Tuesday, 3 April 2007

Bumblebees herald spring

The sight and sound of queen bumblebees is a sure sign of spring. Newly emerged from winter hibernation, these large and fascinating insects will soon be searching for a suitable site in which to establish a nest. With their internal fat stores almost completely exhausted, the first priority is to seek out sources of early season nectar and pollen. Although the buff-tailed bumblebee may be seen visiting the catkins of sallow or garden flowers as early as February, the other common species may not be on the wing quite so early. It is a precarious time; early emergence may use up valuable resources if there is insufficient nectar and pollen to be found. If the weather turns cold, as it did towards the end of March, then the bees will seek shelter and become torpid, re-emerging once the weather improves. By the beginning of April all the common species will be on the wing, engaged in finding food. Interspersed with these bouts of foraging are periods spent sunbathing on exposed leaves, walls or stones, a behaviour that is thought to help with the development of the ovaries in readiness for egg laying.

There is then a change in bumblebee behaviour, with the introduction of low, contour-hugging flights, as individuals prospect for a suitable nest site. Preferred nest sites vary with species; while some bumblebees nest underground others may use old bird nests, nest boxes or tree cavities. The underground nests tend to be in the abandoned burrows made by small mammals, like mice and voles. These can be a scarce resource and competition for nests may be fierce. It is sometimes possible to find several dead queen bumblebees around the entrance of a small mammal nest being used by a seemingly victorious queen. One species, the red-tailed bumblebee, uses holes in bare ground, or adjacent to stones or concrete, which act as heat stores and aid the regulation of temperature within the nest.

The whole process of nest site location and selection may take several weeks but, once established, the queen soon gets on with the business of producing her first batch of eggs. Typically, the queen will line the nest with fine material, before constructing a brood chamber. This chamber is made of wax, extruded from her abdomen. It is into this that she will lay a batch of eggs, usually between eight and twelve in number; the queen having mated back in the autumn before going into hibernation. Alongside the brood chamber the queen will also construct a nectar store. This initial batch of eggs will become the first infertile workers that will tend subsequent broods but the queen will need to tend this first brood herself.

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