A comparison of bumblebees' movements in uniform and aggregated distributions of their forage plant

1. Movements by bumblebees Bombus lapidarius were studied in arrays of plants of oil-seed rape Brassica napus cv Westar. In the arrays, plants were arranged into either a uniformly dispersed configuration or a patchily dispersed configuration. 2. Progress across an array was measured as the distance bumblebees had moved away from an original plant location after eight subsequent, successive plant visits. Movements were analysed in units of both metres and inter-plant spaces. In both units, bumblebees progressed more slowly across patchily dispersed arrays. 3. Plant dispersion had a detectable effect on certain components of bees' individual inter-plant moves (e.g. move length in metres) but not on others (e.g. mean turning angle). In some cases, the effects of aggregation on the components of individual moves were probably too small to detect statistically and only emerged in their cumulative effect on bees' progress. 4. Over short sequences of plant visits, bumblebees revisited plants rarely, but the frequency of revisits was almost twice as high in patchy arrays as in uniform arrays, and economic penalties may result from foraging among highly aggregated plants. 5. The effects of plant dispersion on pollinator movements detected in this study are unlikely to have a major impact on pollen transfer in B. napus because differences in pollinator progress only emerged after several successive inter-plant flights away from a potential pollen source, by which time the limited extent of pollen carryover in this species means that fertilisations from the source plant would be very rare.