of Apicultural Research. Here are some key points I picked up.

Q: Why do many sources say hives need lots of ventilation, but others stress warmth – which depends on minimum ventilation?

A: Much of the research was done around WW2, for example there was a key book published in 1947, and in those days wood was scarce and they did not have access to modern insulators like polystyrene or cellotex. The hives were thin walled and cold. In these conditions, to avoid damp and mould, yes, you need plenty of ventilation! But these days, we can make much warmer hives, which mimic the hollow trees bees evolved for, and in this case high humidity is better (high humidity helps bee egg survival and hampers varroa breeding) and there are no cold walls for damp to condense on and promote mould growth. For a well insulated hive, ventilation is undesirable.Apart from the desirability of high humidity arising from well sealed hives, high temperatures hinder varroa breeding, and suppress nosema and chalk brood, and promote grooming behaviour (which may also help versus varroa). And in general, better thermal control reduces early mortality – chilled brood don’t live long, even if they hatch into adult bees.

Interestingly, although not describing herself as a natural beekeeper, Elaine Mitchell has stopped varroa treatments. Her super-insulated Nationals, which use frames and foundation, have a few varroa but never in high enough numbers to hamper the colonies.

Q. Don’t bees use more energy when they are active? Colonies which are very well insulated don’t seem to cluster in winter – surely they will use up their stores too quickly?

A. No, they don’t use more energy. This is a misconception based on experiments on metabolic rate where bees were held in air at controlled temperatures. If they can warm the hive to a comfortable point, they don’t need much energy to stay alive – so the key is to provide an environment which they can warm at minimum cost. A well insulated cavity, in other words.

Q. What’s the best insulated hive type?

A. The lecturers did some serious testing (2.3 million measurements!) on 8 types of hive (12 hives in total). For a 20 watt input – that’s equivalent to burning 20 teaspoons of honey a day – they saw the following temperature rises (there was a spread of results, this is my estimate of the centreline of the graphs they displayed) –

• Natural tree cavity (6 inch thick walls) – 60 degrees C
• Skep, sealed with cow dung – 22 C
• Polystyrene hive (various types) – 22 C
• Warré, walls just over 1 inch thick, 4 inch thick sawdust quilt on top – 16 C
• Kenyan TBH with thick top bars, directly under a flat roof – 14 C
• Wooden National – 7 C to 10 C

They didn’t have access to WBC’s (a double walled National) or a wooden Langstroth (the American equivalent of a National). The polystyrene hives were: 4 Nationals, 1 Langstroth, 1 Nuc, and one poly 14×12.

Further key points

Colonies use far more energy to gather and evaporate nectar, in Spring and Summer, than to survive through winter. Evaporation works much better at high temperatures. So counterintuitively, insulation is more important in warm weather than cold! It’s making the hney which is energy intensive, not surviving winter. One UK bee farmer has 1,000 wood and 1,000 polystyrene hives. He gets more honey from his poly hives and is changing to them as fast as possible.

One reason skeps are so efficient is that they fit the shape of the cluster well – there is no dead volume to heat, and they have a good surface area-to-volume ratio.

Derek & Elaine will be presenting this lecture again at the BBKA Spring Convention.