6
The Life-cycle
The true Bumblebees are social insects, in the
sense that they show a division of labour
between the reproductive males and females,
and a caste of 'worker' bees. The latter are
(usually) infertile, and their lives are devoted
to foraging for food and tending the larvae
produced by the single female ('queen') which
establishes the nest. Unlike the honey bees,
Bumblebee nests last no longer than one
season. Typically, a fertilised female emerges
from hibernation in early spring, and
commences feeding on nectar and pollen from
available flowers (Salix spp., Lamium album,
and several early-flowering garden plants are
very important for these early-flying females).
Then begins a more-or-less prolonged search
for suitable nesting sites (which vary
considerably from one species to another).
Death-rates among these over-wintered
females are very high indeed, and their dead
bodies arc easy to find on the ground around
food-sources, roadside verges and elsewhere.
Once a nest is established, the female deposits
a clump of pollen in it, and builds a wax cell
into which she lays her first batch of eggs. The
resulting larvae are nurtured exclusively by
the female, who can be observed during the
spring collecting nectar and pollen for this first
brood. Soon this task is taken over by the
newly emerged workers, and the female then
remains in the nest, continuing to lay eggs.
During the late spring and early summer the
number of workers produced in each nest
continues to grow until a remarkable transition
takes place. Instead of workers, a brood of
fertile males and females emerges from the
nest. These mate, and the males continue to
live 'solitary' existences outside the nest,
foraging only for their own subsistence.
Meanwhile, the fertilized females build up an
internal store of nutrients to take them through
their winter hibernation, and begin searching
for suitable hibernation sites (usually
burrowing a few centimeters into a north-
facing bank). However, in favourable seasons
the females of some species establish nests in
mid-summer and complete a second full life-
cycle in the same year. Actively foraging
workers of some species can often be observed
well into October.
Whilst the above features of the life-cycle are
common to all the true Bumblebees, there are
some interesting variations in the details. In
some species (Bombus terrestris, for
example), stores of pollen are maintained in
wax cells or empty pupal cocoons. The larvae
of this group of bees spin separate
compartments in the nest, and are fed
individually by the queen or workers with a
mixture of nectar and pollen from the pollen
stores. This tends to produce workers more-or-
less equal in size. Bumblebees of this group
arc known as 'pollen storers'.
Alternatively, in such species as Bombus
hortorum and Bombus pascuorum, the larvae
stay together in a common brood chamber.
Wax 'pockets' are built in this, and foraging
workers or queens deposit their loads of pollen
into them. The larvae feed themselves from
the resulting mass. These bees are termed
'pocket makers', and because of competition
between larvae for access to food in the brood
chamber, there are said to be bigger
differences in the size of the resulting adult
workers than with the pollen storcrs.
More research is needed into the processes
involved in the seasonal shift from production
of workers to fertile males and females. It
seems likely that the mechanisms vary
considerably from species to species, but two
main groups of species are distinguished. In
so-called 'simple' species (e.g. Bombus
pratorum) the queen appears not to influence
the transition to production of new queens.
Such factors as the ratio of workers to larvae,
the overall density of bees in the nest and
temperature arc thought to lead to some larvae
feeding more and for longer than others, and
these eventually emerge as fertile females. In
the case of 'complex' species (e.g. Bombus
terrestris) it seems that queens can regulate
the timing of the production of new queens by
emitting pheromoncs which influence worker-
activity. In these species there is a more clear
distinction in size between workers and
queens.
Essex Naturalist (New Series) 16 (1999)