Foraging bumblebees avoid flowers already visited by conspecifics or by other bumblebee species.

Honey bees, Apis mellifera, use short-lived repellent scent marks to distinguish and reject flowers that have recently been visited by themselves or by siblings, and so save time that would otherwise be spent in probing empty flowers. Conversely, both honey bees and bumblebees, Bombus spp., can mark rewarding flowers with scent marks that promote probing by conspecifics. We examined detection of recently visited flowers in a mixed community of bumblebees foraging on comfrey, Symphytum officinale, in southern England. When foraging among inflorescences on a plant, three abundant species of Bombus probed fewer inflorescences more than once than would be expected from random foraging. Bees frequently encountered inflorescences but departed without probing them for nectar. Examination of the incidence of such rejections in the two most common species, B. terrestris and B. pascuorum, revealed that the low incidence of multiple probing visits was due to two factors: bees both foraged systematically and selectively rejected inflorescences that they had previously visited. When presented with inflorescences of known history, bees selectively rejected those that had been recently visited by themselves or by conspecifics compared with randomly selected inflorescences. They were also able to distinguish inflorescences that had been visited by other Bombus species. Bees were unable to distinguish and reject inflorescences from which the nectar had been removed artificially. We conclude that these Bombus species are probably using scent marks left by previous visitors. The significance of deposition and detection of interspecific scent marks for competitive interactions between species is discussed. Copyright 1998 The Association for the Study of Animal Behaviour.