by Dave Goulson
The Bumblebee Year
A bee is never as busy as it seems; it’s just that it can’t buzz any slower.
Kin Hubbard (American humourist)
Until the mid-1800s, so little was known about bumblebees that even the basics of their life history had yet to be described. Charles Darwin was one of the first to study bumblebees, but he had diverse interests ranging from barnacles to worms to coral reefs, and bumblebees could necessarily occupy only a small proportion of his attention. The first person to devote himself wholeheartedly to the study of bumblebees was Frederick William Lambart Sladen, the eldest of twelve children born to Lieutenant Colonel Joseph Sladen, commanding officer of the School of Gunnery at Woolwich. Born in 1876, Frederick spent his childhood at the family home, Ripple Court near Dover in Kent. Victorian Britain had a strong tradition of studies of natural history, particularly among those of ‘independent means’, the lucky few who could afford to spend their days skipping through flowery meadows, pinning butterflies and pressing flowers (or at least that is how I imagine it). Schooled at home by private tutors, Sladen seems to have had plenty of time for raking through the fields surrounding his home looking for bumblebee nests, which he dug up and brought home for study. Astonishingly, by the age of sixteen he was the world expert, publishing a short book on bumblebee habits in 1892, and continuing to study them for the next twenty years, culminating in the publication of The Humble-bee; its life history and how to domesticate it in 1912. This was the first proper book devoted to bumblebees, and it is still an invaluable guide, containing many fascinating observations, anecdotes and descriptions of the experiments that he carried out in his garden. Species that are today rare or extinct in Britain, such as the short-haired bumblebee, were familiar to Sladen, and his descriptions of the nests of such species remain pretty much all that we know. No one since has come close to matching Sladen’s knowledge of the nesting habits of bumblebees. Much of the information that follows in this and subsequent chapters was first discovered by Sladen.3
The bumblebee year really begins with the emergence of the first brave queens from hibernation in spring, often as early as February or March. These queens are the biggest of their kind, and since only the very fattest survive hibernation, the first to emerge are zeppelins of the insect world. By this time they are nearly starved, having been in hibernation since the previous summer, eight months or more ago. They drone slowly through the cold air, labouring to find scarce spring flowers. Pussy willows are one of the few plants in flower this early, and these small trees can attract hundreds of hungry queens. The flowers droop under the weight of the enormous bees. Only the female trees produce sugar-rich nectar, and it is this that the queens need first to boost their energy levels after their long sleep.4 By contrast, the male trees produce bright yellow pollen-bearing catkins, and this is where the queens go next to stock up on the protein-rich pollen. Inside their abdomens, their ovaries are shrivelled and need protein to expand and develop their eggs. Sperm is also stored inside each, from a brief tryst the summer before with a now long-dead male.
Over the next few weeks the queens slowly fatten, and as their eggs begin to develop they start to search for nest sites. Many species like to nest underground, but as bumblebees are pretty poor diggers, they often explore existing holes. This nest-searching behaviour is very characteristic, particularly in the buff-tailed bumblebee, one of our commonest and largest species. These huge bees sway from side to side just above the ground, quartering across grassy terrain and along hedge banks in search of a mouse hole or other cavity. They are quite easy to follow and watch. Any dark patch of soil or indentation in the ground will draw their attention, and they will land to investigate. If there is a hole, they will crawl inside. Often they remain underground for many minutes, exploring subterranean chambers and tunnels dug by rodents or moles or rabbits. They may walk for many feet underground (some nests can be 10 feet or more along tunnels). Exactly what they look for we do not know, but presumably they are trying to assess whether the cavity has all the attributes required for a successful nest. Is it large enough? Is it deep enough to be safe from badgers? Will it flood in a rainstorm? Is it dry and free from draughts? Is it already occupied? More often than not they eventually emerge and fly away to try elsewhere, but they must find somewhere or die in the search.
Although bumblebees do not gather nest material from far away like birds, they do need insulation in the form of feathers, hair, dried moss or grass, so this probably plays a large part in their choice of nesting site. Very often they seem to choose places where there is an old bird, rabbit, mouse or vole dwelling. They will enthusiastically rearrange the materials that they find in and around the nest into a nice cosy ball with a hollow centre. Some bumblebees, known as carder bees, will even vigorously comb it with the bristles on their legs. Their name is derived from this, carding being an old-fashioned term for combing wool before spinning. Bumblebees will readily use man-made materials. Loft insulation seems to be just the job, and some bumblebee species nest in roof spaces under the insulation (although modern felted roofs are much harder for them to penetrate). I have even heard of a bumblebee nest in a disused tumble dryer, with the bees using the accumulated fluff in the air filter to make themselves very snug.
Just like birds, different bumblebee species tend to nest in different places. White-tailed bumblebees seem to love settling under the wooden floors of garden sheds, for instance, whereas buff-tailed bumblebees often end up under patios, or will use airbricks to access the wall cavities of houses. Early bumblebees will sometimes nest in old birds’ nests high in trees, and both the early and the red-tailed bumblebees will use tit boxes, but only if they contain an old bird’s nest for them to use as insulation. Queens of the feisty Turkish bumblebee species, Bombus niveatus, can be so bold as to take over occupied redstart nests, driving the birds away even though the redstarts are perhaps fifty times heavier than them. The tree bumblebee, as its name suggests, always nests in holes in tree trunks, and again will often use tit boxes, whereas the carder bumblebees tend to nest just above the soil, in dense tussocks of grass or under piles of dry leaves under bramble thickets. Sadly, one of the few places where bumblebees generally won’t settle is in the bumblebee nest boxes widely sold in garden centres.
Whatever site she chooses, within it the queen bee constructs a loose ball of insulating material with a central cavity and a hole through which she can squeeze. Within this cavity, perhaps the size of a tennis ball, she constructs a thimble-shaped cup from wax produced from special glands on the underside of her body, which is then scraped off and moulded into shape with her legs and mandibles. She fills this cup with honey (honey being concentrated nectar). She also collects pollen, which she forms into a small ball about the size of a pea, using a drop of sticky honey to hold it together. This she also coats in wax. By this time her ovaries should have fully developed, and she is ready to lay eggs, which she fertilises as they pass out from her body, using the sperm stored inside her. Next, she excavates a small hole in her pollen ball, pushes a batch of sixteen eggs into the dough-like material, and then seals them over with wax. The number sixteen is determined by the paired ovaries, each of which can produce eight eggs at a time. The eggs themselves are cream-coloured and shaped like slender sausages.
The mother incubates her laid eggs in much the same way as a bird. She shapes the pollen ball to create a shallow groove along the top into which her body fits snugly. She has thin fur on her belly so there is good contact between her and the brood (just like the brood patch of a bird). Now she begins to shiver, and warms the eggs, keeping them at about 30°C even when the air temperature outside the nest may dip well below freezing on an early spring night. Shivering uses lots of energy, which is why the queen has already placed her pot of honey within easy reach while she sits on the eggs; but this is not enough to keep her going for long. If she leaves her eggs for too long to collect more nectar they will get cold, but if she does not go to fetch food sh
e will starve. A queen may use her own weight in sugar each day to incubate her brood, which may necessitate visiting up to 6,000 flowers. If these flowers are too few and far between she will be away from the nest for much of the day, her brood will cool and as a result develop too slowly, and she will wear herself out in her frantic search for food. Hence the proximity of lots of nectar-rich spring flowers is probably vital.
All being well, after about four days the eggs hatch into tiny comma-shaped white grubs. They have no legs, and are little more than eating machines, with a mouth at one end and an anus at the other. These unattractive creatures remain clustered together, a herd of maggots grazing on the pollen dough. As they use it up, the queen must gather more by visiting flowers, and add it to the brood clump.
To grow, the grubs must also shed their skins. This is a general feature of insect development – namely that their skin cannot grow with them as ours does, but is instead a more or less fixed size. So for the grub to get larger, the old skin must be sloughed off, revealing a new and larger skin underneath. Think of it as a paper bag – after each moult, this new skin will be half-full and wrinkled. As the grub feeds, this skin stretches out until the wrinkles are gone. At this point it cannot grow any larger, for if it eats more, its skin will split – which is exactly what happens. The grubs will shed their skins three times in about two weeks, by the end of which they will have increased enormously in size and will each be roughly the size of a broad bean (although they vary a lot). At this point the grubs each spin themselves a neat egg-shaped cocoon, using silk produced by glands in their mouths. Once inside their cocoon, they shed their skin again, but what emerges is very different: now the pupa has the rough shape of an adult bee, but retains the creamy colour of the grub. The legs, head, antennae, eyes and tongue are clearly visible, and neatly folded against the body. Inside the pupal skin an amazing transformation is taking place. The internal structures of the grub – the nerves, muscles, gut and so on – simply dissolve, and the body rebuilds itself in the entirely different form of an adult bee.
This reconstruction takes about two weeks, at which point the adult bee bursts out from the pupal skin and bites through the cocoon to escape. The first bees to emerge are all females, all the daughters of the queen; they are her workers. When they first emerge they are entirely white, resembling, with a little imagination, rather cuddly miniature polar bears. The wings are initially shrivelled, but in the first few minutes after the bees emerge, blood pumps into their veins to stretch the wings out. Once expanded, they quickly harden. The white appearance lasts for a couple of days, during which time the workers remain within the nest and the queen makes her last few foraging trips. As soon as her first batch of larvae has pupated, however, the queen will have laid a second batch of eggs and by now these should be well-developed grubs. The first generation of grown-up workers now takes over the care of these grubs, helping to keep them warm and feeding them. Once their adult colours have developed, some of these workers venture outside to gather food. At this point the queen stops the risky business of gathering her own food, and from here on she remains in the nest for the rest of her days, being fed by her worker children.
The workers are initially tentative when they leave the nest. Their first flights are short, as they familiarise themselves with the entrance to the nest and surrounding landmarks; if they become lost, then they are doomed. After a few exploratory trips they start to range further, and experiment with visiting flowers. They have innate preferences for blues and yellows, so will tend to visit flowers of these colours first. Finding nectar and pollen in flowers, and learning to gather it efficiently, is harder than it might at first appear, and it takes a few days for the worker bees to perfect their skills. Once they have, however, food begins to flow rapidly into the nest, and the queen starts to lay batches of eggs more regularly. If it is in a good position, with lots of flowers nearby, the nest grows rapidly. The workers also produce wax, like their mother, which they use to build more honey pots, and in some bumblebee species they also build special pollen-storage containers in the form of tall wax pots with a flared rim. Old pupal cells, once their occupants have departed, are remodelled with wax and used for honey storage. Often a wax dome is also built over the nest, which helps to insulate it and keep predators out. Compared to the military precision of a honeybee nest, which contains regular flat sheets of perfectly hexagonal cells, the bumblebee nest is a rather ramshackle, jerry-built affair, but it serves well enough.
By July, the nest may have grown to contain a sisterhood of several hundred worker bees, labouring together to help their mother. In physical size, the nest may now be as big as a half-deflated football, although this varies and some species, such as the early bumblebee, are content with a much smaller nest, more like a tennis ball. At this point in the summer, the queen changes strategy and ceases to produce daughter workers. Up until now she has been producing a chemical signal, a pheromone, which tells the developing grubs to become workers. Now, she ceases producing the pheromone and starts to lay both male and female eggs.5 In the absence of the pheromone signal, these female grubs develop as future queens, growing much larger than the grubs destined to become workers, and so take longer to reach full size. The males are smaller, similar in size to the workers. A big nest might produce a hundred or more future queens, and several hundred males, although most nests seem to produce either mainly new queens or mainly males.
Once they have emerged from their pupae, these new males and queens spend a few days resting, feeding on honey and pollen reserves. They will sometimes go out of the nest on brief exploratory missions, and before long they leave the safety of the nest for ever. The males are doomed to be short-lived for, with summer’s end approaching, they have no way of surviving the winter. Their only role is to mate. In high summer, males can be very common. They sit around on flowers drinking nectar; they prefer flowers with big, sturdy heads such as thistles and knapweeds, and gangs of males can often be seen clustered together, reminiscent of a group of men propping up the bar in a pub.
Mating in bumblebees is a rather enigmatic activity that we will return to later. Even for some very common bumblebee species we are not sure how males and queens find one another as mating is so rarely seen. So far as we can tell, virgin queens are soon accosted by males when they leave the nest, and they tend to mate very quickly. As they enter hibernation soon afterwards, few queens are seen in high summer. In most species, the queens mate only once in their entire lives, and the sperm from this event is stored within them for use the next spring when each builds her own nest. The males will happily mate many times, but because there are usually many more males than queens, it is a very lucky male indeed who mates more than once in his life.
Once mated, the queens must find somewhere to hibernate. This clearly doesn’t take long as, by contrast to their nest-searching behaviour in the spring, they are rarely seen looking for hibernation sites. They hibernate a few centimetres under the ground, but as bumblebees are pretty inept at digging, they tend to choose very loose soil that is easy to burrow into. They will also sometimes go into newly dug soil in gardens, and I have a suspicion that in the countryside they often make use of molehills. Hibernating queens are also frequently found in old compost heaps and in flowerpots full of loose compost.
Back in the old nest, the founding queen is now a little over a year old. She is frail and balding. Her workforce is no longer being replaced, for all the colony’s reserves have gone into producing new queens and males. As the ageing workers die off, the food supply coming in dries up, and before long all of the bees are dead. By September, all that remains is a pungent and loose pile of empty pupal cells, honeypots and bee corpses, which are slowly picked through and eaten by a range of scavengers such as woodlice, maggots and beetles.
The new queens of the aptly named early bumblebee enter hibernation as early as June, while most species do so in July or August. We don’t know why they hibernate so early, but perhaps hibern
ating is safer then being out and about, where the queen might pick up parasites or be eaten by a predator. They have a long wait until the following spring. The fat reserves in the queen’s body must keep her going; any queens that are smaller than average or have slightly smaller stores of fat tend to die during the long hibernation. They are also prone to becoming mouldy in damp weather, or drowning in heavy winter rains, and it is likely that the vast majority of hibernating queens do not make it through to spring.
Fortunately some do, and as the first rays of spring sunshine warm the soil, they burrow upwards to the light, beginning the bumblebee year once again.
CHAPTER THREE
The Hot-blooded Bumblebee
Aerodynamically, the bumble bee shouldn’t be able to fly, but the bumble bee doesn’t know it so it goes on flying anyway.
Mary Kay Ash (American businesswoman and writer)
By 1974 I was nine years old. I became interested in how animals worked, and decided to investigate what they looked like on the inside. Thinking back, I wonder at my parents’ indulgence, for I took to collecting roadkill and dissecting the remains of a variety of mangled beasts in our back garden. Rabbits were the most common, but I also found grey squirrels, a fox, a lovely brown hare, pigeons, frogs and who knows what else. I persuaded my grand-parents on my mother’s side to buy me a dissecting kit for my birthday. I’ve no idea now what I said I wanted it for, but I must have made up some sort of plausible excuse. I very much doubt that I told them I wanted to chop up dead and squashed animals. They were very strait-laced, devout Methodists from a small village in deepest, darkest Norfolk, and not exactly humorous. I can’t imagine that they would have thought this a healthy hobby for a nine-year-old. Nonetheless, on my birthday the dissecting kit arrived (ordered from Watkins & Doncaster, of course), and it was marvellous, containing lethally sharp scalpels of various shapes and sizes, a selection of pointed probes, delicate scissors and a lovely little chrome hand lens, all rolled up in a soft cloth pouch. I boosted its contents by persuading my uncle Ed, a chiropodist from Norwich, to give me some extra instruments including some wickedly curved scalpel blades used for carving ingrowing toenails or bunions or some such.
