Tuesday, 11 August 2015

Plight of the Bumblebee Part 1: Lifecycle

B.terrestris worker
Aside from the honeybee, just about the only wild bee species most people in Britain are able to name are the large, furry bumblebees in the genus Bombus. With 24 species in the UK, and around 250 worldwide, they are a small but conspicuous minority of the several hundred species wild bee species in the UK. Aside from bumblebees and honeybees, the other species are all solitary, with a single female provisioning their nest, usually in a hole which may be excavated in the ground, wood, or simply a hollow stem.

Like honeybees, bumblebees are eusocial, with a colony comprising a queen which lays all the eggs and a group of workers which go out and collect pollen and nectar. Unlike honeybees, colonies are annual and usually only last a few months before dying off, leaving a group of fresh queens to survive the winter.

The lifecycle begins with a mated queen emerging from hibernation in the spring. Depending on the species and weather, this may be as early as February in B.terrestris, or as late as May or even June in B.sylvarum. As soon as they emerge, queens begin prospecting for a nest site where they can raise a brood. Different species will use different types of potential nest, with many in the UK using mouse holes. Other species make nests in tussocks of grass or even holes in trees, but they almost all require an abandoned bird or mammal nest. This is because the nest requires insulation so that the developing brood can be kept warm by the queen.

The early nests are especially vulnerable, because only the queen is available to collect the pollen and nectar that the brood requires, and only she can provide warmth by shivering. Bumblebees are extremely well adapted to cold climates, much better than honeybees which originated in the tropics, and their extensive fur provides insulation for their active metabolisms. In fact, an incubating queen can maintain a body temperature as high as 38o C In order to grow the first batch of workers she must split her time between incubating them and gathering food, and if the nest is not located close to suitable flowers the new colony may fail.

Once the first workers appear the queen remains in the nest permanently while her daughters go out to gather food. Although bumblebees produce wax and store nectar, they do not maintain large stores and prolonged bad weather may be fatal. Different species of bumblebee select different flowers to feed on, which seems to depend on the length of their tongues. Short-tongued species feed mostly on open flowers, while long-tongued species feed on deep flowers such as members of the Fabaceae (the pea and bean family).

As the colony grows they eventually switch to producing the reproductives which will be the parents of next year’s colonies. Males leave soon after emerging, and attract females by leaving scent marks. Young queens feed up after mating to build up reserves and then dig in for hibernation. The hibernation site probably varies by species, but those that frequent gardens may often use flower pots as the compost in them makes for easy digging. Providing they survive the winter (and many do not) they will then emerge the next year to repeat the process.

Next time, the importance of bumblebees in agriculture, and conservation problems associated with them.

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