Range expansion of the tree bumblebee

By Liz Elliott

My PhD focussed on modelling the range expansion of species and I often used butterflies as examples of species expanding their range in Britain but I didn’t know much about other pollinators and so I thought it would be great to do a bit more research into this.  The first species that sprung to mind to find a bit more about was the tree bumblebee (Bombus hypnorum).  Until 2001 this bee was not present in the UK, which was surprising as it is widespread on mainland Europe.  The first sighting was made in the New Forest and since then this bee has rapidly spread around the country with sightings from Northumberland to Wales and Cornwall.  Have you seen a tree bumblebee near you?  If so help BWARS to map its spread by visiting: http://www.bwars.com/index.php?q=content/bombus-hypnorum-mapping-project

The tree bee is easy to identify as it looks very different to other bumblebees with its ginger thorax, black abdomen and white tail, and (helpfully for ID!) the queens, males and workers all have the same colouring although vary in size.  The queens of this bee emerge from hibernation in February or early March and workers are active throughout spring.  Males are produced in late May and June and a partial second brood may be active in the summer. It is possible for sightings of late flying queens to be made into November and December, so you may spot this bee out and about nearly all year round!

Identifying features of the tree bumblebee

  

This bumblebee is found in a wide range of habitats, including woodlands, gardens and parks.  Unlike other bees in the UK, which nest at ground level, this species nests above ground in cavities, such as holes in trees and roof spaces, and indeed last year at Leeds University a nest was observed in a gap in one the buildings in the Biology department!  The tree bumblebee is also often found nesting in bird boxes, so even if you don’t have birds nesting in your garden, a bird box could be a really important nest site for these bees instead.

Nesting in a bird box in an allotment in Leeds - can you spot the bee zooming in on the entrance?!

The nest in the Biology department at Leeds University - a little blurry but you can just make out a few in the gap in the wall.

A nest in a garage roof.

 

The males of this bee do a dance outside nesting sites, waiting to mate with the new emerging queens.  Last year the Leeds team were lucky to observe this behaviour outside the nest in the garage roof.

Males dancing outside the nest.

 

Bees are really good to encourage into your garden and the tree bumblebee in particular is an important wild pollinator of fruit trees and is often found visiting their flowers and those of other plants that produce fruit such as bramble and raspberry.  While out and about in Leeds we have often seen them on flowering currant, so if you have one of these in your garden have a look and see if you can spot a tree bumblebee on it – I had a look at the one in my garden and was delighted to find there were some on mine!

Tree bee feeding on a flowering currant (Ribes sangiuneum)

 

This bee is expanding its range when other bees are declining because it is often associated with open clearings in woodlands but also because urban gardens are a good source of readily available foraging and nesting resources.  It also has relatively high dispersal ability and so is able to fly between patches of suitable habitat.  This high dispersal ability may be how it colonised the UK, flying across the English Channel, potentially with the aid of strong winds.    As this bee has different nesting sites to other bumblebees in the UK it is not competing for resources and so is unlikely to impact negatively on other species.  The rapid expansion of this species may mean that it will soon be found across the whole of Great Britain, so watch this space!

We rescued this rather drowsy tree bee from somebody's drive earlier this week!