Bombus hypnorum, the tree bumblebee.
© entomart, Attribution licence, via Wikimedia Commons

Bombus hypnorum Linnaeus, 1758, the tree bumblebee, is one of Britain's most remarkable conservation success stories. First recorded in the UK in 2001, it has spread across England and Wales and into Scotland within two decades, one of the fastest range expansions of any British insect. It is instantly recognisable by its three-colour pattern, shared by queens, workers and males alike: a chestnut-brown thorax, a jet-black abdomen and a clean white tail. Unlike most British bumblebees, which nest underground, the tree bumblebee nests above ground in tree cavities, roof spaces and, most famously, garden bird boxes, which is how most people first meet it. See where it fits among Britain's wild bees on the UK Native Bee Species Map, or explore the wider genus in the World Bee Atlas.

Taxonomy and Classification

Carl Linnaeus described the tree bumblebee in 1758 in the tenth edition of Systema Naturae.^[1] It belongs to the family Apidae, genus Bombus, and the subgenus Pyrobombus, the "fiery-tailed" bumblebees, of which B. hypnorum is the type species.^[2] Pyrobombus is a large and successful subgenus accounting for nearly a fifth of all bumblebee species, with centres of diversity in Central Asia and North America. In Britain its close relatives include the early bumblebee (Bombus pratorum), with which it shares the pratorum-group.

Physical Description

The tree bumblebee is one of the easiest British bumblebees to identify because all three castes, queens, workers and males, share the same distinctive three-band pattern: a chestnut or ginger-brown thorax, a black abdomen, and a pure white tail.^[4] The combination is unique among British bumblebees. The white tail is the clinching feature, separating it instantly from the all-ginger common carder bee (Bombus pascuorum), with which a faded individual might otherwise be confused.

Queens are roughly the size of a buff-tailed or white-tailed bumblebee queen, while workers are often noticeably small and males fall in between. There is natural variation: the brown of the thorax can darken from the centre outward, and in extreme cases a bee can appear almost entirely black except for the white tail. Males often carry a tuft of chestnut hairs on the top of the head and may show ginger hairs mixed into the abdomen. The species has a short proboscis and a characteristically rounded head.^[4]

A New Arrival: The UK Range Expansion

The tree bumblebee is genuinely new to Britain. The first confirmed British record came in 2001 from near Southampton in Hampshire.^[5] From that single foothold it has undergone one of the most rapid range expansions ever documented for a British insect, spreading across most of England and Wales, reaching Scotland by 2013, and being added to the Irish list in 2017. Within roughly two decades it went from absent to one of the commonest bumblebees in many gardens.

Its success runs against the grain of the wider bumblebee story, which is largely one of decline. The reason lies in its ecology. The tree bumblebee is a species of open woodland and woodland edge, and the modern suburban garden, with its mix of trees, shrubs, flowers and cavity nesting sites, mimics that habitat almost perfectly.^[6] Research on the colonising UK population found its density was positively associated with both urban and woodland cover, a combination unique among the bumblebees studied, which helps explain why it has thrived where others struggle. You can see how its spreading range overlaps with Britain's resident bees on the UK Native Bee Species Map.

Nesting: Bird Boxes and the Buzz at the Entrance

The single most distinctive thing about the tree bumblebee is where it nests. While most British bumblebees nest underground in old mammal burrows, the tree bumblebee nests above ground, in tree cavities, roof eaves, behind soffit boards, in cavity walls, and very commonly in garden bird boxes, frequently reusing old birds' nests as a foundation.^[5] Queens have even been recorded evicting blue tits to take over an occupied box. This habit, more than anything else, is what brings the species to public attention.

Most homeowners first notice a nest because of the bees hovering and darting around the entrance hole. These are almost always males engaged in what is called nest surveillance: groups of males patrol outside the nest waiting for new queens to emerge so they can mate. It can look alarming, like a swarm, but it is harmless. Male bumblebees have no sting at all.^[5] The behaviour fades once the new queens have left and the colony naturally winds down.

Behaviour and Life Cycle

Queens emerge from hibernation early, in late February or March, and begin searching for an aerial nest site. About six weeks after a queen establishes her nest, the first workers emerge and take over foraging. Colonies are larger than those of most related Pyrobombus species, typically reaching around 150 workers at their peak and occasionally up to 300 or 400.^[5] In the south of England the species can sometimes fit two generations into a single year. New queens and males are produced as the colony matures, after which the founding colony dies and only the mated new queens survive the winter.

As a forager the tree bumblebee is polylectic, visiting a very wide range of flowers, but it has clear favourites. It is strongly drawn to soft fruit blossom such as raspberry and bramble, along with comfrey, cotoneaster, fruit trees and garden shrubs, while showing little interest in the legume flowers favoured by many other bumblebees.^[4] Strikingly, it forages over very short distances: a genetic study of a UK population estimated a mean worker foraging distance of only about 104 metres from the nest, far shorter than most bumblebees, which suits a bee that nests in flower-rich gardens.^[7]

Conservation Status

The tree bumblebee is assessed as Least Concern and is one of the few bumblebees whose range and numbers are increasing rather than falling.^[2] In an era of widespread pollinator decline it is a genuine bright spot, demonstrating that some species can adapt to and even benefit from human-altered landscapes when the right nesting and foraging resources are present. As an effective pollinator of soft fruit and garden plants, its spread brings a useful new pollination service to British gardens and allotments.

Its arrival has prompted some discussion about possible competition with resident bumblebees for nest sites and forage, but to date there is no clear evidence that the tree bumblebee has harmed native species, and its preference for aerial nest sites means it largely avoids competing for the underground cavities that most native bumblebees use.

Relationship to Honey and Pollination

Like all bumblebees, the tree bumblebee does not produce harvestable honey. Its colonies are annual and store only small quantities of nectar to ride out poor weather, so there is no surplus to collect. Its contribution is pollination, and as an early-emerging, soft-fruit-loving garden specialist it is a particularly valuable one for raspberries, blackberries, currants and orchard fruit.

For us as a honey company, the tree bumblebee is a reminder that a healthy garden supports many kinds of bee at once. The honey in our jars comes from managed Apis mellifera colonies, but those hives share gardens and hedgerows with bumblebees like B. hypnorum, and all of them depend on continuous flowering forage. Our British Wildflower Honey, gathered in the Midlands from bramble, clover, hawthorn and other native blossom, comes from the same kind of mixed forage that tree bumblebees thrive on. To understand what a world with fewer pollinators would look like, visit Your Plate Without Bees.

Apis mellifera | British Midlands

British Wildflower Honey | £10.99

Raw, cold-extracted honey from the rolling Midlands, gathered from the bramble, clover and hawthorn that garden bees forage too. SALSA-certified British supplier. 280g.

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