The foraging ecology of bumblebees

This thesis investigates how intrinsic and extrinsic factors affect bumblebee foraging ecology and how bumblebee behaviour can influence plant reproductive success. Research focused on the behaviour of six common species of bumblebee in southern England (Bombu hortorum, B. lapidarius, B. lucorum, B. pascuoru, B. pratorum and B. terrestris). I found that foraging bumblebees could detect scent marks left on flowers by conspecifics and congeners and avoid flowers which had recently been visited and depleted. Very small amounts of long straight-chain hydrocarbons secreted from the tarsi caused the repellent effect. Repellency faded as nectar accumulated in flowers; in Symphytum oficncale, this takes 20-60 minutes. Bees of three different species (Bombus hortorum, B. pascuorum and B. terrestris) were found to display distinct behaviour patterns when foraging on different plant species and were affected by plant abundance in different ways. When foraging on two-species arrays, relative plant density and the identity of the competing plant species also had significant effects on pollinator visitation, the proportion of flowers visited per plant and flower constancy. Rare plants tended to attract more bumblebee visits, but pollen transfer among conspecifics may have been reduced. Morphologically complex flowers, predicted to be at more of a disadvantage when rare compared with simple flowers, attracted as many pollinators as simple flowers. The foraging niches of the same three species of bumblebee overlapped considerably when bees were observed foraging in flight cages, and did not significantly change when heterospecies were removed from the environment. This was despite interspecific differences in tongue length and was possibly because of a superabundance of resources in the flight cages. Bumblebees of the same species, but of different sizes also employed different foraging strategies. Large bumblebees were unable to trigger the pollination mechanism of the morphologically complex, nectarless flowers of Cytisus scoparius.