This season I have left BeeCosies on all my colonies to encourage brood-rearing and thereby make increase through splits and swarms. The BeeCosies also help the bees work the abundant heather we have close by.
As colonies keep their brood nest at around 30C when they are brood-rearing, my logic was there would be no harm in leaving BeeCosies on unless temperatures got above this level. In the Yorkshire Dales this is highly unlikely!
In July even when temperatures soared to nearly 30C - the Cosies were left on to provide insulation from the hot conditions. To test the merits of this approach, on one particularly hot day, I took off a BeeCosy, returned an hour later and measured the temperature with a surface thermometer on the hot metal roof as 52C! By leaving the BeeCosies on I hoped to slow down the build-up of excessive heat in the hive at midday, increase average night time temperatures and so flatten out temperature extremes.
The strategy has definitely helped me grow from 4 to 10 colonies this year and all 10 colonies have feasted on abundant heather we had in late August. The BeeCosies have raised hive temperatures which has helped the bees reduce the water content of nectar and so make heather honey for longer in the evenings. In the last 4 days of August, 10 colonies added a total of 9Kgs – the largest adding 2Kgs. So we are hoping for a good heather harvest from around 5 of the colonies, the other smaller ones being able to build up a good supply of heather honey supplies for the winter.