The life cycle of a bumblebee is squeezed into a few short months, but they certainly pack a lot into that time. No wonder they are called busy!
Bumblebees, like honey bees and ants, are social insects and live together in colonies; unlike those of the other two, however, bumblebees’ colonies are annual ones, newly-built each year.
Bumble bees do not store honey to tide them over winter, so in autumn all members of the colony dies except the young mated queens which hibernate until spring in the shelter of a hedgerow, heap of moss, or pile of leaves, normally burrowing a few inches below the surface of the soil. The rest of the life cycle is squeezed into our few spring and summer months.
In the early spring you can often see a queen bee, newly-emerged from hibernation, blundering around close to the ground as if she were looking for something. She is: a suitable place to nest. Some species choose underground sites – a leaf-filled hole beneath the roots of a decaying tree, or a disused hedgehog or mouse nest – while others, the carder bees, make an above-ground, ball-shaped nest of moss, grass and leaves which they weave or ‘card’ together.
Once the nesting site is established, the queen makes a circular chamber, about 3cm (1¼in) in diameter, in the centre and dries it out with the warmth of her own body. She lines the chamber with a layer of wax produced from special glands on the underside of her abdomen, then makes a cell to receive the eggs which have developed inside her from her mating the previous autumn. The cup-shaped cell is moulded out of a mixture of wax and pollen and is filled with yet more pollen.
Finally the queen lays ten or more eggs inside the cell and seals it with a lid of wax.
The queen bumble bee is a careful and provident mother; she incubates her eggs by sitting on the cell to keep it warm, and also continues to search for food. She brings in more nectar and pollen (the sole food of adult and larvae) than she herself needs and stores them in a specially constructed ‘honey pot’ made from wax so that her brood will not go short when cold, wet weather confines it to the nest.
The larvae, which look like white grubs, hatch in about five days and feed on the pollen provided in their cell. The queen replenishes nectar and pollen and the larvae grow quickly on the rich diet. They moult several times, then spin silken cocoons around themselves and pupate inside them.
At this stage the queen removes the wax round the cocoons and uses it to build new cells.
After about two weeks the very small adults – all unfertilised, subordinate
females – gnaw their way out of the cocoons. They are the workers whose job in life is to keep the queen and her growing colony supplied with food and to defend the nest and assist with cell-building. They never mate but they can lay eggs which, since they are not fertilised, produce only males (drones).
The workers, which usually live for several weeks, go out to forage every day except when it is cold and rainy. While the workers forage, the queen stays in the nest, building new cells and laying more eggs. Eventually a colony of up to several hundred workers may develop; but the whole time, the cluster of cells – the comb – is rarely larger than the palm of a man’s hand. As the food supply increases and the nest is enlarged, the workers also increase in size.
Towards the end of summer, the queen lays a number of unfertilised eggs which develop into drones. Their sole task is to mate with the queens – they never work. At the same time the fertilised eggs the queen lays develop not into workers but into new queens – up to 200 in large nests. These larvae are fed by numerous workers and receive an abundance of food; it is this level of nutrition which determines whether fertilised eggs become workers or queens.
In late summer the young queens and drones disperse to find mates from other colonies. By late autumn the old queen, workers and drones have all died and only the young fertilised queens go into hibernation, ready to begin the cycle again in spring.
Bumblebees get their name from the slow bumbling flight; the body looks too heavy for the small wing. There is a saying that their flight defies physics and no one can work out how they achieve it – but that hasn’t been true for many years.
The thrust achieved by the way the winds beat are more than enough to lift the bee and its load. It’s all in the twist… The wings twist on every stroke, are clapped together at the top of the up-stroke, and are then opened rapidly, so creating a vortex which increases the lift on the next down stroke.
The humming or buzzing you hear as the bees fly from flower to flower is produced by the vibrations of the wings which beat increasingly fast, at a rate of 130 to 240 beats per second according to the size of the bee (smaller bees have faster wing beats).
Neither queen nor worker bumblebee needs to be taught how to go about gathering food, they instinctively know what to look for.
Their long tongues enable them to reach the nectar stored deep inside long-throated flowers such as foxgloves and white dead-nettle. By bringing forward their proboscis and alternately shortening and lengthening the tongue nectar is drawn up by a capillary action through a deep channel enclosed by hairs on the tongue. The nectar then passes to the honey stomach, where it is stored until the bee returns to its nest.
Different types of bumblebee require different types of flowers to visit, because some have short tongues and others have longer ones.
Bumblebees with short tongues will visit flowers that are very open and have nectar within easy reach, such as daisies, white clover, lavender, and comfrey.
Longer-tongued bumblebee species are attracted to honeysuckle,
The Power of Different Flowers for the Bumblebee
delphiniums, white dead-nettle, foxglove, and catmint, for example. Deeper flowers tend to offer more reward for the extra effort involved by having more nectar per flower. However, there are some cunning short-tongued types of bumblebee that have evolved to sneakily take advantage of this by cutting a hole into the base of the flower in order to steal the nectar!
Pollen is another important food source. The hairs on bumblebees’ bodies brush masses of pollen from the flower stamens; you can sometimes see bumblebees almost completely covered in a dusting. The pollen is held by special storage baskets on the bee’s back legs – some bees are known to have carried up to 60 per cent of their own weight in pollen, but the more usual load is 20 per cent.
The female bumblebee and the honey bee both collect pollen to take back to the nest. Using her front and middle legs, a bee brushes pollen off the hairs on her head and front part of her body forwards to her mouth. She moistens the pollen with nectar and then passes it to the pollen brush on one hind leg and finally to the pollen basket on the other leg. It is then pressed into position and held in the basket by the fringe of long, curved hairs.
To learn the best flowers to plant in your garden to attract bumblebees, click here to visit Wild About Gardens.
The Buff-Tailed Bumblebee (bombus terrestris, above) is our largest species, at up to 25mm. It is too big for deeper flowers, preferring to visit open flowers such as daisies and cherry and apple blossom, but is a cutting 'thief' of foxgloves, etc. Queens are frequent visitors to sallow catkins in March and April. Nests are built well below ground level.
Bombus Pascuorum (right) is the only garden bee that has the bright golden thorax. Although long-tongued, and therefore most often seen visiting tube flowers such as foxgloves (pictured below) and white dead-nettle flowers, it is also especially fond of lavender.
It tends to built nests in long grass at ground level, but has also been known to use old birds' nests or bird boxes. Queens usually appear in late March, and the colonies continue well into autumn, and can still be seen long after most other bumblebees have died off for the year.