--- title: "The tree bumblebee in September: rare, until it wasn't" slug: "bumble-bee/tree-in-september" canonical: "https://www.thebritographer.co.uk/journal/bumble-bee/tree-in-september/" date: "2026-09-07" subject: "Tree bumblebee" wordCount: 994 readingTime: "PT5M" author: "The Britographer" publisher: "The Britographer" license: "https://www.thebritographer.co.uk/privacy/#license" --- # The tree bumblebee in September: rare, until it wasn't She is hanging upside down from a buddleia spike, back legs hooked over a floret, and the first thing you notice is that she is working. Not resting, not feeding for herself. There is a load on the hind leg the size of a grain of barley, pale yellow, packed and rounded, and she is still pushing her face into the next tube along. Ginger over the shoulders. Dark through the body. The purple round her is going over at the edges — brown at the tips of the panicle, the lower florets already dropped — and it is the first week of September. She is a small specimen, loaded up so full that you would be fooled into thinking her wings might refuse to carry her. *Bombus hypnorum.* The one that was not here twenty-five years ago, the one in your bird box, the one that has just made a September worker with a full corbicula into an ordinary thing to see. ## What the load on her leg says A bumblebee with pollen on her legs is a worker provisioning a nest. A gyne feeding up for winter eats what pollen she wants and packs none of it. A clear demonstration that somewhere within a few hundred yards there is a nest with larvae in it, still being fed, in September. For most of our bumblebees that is late. A colony founded in March has usually done its work by July or August — reared its gynes and its males, which left the nest while the workers left behind slowly dwindled, the foundress dead in her nest. By September what is left working the flowers is mostly the next generation feeding itself, and the last worn workers of colonies coming to an end. A worker with a load that size, working a flower like that, is a colony still in the middle of its year, and obviously still has a brood to feed. ## Two years in one The tree bumblebee is partially bivoltine. Some of the gynes that leave a nest in June do not go to ground. They mate, skip diapause altogether, and found a colony of their own at once, in the same year their mother did. Those non-diapause queens have a nest of first workers by July and are still provisioning in September, and it is their gynes, reared in the back half of the year, that hibernate and come up in February. Two colonies, one year, the second working a later flush of the same flowers, or whatever else becomes available. A warm year helps it. A long season, a dry August, buddleia and ivy running on into October — all of it widens the window a July colony has to rear its gynes before the cold. But the habit is not made by the heat alone. It is in the bee. It runs a second colony in an ordinary British summer too, and has done since it arrived. ## How rare it used to be It used to be worth writing down. Sladen, in 1912, noted the early bumblebee, *Bombus pratorum*, whose colonies break up in July, still sending workers to the runner beans in the first week of September 1911, when every other species was on the eve of dissolution — a note in the margin, a thing a careful man had seen once and thought odd enough to keep. For the best part of a century that is what a second generation was: an occasional trick of one small species in a good year. Then, from the 1990s, buff-tailed colonies in the south of England started running through the winter on ivy and winter-flowering garden shrubs. A clear change in behaviour. Within twenty years the tree bumblebee became one of the eight bumblebees you are likeliest to meet. Its second generation came with it, genetically and behaviourally. What had been an entry in Sladen became, in a single human generation, the bee on the flowers in September. > The rare thing did not become common. A common thing arrived that did it naturally when conditions aligned. ## The arithmetic of a second colony There is a cost, and the colony pays it. A gyne that skips diapause is gambling summer against an autumn hibernation. If the season holds, she has doubled the genetic line's chances for the year. If it turns in September, she has spent herself and a nestful of workers rearing queens that never fly, while her sisters who went to ground in June sleep through the failure and come up in February regardless. The species carries both strategies at once, in the same brood. This is not the colony hedging. It is a switch, thrown or not thrown in a June gyne by whatever cues it — daylength, condition, the nest she came from — and the sum is settled afterwards by the weather. ## On the spike Which is why the load on that leg is worth a second look. It is not a rarity any more. It is the sign of a colony that bet on the back half of the year, and this year, so far, the bet is holding. The buddleia is browning, the ivy has not opened yet, and there is a fortnight in between where a July nest lives on whatever is left in the gardens or the verge. Some of the early flowers are still offering a new flower every now and then. Red clover, left alone to grow on the verge, continues to flower, as does a flush of hawkweed and hawkbit. This is a good year to notice the second brood. On the last purple spike of buddleia, on the Michaelmas daisies, on the first ivy flower when it comes. Ginger, black, white; a load on the leg; in no hurry. The weather, predation, and some degree of luck will see the outcome of this colony, unaware of the change of weather that the autumn eventually brings. --- Source: The Britographer — https://www.thebritographer.co.uk/journal/bumble-bee/tree-in-september/. Cite as: The Britographer, "The tree bumblebee in September: rare, until it wasn't".