A welcome migrant?

By Athayde Tonhasca

If you spot a bumble bee in a garden, park or street from now until July, there’s a good chance it will be a tree bumble bee (Bombus hypnorum). Which is remarkable, considering this species was not even recorded in Britain until 2001. From the first sightings near Southampton, the tree bumble bee took Britain by storm: it has spread throughout England, Wales and much of Scotland. This represents an expansion of about 56 km/year. Today this species is often the most abundant bumble bee in urban and semi-urban environments. 

The tree bumble bee has a unique colour pattern among British species: ginger-brown thorax, black abdomen and white tail © Graham Ekins, The Essex Field Club

One of the characteristics that helped the tree bumble bee make itself at home in Britain is being a synanthrope. From the Greek syn (together) and anthropos (man), this term refers to species that benefit from humans and their habitats. Cockroaches, house sparrows, pigeons and the brown rat are quintessential synanthropes: their distribution and even survival depend on people. The tree bumble bee doesn’t go that far, but its life is greatly improved by our presence. Most bumble bees – most bees, in fact – nest on the ground, and appropriate sites are sometimes scarce. The tree bumble bee, as its name suggests, prefers to nest high up in a tree cavity. But many manmade structures above ground level such as bird boxes, compost bins, gaps under roof tiles, house eaves, and holes in walls and fences are perfectly suitable substitutes. And these sites are usually not disputed by other bees.

Compared to other bumble bees, the tree bumble bee starts the season early and forages at short distances from the nest. It also feeds on a wide variety of flowers, has a high nest density and may have two generations per year. Some or all these factors may have contributed to its success. Whatever the reasons, the speed and reach of tree bumble bee invasion have few parallels among alien species. 

An endearing creature such as the tree bumble bee is frequently referred to as ‘a recent arrival’, ‘a welcome addition to the UK’s fauna’ or ‘a newcomer’, because ‘alien’ has bad connotations. Alien species (those introduced outside their normal distributions) are often assumed to be harmful, therefore demanding eradication or control. But this is misguided. Most introduced species are neutral, as they neither damage nor help the environment. Some are beneficial, improving ecological services or providing food for native species.  Others become invasive, that is, they are harmful to habitats or other species. And bees can be invasive, such as the buff-tailed bumble bee in Asia and South America or the giant resin bee (Megachile sculpturalis), which is spreading in continental Europe and is known to displace native bees. 

So is the alien tree bumble bee a non-invasive species? Apparently, yes. So far it seems to cause no harm to other bees, and it may contribute to the pollination of our crops and wild flowers. But if its numbers keep increasing, other species may feel the nudge of competition. We have to wait and see.

Other bees seem to be fine with the newcomer, but its synanthropic nature has raised some human eyebrows. Tree bumble bees may take over bird boxes by expelling existing occupants, and their nesting sites may feel too close for comfort. Some people are also apprehensive at tree bumble bees’ apparent ‘swarming’. These are drones (male bees) hovering near a nest entrance, waiting for the opportunity to mate with a virgin queen flying from or into the nest – watch it. These events are called nest surveillances, and can last several weeks. But since drones are stingless, like all male Hymenoptera (bees, wasps and ants), nest surveillances are harmless.

A local arrival of tree bumble bees provokes spikes in calls to local authorities, beekeepers and pest controllers from people who want reassurance or, in a handful of cases, to get rid of bees buzzing nearby. Although most people are thrilled to have bumble bees as their close neighbours, this may change if bee numbers increase much further. Once again, time will tell.

The tree bumble bee, just like the ivy bee, seems to be a benign alien. But the consequences of its expansion are being watched closely.