Patch use by bumblebees (Hymenoptera Apidae): temperature, wind, flower density and traplining

The foraging behaviour of the bumblebees Bombus hortorum (Linnaeus)/ Bombus ruderatus (Fabricius), Bombus pascuorum (Scopoli) and Bombus mesomelas Gerstaecker in a monospecific patch of Digitalis ferruginea Linnaeus has been investigated in relation to temperature, wind direction, plant density and patch configuration. D. ferruginea proved to be very attractive for bumblebees, aggregating a large number of foragers (up to two bumblebees per inflorescence at the foraging peak). The number of active bees showed a positive correlation (P < 0.01) with temperature. On windy days, flight distances were longer downwind than upwind (P = 0.0194), while on calm days or in gentle wind the bees made long flights upwind as well as downwind. In two subunits of the patch, differing in plant density, a larger number (P = 0.0026) of bees foraged in the high plant density unit, and a lower infloescence/bee ratio (P = 0.0367) was found there. The estimated total number of bees foraging at any one time in the two patch subunits differed significantly (5 out of 7 times, P < 0.01 or < 0.05) from the values expected if the bees were distributed in proportion to the number of inflorescences in each patch. Bees foraged within individual undefended and interlaced areas, and flight routes within these areas were maintained for up to at least 10 days. When part of their foraging area was screened by bagging some plants, the workers did not leave the patch, but most of them responded by visiting some adjacent plants or plant groups.