We have a new bumblebee on the block, the tree bumblebee (Bombus hypnorum) has arrived from Europe, probably because of climate change. First seen in Wiltshire in 2001, in the next twenty years they spread throughout England and Wales and well into Scotland. The first Devon record was in 2010, and Tony Dustan-Smith recorded one in Sidmouth in March 2012.
Unlike some non-native newcomers such as 3-cornered leek and Himalayan balsam, tree bumblebees can be welcomed. It is a successful pollinator species and shows no sign of affecting other bumblebee success.
Tree bumblebees are easy to identify. They have a very woolly ginger thorax and a black abdomen with a clear white tip. The only other bumblebee that might confuse is the common carder bee which also has a woolly ginger thorax but the abdomen is rarely dark and never has a white tip.
Originally a woodland edge species nesting in holes in banks and in trees, hence the name, tree bumblebees have taken to town life and will nest in roof spaces and sheds. As with hole nesting birds such as blue tits, they are happy to set up home in nest boxes put up for garden birds. The nests only last for a short season and so do no harm.
Occasionally a nest in a bedroom roof space may give out an audible buzzing noise at night, and if you disturb the nest by poking about, the workers may defend it, but their stings are quite mild for most people.
The tree bumblebee year begins in late February to March when hibernating queens wake. They top up their food reserves and begin scouting for a suitable hole to build a nest. Four to five weeks later the first female workers emerge. Some will remain in the nest to do the housework while others are the hunter gatherers collecting pollen and nectar. Tree bumblebees have short tongues and so feed from open flowers, particularly members of the Rosaceae such as bramble, apple and cotoneaster. Willow catkins also provide copious and easily accessible pollen.
Most nests have up to 150 workers at the peak. In late May and into June a generation of reproductive bees are born, males and fertile queens. The males will scout the neighbourhood looking for nests with queens. If they find one, they crowd in a mini-swarm outside the nest hole waiting their chance. If a queen emerges they will compete to get hold and mating takes place in mid-air.
At the end of the season in July the old queen and workers die off, their work done. Males will live an independent life for a few weeks and then die. Fertilised queens will spend a few weeks fattening themselves and then they will find a safe place to hibernate, ready to restart the cycle the following spring.
Ed Dolphin
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