Tree bumblebees: a growing buzz in our gardens Unless you've been on Mars for the last decade, you will know by now that Britain's pollinating insects are in trouble, and bumblebees are no exception. Of the UK's 24 species, two have become nationally extinct in the past century, and a further six are cause for serious concern. The problem, as for much of the rest of our native wildlife, is the spread of intensive farming, and especially the loss of flower-rich grasslands. Bumblebees depend particularly on protein-rich pollen from clovers and other members of the pea family. And yet, curiously, six species of bumblebees (the "big six") are doing very well indeed, at least partly because they thrive in gardens. You almost certainly have all six in your garden some of the time, even if not all at once. But the big six don't have our gardens to themselves any more, because Britain has a new bumblebee. Several bumblebees are found in Europe, but not in Britain, and one that many naturalists always thought "should" be here is the tree bumblebee, Bombus hypnorum. The tree bumblebee is found throughout much of Europe, as far north as the Arctic Circle. It also seems to like gardens, and has generally become more abundant during the 20th century. So there was no great surprise among the bee cognoscenti when a specimen was captured on the northern edge of the New Forest in 2001. Because bumblebees are social - that is, they live in colonies - if you find one bumblebee there are inevitably plenty more somewhere nearby. Since 2001, the tree bumblebee has spread rapidly, with the first records from Wales in 2009. I saw it for the first time in my Sheffield garden in 2010, and in June last year (the peak month for activity of this species) it was by far the commonest bumblebee in my neighbourhood. There seems no reason why it shouldn't eventually colonise the whole country, but it isn't everywhere yet. I looked for it on the south coast of Devon in June last year, and didn't see any. British bumblebees generally nest in holes in the ground, or on the surface in tussocky grassland. The tree bumblebee, as its name suggests, nests in holes in trees and is particularly fond of nest boxes provided for birds. These nesting habits are so different from any native bee that there is no reason to expect it to compete with them. Like all bumblebees, B. hypnorum is a useful pollinator and completely harmless as long as you leave it alone. Do you have tree bumblebees in your garden? Four of the big six are variations on the familiar black-and-yellow-striped theme, one is black all over with a red bottom, and the sixth is tawny all over. Fortunately the tree bumblebee looks nothing like any of them, with a ginger thorax, black abdomen and white tail. If you have trouble remembering that, just imagine a furry mint humbug; no other British bee looks anything like it. If you see one, and especially if you live in Scotland, Ireland or the Isle of Man, which are still waiting for their first records, the Bees, Wasps and Ants Recording Society (bwars.com) would love to hear from you. I know some people don't like exotic "invasive" species on principle, but even they can relax. Although our original tree bumblebees presumably came from France or Belgium, as far as we know they got here on their own. No difficulty about that - bumblebees are strong fliers and the Channel is no problem, certainly not with a following wind. So technically that makes the tree bumblebee a native species, because it spread naturally from an area where it was already native, unlike alien species that were introduced into Britain by man. The only surprise is that it took so long. Ken Thompson is a plant biologist with a keen interest in the science of gardening. He has written four gardening books, the latest of which is Do We Need Pandas? The Uncomfortable Truth About Biodiversity