First Brood

One day in late-February is warmer than the rest. There was no frost last night and the winter sun is warming a small patch of ground, still bare from the harsh winter conditions. The heat moves into the soil, seeping down into a small hollow. Inside is a small, decidedly furry creature who has been here for almost seven months. As the warmth penetrates her hard exoskeleton, chemical reactions inside her body begin to occur. The enzyme fructose bisphosphatase breaks down sugars and this metabolic reaction releases heat, warming her further. She can also dislocate her wings from her wing muscles. She will vibrate the muscles and this will release heat energy, warming her. She now has enough energy to push herself up through the soil, using her head like a miniature battering ram and her tiny feet as trowels to remove loose earth. She finally emerges into the bright sunshine and is revealed as an Early Bumblebee, Bombus pratorum.

Bumblebees often herald the arrival of Spring

Bumblebees often herald the arrival of Spring

Feeding on the early spring flowers, she quickly builds up her energy reserves before beginning the process of starting a nest.

Once the bumblebee queen has made her nest she lays her small brood of eggs, fertilised with sperm saved from the previous autumn. Bumblebee queens are attentive mothers. The Carder Bees (the most common UK species being the Common Carder Bombus pascuorum) are so named because of the technique they use to cover young. The bees use comb-like bristles on their legs to card material into a soft covering, which they then drape over the larval cell, keeping the young bee warm. Among other species, the Red Tailed Bumblebee queen Bombus lapidarius provisions her nest with pollen which she moulds into a lump covered in wax, within which she lays her eggs. She sits in a groove on top of the pollen lump and shivers continuously. Her movement maintains an internal temperature of 37-39˚C which maintains her brood at a constant temperature of around 32˚C keeping her young bees warm. As this continual heat provision requires a lot of energy, this is a precarious time for the queen, made all the more so by the lack of nectar rich flowers in early spring. To combat this, she crafts a pot which she places within tongue length in her nest and fills it with nectar.

The queen’s first offspring are almost invariably worker bees who will help the queen tend to the colony. Freshly emerged they are covered in pure white hairs, with their colouration developing within 24hrs. These new workers will have to help the queen maintain the nest for the rest of their lives. At this early stage this means that the workers have to provide enough food energy for the queen to lay lots of eggs. The workers have to go out and forage.

Feeding

Early Bumblebees Bombus pratorum have fairly short proboscis (8-12mm) and cannot sip nectar out of all flower types. However, these bumblebees have a narrow, arrow-like head which allows them to push into the flower and get at the nectar their proboscis could not reach.

Head-on view of a Bumblebee showing mouth parts

Head-on view of a Bumblebee showing mouth parts

Bumblebees have a range of proboscis lengths and one reason for this is due to inter-specific competition forcing the bees to exploit new resources. The Garden Bumblebee Bombus hortorum for example, has a long proboscis and can therefore sip nectar from flowers with longer corolla tubes (the section of flower where the nectar lies) such as Sage Salvia oficinalis. As the corolla tube of the Sage is longer than most UK bumblebee proboscises (it varies between 12-18mm in length) the Garden Bumblebee can feed on the Sage with very little competition from other bumblebee species.

Some bumblebees such as the Buff-tailed Bumblebee Bombus terrestris have a trick up a few of their six sleeves. To lap the nectar from flowers with corolla tubes longer than their proboscis, these bumbles nibble through the side of the tube and sip the nectar from the side! This is called nectar robbing and the Buff-tailed’s and White-tailed’s are really the only UK bumbles that exhibit this underhand technique.

Alloethism

Within a growing colony there is a vast variation in size related to role. This is known as Alloethism. The most obvious example of this is the queen. She is larger than the rest of her brood due to her role in surviving the winter and reproducing continually. She is also heavier than the workers as instead of a nectar stomach she has fat reserves. Interestingly, the worker bumblebees size seems also to be related to their role within the colony. The bees that emerge early to begin foraging are generally larger than the nest bees that help maintain their home. There are a few advantages to differing sizes. Larger forager bees have disproportionately smaller limbs than the smaller nest bees and this leads to a lower surface area to volume ratio. This adaptation means that less heat is lost from the bumblebee via conduction and the bumblebee can therefore begin working earlier in the morning despite the low temperature, thus increasing hours of productivity for the nest. The larger bees also tend to have larger eyes and ocelli (light sensing cell clusters on their forehead) allowing foraging to occur despite low light levels and also helping in predator avoidance strategy. They also have more olfactory sensilla (basically odour detectors) in their antennae.

These are now known to be also electrostatic energy receptors. Daniel Robert, a Swiss scientist at Bristol university, is investigating the use of electrostatic energy in determining the productivity of a flower. Simply, his findings are as follows; a bee flying along has a positive electrostatic charge. When the bee visits a flower (negatively charged) and drinks the nectar the flower’s charge reverses. Bees detect this change and will not visit this flower as they can detect the fact that there is no nectar and so investigating the flower would be wasted energy. However, as the nectar is replenished within the flower the electrostatic charge of the flower returns to being negative again, and the bees return to forage from it.

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Bumblebee parasites

Bumblebees also have to worry about a range of cleptoparasites such as the Cuckoo Bee. Bombus vestalis which is a parasite of the Buff-tailed Bumblebee. To ensure the survival of her brood the Cuckoo Bee has to possess some morphological adaptations and differences to the social bumblebees. She is larger and has a thicker cuticle to allow more muscle attachments to her limbs. Flying with a deeper hum than her social cousins, she identifies a nest of the Buff-tailed Bumblebee. Darting inside, she hides at the bottom of the nest, well away from the buzz of the colony. She hides because the hydrocarbons on her body are different to that of the colony and so she is easily detectable as an intruder. A few days in the colony will ensure that she begins to smell like the other bees. She now launches her attack. The bigger, stronger cuckoo bee overpowers the original queen, either killing her or, more rarely, forcing her into subservience. Now in charge, the cuckoo will lay her eggs and the enslaved workers will tend to them for her. All the eggs are fertile and will leave as they hatch. This means the colony will not survive much longer as workers are not replaced. However, the cuckoo will have passed on her genes to the next generation.

Other organisms also parasitise bumblebee nests. The hoverfies Vollucella bombylans and Pocota personata are incredible bumblebee mimics. V. bombylans parasitises both Red-tailed and Garden Bumblebees as it has two different colour morphs. The hoverfly needs this mullerian mimicry as her life history depends upon it. The hoverfly lays her eggs within the bumblebee nest and her young lie at the bottom, eating the debris that falls from the bustling hive above. However, the guard bees that sit at the nest entrance will not tolerate intruders and so her disguise has to be perfect. If she is uncovered, however, she has a final trick to play. In her death throes she will lay hundreds off eggs which will tumble deep into the nest. Here they will hatch and her genes will continue to the next generation.

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Bumblebee genetics

Bumblebees are well known for their social tranquility within the nest. With sometimes 400 workers in the nest, working in unison is key. Bees, belonging to the insect group hymenoptera, are social insects and all social insects have quite unusual genetics. The sex of a bee is determined by a single gene; if a bee has two different copies of this gene it is a female whereas if it has two identical (or just one) copy of the gene it will be a male. To produce a son, the bee has just to lay an unfertilized egg and the haploid gamete develops into a healthy boy bee which, like all other male bees, has no father. The egg must be fertilized for a daughter to be produced and the sperm used for this is stored within the queen from her mating flight the previous summer.

The male bee that the queen mated with at the end of the previous summer will be 100% related to all of his offspring (he is haploid so all of his genes are passed on) but the queen is only 50% related to them as she is diploid (two sets of genes) and only one set is passed on. Worker bees also are only 50% related to their offspring as only half of their genes are needed to produce a son and they cannot produce a daughter. Worker bees are 50% related to their mother and 100% related to their father.

This means that the workers are 75% related to each other and, therefore, it is in their interests to help each other rather than produce offspring (that they would only be 50% related to).

Reproduction

Towards the end of the summer, the queen will turn off the pheromone that she has been producing which instructs her eggs to develop into workers rather than queens in the hope that these newly produced queens will survive the winter and continue her dynasty. She will also produce males at this time and this is where the peaceful nature of the nest breaks down. The males that the queen produces are 50% related to her but therefore only 25% related to her workers. The workers would much prefer to raise male offspring of their own (as they would be 50% related to them) and so they will try and lay their own eggs in the nest. The queen hates this and will bite any eggs laid by a worker and will also attack any worker she finds laying eggs. However, this takes a large amount of effort and the queen will generally not last long thereafter. But such is her power that only around 10% of all males produced by the nest are laid by workers.

There are, on average, seven males to every queen produced at the end of the summer. The new queen must choose a suitable male who has fit enough genes to ensure a successful colony next year. To do this, male and female bees utilise a phenomenon called ‘hill topping’. Male bees will dual for position closest to the summit of steep hills and the queen can now fly to the top of the hill and mate with the male at the peak, assuming that he is the fittest.

Now mated, the new queen will spend the last few warm days of autumn feeding heavily on Mahonia and winter-flowering Honeysuckle before landing on the soft earth. She determinedly pushes her head into the ground, burrowing her way into the soil. Moulding a small chamber, she rests her head on a root and drifts off into sleep, awaking only when the soft tendrils of spring nudge her into forming a colony of her own.