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Major pollinators apart from the honeybee are suffering steep decline worldwide,
chief among them the bumblebee, and neonicotinoid pesticides are a major culprit
that should be banned Prof. Joe Cummins
There has been a huge amount written about the decline of the honeybee, Apis
mellifera , but relatively scant attention has been paid to other important
pollinators that have also been affected simultaneously, in particular, the
bumblebee, Bombus terrestris, which has been experiencing a precipitous global
decline. The bumble bee, an important pollinator, has received much less
attention than the honeybee because the bumblebee does not produce marketable
quantities of honey.
Is the bumblebee doomed?
As early as six years ago, there was clear evidence that the bumblebee was in
deep trouble in North America. A survey of bumblebee species reached the
following conclusion [1]: “The bumblebee subgenus Bombus is represented by five
species in North America. Of these, one, B. franklini, may be extinct, and two
others, the western B. occidentalis and the eastern B. affinis, appear to be in
steep decline. For all of these species, habitat loss and degradation and
extensive pesticide use are threats faced daily. However, circumstantial
evidence indicates that the principal cause for these population declines is the
introduction of exotic disease organisms and pathogens via trafficking in
commercial bumblebee queens and colonies for greenhouse pollination of
tomatoes.”
In 2009 the decline in Midwestern North American bumblebee was documented and
large scale agricultural intensification implicated as the cause [2]. A 2010
report showed that the relative abundances of four species have declined by up
to 96 percent and that their geographic ranges contracted by 23–87 percent
within the last 20 years [3]. It also showed that declining populations have
significantly higher infection levels of the microsporidian pathogen Nosema
bombi and lower genetic diversity compared with co-occurring populations of the
stable (non-declining) species. It concluded [3]: “Higher pathogen prevalence
and reduced genetic diversity are, thus, realistic predictors of these alarming
patterns of decline in North America, although cause and effect remain
uncertain.”
A number of causes have been put forward for the decline of the bumble bee,
these include intensification of land use, introduction of pathogen-bearing
bees, pathogens from honey bees, inbreeding, and exposure to pesticides. Before
discussing those causes, I describe the lifestyle of the bumblebee.
Lifestyle of the bumble bee
The bumblebee may look like a large economy-sized honeybee, but it has a
distinctly different lifestyle [4]. Most species live in small colonies, usually
underground, often in an old mouse hole. The queen lays her eggs in a hollow
nest of moss or grass at the beginning of the season. The larvae are fed on
pollen and honey, and develop into workers. All the bees die at the end of the
season except fertilized females, which hibernate and produce fresh colonies in
the spring. Bumblebees form colonies. These colonies are usually much less
extensive than those of honeybees. 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. Bumblebee nests may be
found within tunnels in the ground made by other animals, or in tussock grass.
Bumblebees sometimes construct a wax canopy (“involucrum”) over the top of their
nest for protection 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 that overwinter separately in protected spots.
The queens can live up to one year, possibly longer in tropical species.
Bumblebee nests are first constructed by over-wintered queens in the spring (in
temperate areas). 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 bumblebee species, with some species 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 isometrically into a clump of brood cells.
After the first or second group of workers emerge, they 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 other
environmental factors. Unlike the workers of more advanced social insects,
bumblebee workers are not reproductively sterile and are able to lay haploid
eggs (with one set of chromosomes) that develop into viable male bumble bees.
Only fertilized queens can lay diploid eggs (with two sets of chromosomes) that
mature into workers and new queens. 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 suitable location for diapause (dormancy).
Bumblebees generally visit flowers that form recognizable groups according to
pollinator type. They can visit patches of flowers up to 1–2 kilometers from
their colony. Bumblebees will also tend to visit the same patches of flowers
every day, as long as nectar and pollen continue to be available, a habit known
as pollinator or flower constancy. While foraging, bumblebees can reach ground
speeds of up to 15 metres per second (54 km/h). Once they have collected nectar
and pollen, they return to the nest and deposit the harvested nectar and pollen
into brood cells, or into wax cells for storage. Unlike honey bees, bumblebees
only store a few days’ worth of food and so are much more vulnerable to food
shortage.
Land use
The decline of bumblebee worldwide was brought to light when the impact of
particular land use practices was deemed to require fuller study to pinpoint
those practices that would benefit the survival of the bumblebee [5]. Organic
farming practices increased bumblebee species richness over conventional farming
practices [6].
Read the rest of this article on the ISIS website
http://www.i-sis.org.uk/Plight_of_the_Bumblebee.php
Or find out more details about disappearing bees
http://www.i-sis.org.uk/disappearingBees.php
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