Tree bumblebees
Tree bumblebees first reached the UK two decades ago and have since become one of our most prevalent species. How are these enterprising insects managing to thrive?
Amidst widespread pollinator decline, we investigate the newcomer bee that’s bucking the trend in Britain
Bumblebees are being hit hard. Agricultural intensification, the loss of flower-rich meadows, shock heatwaves and surprise frosts are driving global populations into decline.
But curiously, not all species are affected. One in particular is the tree bumblebee, which has recently taken up residence in the UK. Scientists are now getting to the bottom of how this bumblebee has not only claimed new ground – but is thriving.
“The tree bumblebee is one of these amazing species that, in the face of other bumblebee declines, is expanding massively in range,” says Ryan Brock, from the University of East Anglia, who is studying the UK population.
Tree bumblebees were first recorded in the UK in 2001, when a lone individual was found on a bramble flower on the outskirts of the New Forest. Native to continental Europe and Asia, their abundance had steadily increased through the 20th century in nearby countries such as Belgium and Germany. To experts, the bee’s appearance in the UK was intriguing, but foreseeable.
However, what no one could have predicted is the astonishing success of tree bumblebees once they reached the UK. Spreading at an average rate of roughly 11,500km2 per year, they rapidly colonised England and Wales. They had buzzed north to Scotland by 2013 and winged over to Ireland by 2017. Now, tree bumblebees are one of the UK’s commonest and most widespread species.
Sporting a russet thorax set against a dark head and abdomen, tree bumblebees look like miniature caped superheroes. And like all good superheroes, they could save the day. They are a welcome addition to the UK’s flagging fleet of native bees, which are economically and ecologically vital as pollinators of food crops and wildflowers.
Furthermore, they pose no threat to native UK bumblebees by competing for nesting spaces or food resources – after all, we have no endemic bumblebees, so native UK species coexist harmoniously with tree bumblebees in other parts of Europe. No one knows exactly how the first tree bumblebees reached the UK. Initial research suggested that one or two queens arrived through a chance event, and that the UK population spread from there. Those pioneers could have been stowaways with imported goods from mainland Europe, or perhaps a plucky female was whisked by winds across the Channel.
Inbreeding issues
Early research raised concerns that the UK tree bumblebee population was suffering from low genetic diversity as a result of a genetic bottleneck. Such events typically occur when a small number of individuals, embodying only a portion of the source population’s full genetic diversity, colonises a new range. This can doom new populations to crash due to inbreeding or because of their inability to adapt to changes in the environment.
However, subsequent studies began to reveal higher levels of genetic diversity than would be expected in a genetically impoverished population. “Something didn’t quite add up,” says Brock. To get to the bottom of this mystery, researchers at the University of East Anglia delved deeper into the tree bumblebee’s genetics.
Their study confirmed that the genetic diversity of the UK tree bumblebee population is actually on a par with several other UK bumblebee species. This implies that the tree bumblebee’s arrival in the UK was not a one-off event, but part of a large-scale westwards range expansion. “It is likely that there is ongoing immigration between the UK and mainland European populations,” says Brock.
A study published in 2021 provides further evidence that inbreeding will not pose a problem for UK tree bumblebees. Results indicate that the UK tree bumblebee has similar levels of genetic diversity to the mainland European population. “This has impacts on the population’s evolutionary potential and how well it may cope in the
Unusual nesting habits and an affinity to urban environments might be behind the tree bumblebee’s success in the UK.
future,” says University of Plymouth’s Vanessa Huml, lead author of the study.
By modelling colonisation scenarios, the researchers revealed that bees arrived in the UK from mainland Europe via several routes, ranging from the south to the north-east of England.
Conservationists welcome the news that the UK tree bumblebee population is genetically healthy. “The genetic underpinning of populations is important for conservation,” says Richard Comont, science manager for the Bumblebee Conservation Trust. “It is good to know that they don’t have that timebomb of inbreeding going on and that they are likely to continue doing well without any need for help.”
Novel nesters
Tree bumblebees are aptly named for their habit of nesting high above ground, traditionally in tree cavities. But now, you are as likely to find them nesting in vacant bird boxes or in the eaves of houses. They have even been known to brazenly take up home in fluff-filled tumble-drier vents. In contrast, other UK bumblebee species prefer to hole-up underground or in tussocky vegetation.
Experts think that these unusual nesting habits and an affinity to urban environments might be behind the tree bumblebee’s extraordinary success in the UK. “They are colonising what you might call an empty niche,” says Mark Brown, professor of evolutionary ecology and conservation at Royal Holloway, University of London. “We don’t have any other bumblebee species that nest in the same way that they do.”
The tree bumblebee’s spread through the UK has gone hand-in-hand with several decades of frenzied urban expansion. As humans have created new suitable nesting habitats, the bees have graciously moved in. This ability to thrive in built-up settings also offsets factors causing many other bumblebees to decline. For example, the loss of rural hay meadows does not affect tree bumblebees.
“Tree bumblebees are generalist foragers, so that means that they can take advantage of human-modified landscapes that have a broad diversity of floral resources in them,” says Brown. In early spring, queens may seek nectar and pollen from willow catkins, crocuses and daffodils in parks and gardens; later in the year, worker bees favour soft fruits including raspberries and brambles in allotments and hedgerows.
While many bumblebee species emerge from hibernation at fixed times of the year, the tree bumblebee’s opportunistic
habits afford it the flexibility to cope with fluctuating weather patterns. Tree bumblebee queens can simply wait out cold snaps in early spring and they are among the first to emerge in warmer years, taking full and uncontested advantage of plentiful available resources.
Furthermore, unlike most UK bumblebees, tree bumblebee queens produce two generations a year, so their numbers ramp up quickly.
Another quirk of the tree bumblebee is its resistance to a parasitic nematode, Sphaerularia bombi, that occurs naturally in UK queen bumblebees, curtailing its host’s breeding cycle and hindering population growth. “The parasite hits most species really hard, but the tree bumblebee has a ‘get out of jail’ card,” says Brown.
Flower power
While the tree bumblebee’s success is good news all round – it lives happily alongside people, causes no harm to native species and helps to pollinate plants – our other bumblebees are struggling.
Of the UK’s 24 bumblebee species, one-third are now conservation priority species due to large-scale declines, according to the Bumblebee Conservation Trust. Many declines are driven by shifting land use and agricultural intensification – since 1940, we have lost 98 per cent of UK flower meadows.
“The big problem is loss of habitat, loss of flowers,” says Comont from the Bumblebee Conservation Trust. “Particularly at either end of the season, in early spring and late summer.”
Restoring flower-rich habitats is thus a clear priority. With about 70 per cent of the UK’s land area occupied by farmland, policies that incentivise flower-friendly farming practices will go a long way toward solving the plight of the bumblebee. But everybody can make a difference. “We just need to plant more flowers all the way through the bumblebee flight season [March– October],” says Comont. “If everyone does a little bit, that is when we will start to see results.”