Top left: Buff-tailed bumblebee Bombus terrestris Top right: Red-tailed bumblebee Bombus lapidarius
Bottom Left: Cuckoo bumblebees (probably B.vestalis) Bottom right: Tree bumblebee Bombus hypnorum
Bumblebees are social insects but, unlike honeybees, bumblebee colonies die out in late summer-early autumn and do not persist overwinter. Only young fertilised queen bumblebees survive the winter. They bury themselves in the soil and emerge in spring. For most bumblebees, an ideal potential nest site is an abandoned tunnel in the soil made by a small rodent (eg Mrs Tittlemouse
). A location such as this may contain some nest material left by the mouse or vole that will provide insulation for the nest started by the queen. Bumblebee nests are often located in the sides of ditches and banks, in hedge bottoms and compost heaps. The Tree bumblebee often nests in bird nest boxes and cavities in trees. The Common carder bumblebee makes its nests on the soil surface, often in the base of grass tussocks.
Six of the 25 bumblebee species found in the British Isles are cuckoo bumblebees. Females of these species are unable to found their own colonies. They become active later in the spring than other bumblebee queens. They seek out and enter an established nest of a social bumblebee species. The cuckoo bumblebee queen kills or displaces the other queen that founded the nest and takes over, using the worker bumblebees of the host colony to rear larvae of the cuckoo bumblebee. The Wood cuckoo bumblebee invades the nests of the Early bumblebee; the Field cuckoo bumblebee goes into nests of the Common carder bumblebee; the Southern cuckoo bumblebee goes into nests of the Buff-tailed bumblebee.
Cuckoo bumblebees are similar in appearance to their host species but are of a slightly larger average size and tend to have more sparse hairs through which their hard, shiny bodies can be seen. Their most distinguishing characteristics are square, box-shaped heads, dark wings and as they do not raise their own young, female cuckoo bees lack the pollen baskets visible on the rear legs of queens and workers of true bumblebee species.
When a queen bumblebee has selected a suitable nest site, she secretes some wax that will form a pouch where she will lay a small batch of eggs. Although bumblebees are social insects, initially the queen is on her own and she has to forage for nectar and pollen, as well as constructing the nest and raising the first brood of larvae. In spring and early summer, the queen lays fertilised eggs that will become infertile females, known as worker bumblebees. The larvae are fed by the queen until they are fully grown and ready to pupate, and these offspring of a single mother are smaller than the later workers which have been well fed as larvae by their aunts. Once adult worker bumblebees have been raised, they can take over the tasks of foraging and rearing the larvae, leaving the queen to concentrate on egg laying. Worker bumblebees produced during the early stage of colony development are much smaller than those produced in mid summer, when there is more food available for the larvae.
In mid to late summer, the bumblebee colony reaches peak strength and may contain up to about 100 adult bees. At that time, if the colony is strong enough, the queen will lay some eggs that have not been fertilised. The larvae from these eggs will become male bumblebees. Some of the larvae from fertilised eggs will be given extra food and this enables the resulting adult bees to be queens with functioning ovaries. Mating takes place in late summer, so the overwintered young queens are ready to start reproducing in the following spring. In late summer, the old queen, the workers and the males die, while the young queens burrow into the soil where they overwinter. Common carder bumblebee nests are usually the last to die out, with workers still active in October.
Cuckoo bumblebees do not produce a worker caste
. All of their eggs will develop as larvae that become young queens or males. They do not need worker bumblebees of their own as they exploit those of the host bumblebee species to provide food for the larvae.