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Population decline

Bumblebee species are declining in Europe, North America, and Asia due to a number of factors, including land-use change that reduces their food plants. In North America, pathogens are possibly having a stronger negative effect especially for the subgenus Bombus. A major impact on bumblebees was caused by the mechanisation of agriculture, accelerated by the urgent need to increase food production during the Second World War. Small farms depended on horses to pull implements and carts. The horses were fed on clover and hay, both of which were permanently grown on a typical farm. Little artificial fertiliser was used. Farms thus provided flowering clover and flower-rich meadows, favouring bumblebees. Mechanisation removed the need for horses and most of the clover; artificial fertilisers encouraged the growth of taller grasses, outcompeting the meadow flowers. Most of the flowers, and the bumblebees that fed on them, disappeared from Britain by the early 1980s. The last native British short-haired bumblebee was captured near Dungeness in 1988. This significant increase in pesticide and fertilizer use associated with the industrialization of agriculture has had adverse effects on the genus Bombus. The bees are directly exposed to the chemicals in two ways: by consuming nectar that has been directly treated with pesticide, or through physical contact with treated plants and flowers. The species Bombus hortorum in particular has been found to be affected by the pesticides; their brood development has been reduced and their memory has been negatively affected. Additionally, pesticide use negatively affects colony development and size.

Bumblebees are in danger in many developed countries due to habitat destruction and collateral pesticide damage. The European Food Safety Authority ruled that three neonicotinoid pesticides (clothianidin, imidacloprid, and thiamethoxam) presented a high risk for bees. While most work on neonicotinoid toxicity has looked at honeybees, a study on B. terrestris showed that "field-realistic" levels of imidacloprid significantly reduced growth rate and cut production of new queens by 85%, implying a "considerable negative effect" on wild bumblebee populations throughout the developed world. Another study on B. terrestris had results suggesting that use of neonicotinoid pesticides can affect how well bumblebees are able to forage and pollinate. Foragers from bee colonies that had been affected by the pesticide took longer to learn to manipulate flowers and visited flowers with less nutritious pollen. In another study, chronic exposure in a laboratory setting to field-realistic levels of the neonicotinoid pesticide thiamethoxam did not affect colony weight gain or the number or mass of sexuals produced. Low levels of neonicotinoids can reduce the number of bumblebees in a colony by as much as 55%, and cause dysfunction in the bumblebees' brains. The Bumblebee Conservation Trust considers this evidence of reduced brain function "particularly alarming given that bumblebees rely upon their intelligence to go about their daily tasks."

Of 19 species of native nestmaking bumblebees and six species of cuckoo bumblebees formerly widespread in Britain, three have been extirpated, eight are in serious decline, and only six remain widespread. Similar declines have been reported in Ireland, with four species designated endangered, and another two considered vulnerable to extinction. A decline in bumblebee numbers could cause large-scale changes to the countryside, resulting from inadequate pollination of certain plants.

Some bumblebees native to North America are also vanishing, such as Bombus balteatus, Bombus terricola, Bombus affinis, and Bombus occidentalis; one, Bombus franklini, may be extinct. In South America, Bombus bellicosus was extirpated in the northern limit of its distribution range, probably due to intense land use and climate change effects.