Increased density of honeybee colonies affects foraging bumblebees

The increasing numbers of endangered wild bee species highlights the need for quantifying potential adverse effects of foraging honeybees. We analysed the response of bumblebees at genus and species level to experimentally increased honeybee density. The study was carried out on a Phacelia tanacetifolia field and adjacent patches of wild plants located in an agricultural landscape. Addition of one or ten A. m. ligustica colonies substantially increased total honeybee density not only on Phacelia but also on neighbouring patches of wild plants. The response of bumblebees (Bombus spp.) differed among species. Only minor spatial changes in the abundance of the short-tongued B. terrestris-group were observed on Phacelia. At wild plant patches, the B. lapidarius-group and the longer tongued bumblebee species (B. muscorum, B. sylvarum, B. pascuorum) responded with a shift between plant species. Limitations of competition avoidance in flower-impoverished landscapes are discussed.