A loud hum drew my eyes upwards to a swirling ball of insects dancing around the southern wall of our house level with the eaves. They were not wasps as I initially suspected for these looked bigger and the hum was louder. Inspection with binoculars confirmed that these were bumblebees with white tails and ginger waistcoats.
Occasionally one would enter or leave a crevice between the top of the wall and the wooden eaves but their main activity seemed to be their manic zooming around the space before the wall. Most bumblebees nest below ground or in dense grass or leaf-litter just above ground and I had never come across wall-nesting bees before.
Some research identified them as Tree Bumblebees, a species first recorded in Britain as recently as 2001 in the New Forest and which has spread rapidly northwards as far as southern Scotland. Their normal nest sites are holes in trees but it has been suggested that their advance here is partly due to nesting opportunities provided by the thousands of nest boxes in British gardens.
As with other bumblebees the queens, large at almost an inch long, emerge from hibernation in early spring and lay eggs that hatch into worker bees, which are all female. These bring nectar and pollen to the nest to feed later broods, from some of which hatch sting-less males or drones while others are new queens.
Male tree bumblebees, unlike other bumblebees, do not patrol circuits laying down scent markers to attract females but congregate around the nest entrance waiting for virgin queens to emerge with frenetic behaviour understandable given that only one in seven is likely to achieve their one goal of mating. For the lucky few this usually takes place while resting on vegetation (pictured). Once mated the queens then need to stock up on nectar to prepare for hibernation.
For feeding they are said to prefer wide-open flowers such as daisies rather than tube-like flowers like foxgloves. In our garden their favourites have been the tiny pink flowers of cotoneasters where they feed happily alongside the other bumblebees with which they are thought not to compete while providing extra pollination services.
As with other bumblebees they are not aggressive as long as their nests are not interfered with and the nests are likely to be vacated within a matter of weeks having peaked at up to 500 individuals. Certainly the activity around the nest in our wall has decreased markedly. I will be sorry to see them go.
By Denis O’Connor