Meetings of the Royal Botanical Society of the Netherlands

During a ten years’ study on the pollination of large-flowered Scrophulariaceae, numerous observations of bumblebee foraging were made. In this paper constancy is considered to include both flower constancy and constancy to a forage area. Bumblebees were individually marked for easy identification and followed during their foraging trips. Constancy could be demonstrated during a single foraging trip or during various trips on the same day or on different days. Bumblebees showed a high constancy to certain foraging areas. Large distances to new foraging sites (up to 1.8 km) were observed only if the preferred food plants were unavailable more closely. Some species, e.g., Bombus hortorum and B. pascuorum, were spotted more often than other ones, such as B. hypnorum and B. lapidarius. Distances flown within a given area were never of any appreciable length. At low plant densities (not more than 98 Rhinanthus flowers/m2) 45%, and at high densities (up to 473 flowers/m2) 30% of the flight length did not exceed 10 cm, and only 10% of the flight lengths were over 40 cm at both densities. Rhinanthus serotinus was the plant species preferred by most of the bumblebees except Bombus lapidarius and B. pascuorum, which showed a preference for Trifolium pratense and Cirsium palustre. Pollen-gathering individuals showed a lower constancy to Rhinanthus than nectar-collecting ones.