mperature, via solar radiation, internal mechanisms of "shivering" and cooling from the abdomen (called heterothermy).
Bumblebee colonies are usually much less extensive than those of honey bees. This is due to a number of factors including: the small physical size of the nest cavity, a single female is responsible for the initial construction and reproduction that happens within the nest, and the restriction of the colony to a single season (in most species). Often, mature bumblebee nests will hold fewer than 50 individuals and are typically found below ground (Figure 43). They sometimes construct a wax canopy over the top of their nest for protection from moisture and insulation. Bumblebees do not often preserve their nests through the winter, though some tropical species live in their nests for several years (and their colonies can grow quite large, depending on the size of the nest cavity). In temperate species, the last generation of summer includes a number of queens who overwinter separately in protected spots.
Bumblebees Nest in a Shed. Image Courtesy Pete Griggs.
Bumblebee nests are first constructed by over-wintered queens in the spring. Upon emerging from hibernation, the queen collects pollen and nectar from flowers and searches for a suitable nest site. The characteristics of the nest site vary among species, with some preferring to nest in underground holes and others in tussock grass or directly on the ground. Once the queen has found a site, she prepares wax pots to store food and wax cells into which eggs are laid. These eggs then hatch into larvae, which cause the wax cells to expand into a clump of brood cells.
These larvae need to be fed both nectar for carbohydrates and protein in order to develop. Bumblebees feed nectar to the larvae by chewing a small hole in the brood cell into which nectar is regurgitated. Larvae are fed pollen in one of two ways, depending on the species. So called "pocket-maker" bumblebees create pockets of pollen at the base of the brood cell clump from which the larvae can feed themselves. Conversely, "pollen-storers" store pollen in separate wax pots and feed it to the larvae in the same fashion as nectar. Bumble bees are incapable of trophallaxis (direct transfer of food from one bee to another) which is common in other hymenoptera such as ants.
After the emergence of the first or second group of workers, workers take over the task of foraging and the queen spends most of her time laying eggs and caring for larvae. The colony grows progressively larger and at some point will begin to produce males and new queens. The point at which this occurs varies among species and is heavily dependent on resource availability and environmental factors. Unlike the workers of more advanced social insects, bumblebee workers are not physically reproductively sterile and are able to lay haploid eggs that develop into viable male bumble bees. Only fertilized queens can lay diploid eggs that mature into workers and new queens.
Early in the colony cycle, the queen bumblebee compensates for potential reproductive competition from workers by suppressing their egg-laying by way of physical aggression and pheromonal signals. Thus, the queen will usually be the mother of all of the first males laid. Workers eventually begin to lay males later in the season when the queen's ability to suppress their reproduction diminishes. The reproductive competition between workers and the queen is one reason that bumble bees are considered "primitively social insects".
New queens and males leave the colony after maturation. Males in particular are forcibly driven out by the workers. Away from the colony, the new queens and males live off nectar and pollen and spend the night on flowers or in holes. The queens are eventually mated (often more than once) and search for a suitable location for diapause (dormancy). The following season they will form new colonies.
Bumblebees generally visit flowers exhibiting the bee pollination syndrome. They can visit patches of flowers up to 1-2 kilometers from their colony. Bumbl