Bumblebees

Image: Andy Potter

Spreading along the base of the house wall is a large and old-established clump of Aubrieta, making a mass of small, purplish-lavender flowers. Because the wall is sheltered and faces south, the flowers always come out early and attract any bees that may be in the vicinity.

At lunchtime, when the sun was especially warm, a bumblebee was clambering from flower to flower, taking nectar and pollen. It was a large bee, a queen, with long, silky and luxuriant fur, black except for the buff collar, belt, and tail. Close inspection revealed that the hairy “pollen baskets” on the hind legs were not being filled. Instead she was feeding directly, as if stoking up after a long winter spent underground.

Although a few individuals of some species break their hibernation as early as February if the weather is mild, most emerge in April or early May. Like the smaller honey-bee, the bumblebee is a social insect and nests in colonies. Unlike the honey-bee, though, no worker bumblebees survive the winter. When she emerges in spring, the queen bumblebee must found a new colony from scratch.

Many of the really common species nest underground. Once she has replenished her reserves of fat, the queen searches for a likely site. Often an old mouse- or vole-nest will be taken over, and the bedding left by the previous occupants is rearranged according to her taste, forming a hollow chamber about the size of a fist, lined with the finest of the nest material.

Insects are cold-blooded, but bumblebees are able to generate heat, both by changes in body chemistry and by a muscular movement rather like shivering. During and just after nest-building, the queen becomes “broody” and her body temperature rises. She may sit in the nest chamber for long periods before the eggs are laid, and her body warmth removes any trace of dampness from the bedding.

Once the chamber is ready, she goes out foraging for nectar, which she brings back in her crop. Some of this is smeared on the inside walls of the chamber, helping to consolidate the nest material, and serving also as an emergency supply of food in the event of bad weather.

The eggs of a bumblebee are white and sausage-shaped and about two millimetres long. They develop in eight tubes which arise from the two ovaries, so that, in some species, only eight eggs are laid at a time; others may lay sixteen.

During her foraging the queen also brings back pollen, which she moulds into a lump in the centre of the chamber. The eggs are laid in this pollen, and then covered with the wax which she secretes from glands in her abdomen. The wax canopy extends to the floor of the chamber, helping to hold the egg-clump in place. The queen sits on top of the clump, using her body-heat to brood it.

Wax is also used to make a special pot for holding a reserve of nectar. The pot is usually placed near the entrance, so that the queen, while she is brooding, can extend her tongue to feed. She faces the entrance as she broods so that she can repel intruders – predators, other queens who may want to take over the nest, or queens of the cuckoo-bumblebees which will destroy her own brood and put another in its place.

The eggs hatch after about five days. The grubs feed on the pollen in the clump, which the queen now replenishes with further foraging trips. She must not spend too long away from the nest at this period, or the temperature of the clump will fall and development of the brood may be retarded. If the temperature drops below 10°C they may even die.

The grubs turn into pupae, spinning papery cocoons for themselves. At this stage the queen scrapes most of the wax from the clump and constructs a new layer or row of cells in which the next batch of eggs is laid.

The pupal stage lasts about a fortnight. About five weeks after the first eggs were laid, the first generation of young bees emerges. These then become the first workers, helping with the care of the later broods. The workers are always females, which are produced as a result of sexual reproduction. The males, or drones, are produced from eggs which are not fertilized. They are born towards the end of the life of the colony, and play no part in foraging or nest-maintenance.

At about this time the new generation of queens is also raised. Queens differ from ordinary females only in the care that they receive in their early stage, being supplied with very much more food than a developing worker.

The males leave the nest almost as soon as they are able to fly, and spend an idyllic, if short, existence, lazing about on flowers, needing to feed only themselves, and engaging in elaborate ceremonial flights designed to attract a mate. When the young queens emerge from the nest, they locate the males at visiting-places which are usually marked by scent and may have been used for the purpose by generations of bees. Most females will mate only once, and then, a supply of sperm stored in their bodies, seek out places in which to spend the winter months.

Meanwhile the workers of the old colony gradually die out – foraging is a dangerous business, with many casualties – and, by the onset of autumn, the queen herself, exhausted after all her efforts, is also dead. The first frosts kill off any stragglers, leaving only the young queens to carry on the species.

And now, after a winter spent in suspended animation, they are groggily emerging, like the queen feeding on the Aubrieta by the wall. Soon she will have to find a nest, and then it will begin all over again.

(Introduction to these pieces; see all)