Hives & Humidity by Derek Mitchell

Derek is carrying out PhD research into humidity and heat transfer which involves comparing the performance of man-made hives with natural tree cavities. He uses mathematical computer models constructed from thermofluid theory and checks the results against practical, field measurements.

Perhaps, the principal interest for beekeepers is his focus on the control of humidity levels within the colony cavity. Bees need to regulate the humidity levels for the benefit of their brood (eggs and larvae) and also to assist with the reduction of water levels in the collected nectar. So the brood needs a minimum humidity for survival, while the honey demands a lower level to effect the desiccation of nectar to prevent its fermentation.

Changing humidity by extracting water is quite difficult but the bees achieve their aim by heating the super cavities which reduces the relative humidity levels, thus enabling the nectar to be dried out from around 40% water down to 20%. Derek and Elaine clearly demonstrated the large amount of water involved in this manipulation by using a set of large plastic bottles! It was also emphasized that the muscular efforts required to generate heat and fan the air also delivers a similar amount of metabolic waste water.

Derek explained that hive condensation, which is traditionally viewed by beekeepers as "cold and bad", can also occur at warm temperatures when the relevant dew point is exceeded. Condensation helps to eliminate moisture from the supers, but the operational temperature then becomes significant, too.

Rather as an aside, Derek drew our attention to the effect humidity levels have on varroa fertility: the higher the humidity, the lower the fertility. The numbers read: 59-68% relative humidity, 53% of mites produced offspring, but  with 79-85% relative humidity, only 2% of the mites produced offspring. A startling indicator of how humidity levels could be affecting the health of our colonies.

And finally, Derek strongly recommended that we follow the lead of wild bees and fully insulate our hives throughout the year. This assists the bees in their efforts to regulate both humidity and temperature levels. He considered the benefits of open, mesh floors to be completely unsupported by any decent evidence and they simply make the bees' work a lot harder.

This was a fascinating talk which could truly be characterised as "disruptive". Much discussion over cups of tea followed the Q&A session.

Rob Williams