Repellent scent-marking of ̄owers by a guild of foraging bumblebees (Bombus spp.)

We have found that foraging bumblebees (Bombus hortorum, B. pascuorum, B. pratorum and B. terrestris) not only avoid ̄owers of Symphytum ocinale that have recently been visited by conspeci®cs but also those that have been recently visited by heterospeci®cs. We propose that the decision whether to reject or accept a ̄ower is in ̄uenced by a chemical odour that is left on the corolla by a forager, which temporarily repels subsequent foragers. Honeybees and carpenter bees have previously been shown to use similar repellent foragemarking scents. We found that ̄owers were repellent to other bumblebee foragers for approximately 20 min and also that after this time nectar levels in S. ocinale ̄owers had largely replenished. Thus bumblebees could forage more eciently by avoiding ̄owers with low rewards. Flowers to which extracts of tarsal components were applied were more often rejected by wild B. terrestris workers than ̄owers that had head extracts applied, which in turn were more often rejected than ̄owers that had body extracts applied. Extracts from four Bombus species were equally repellent to foragers. The sites of production of the repellent scent and its evolutionary origins are discussed.