The use of conspecific and interspecific scent marks by foraging bumblebees and honeybees

Bumblebees (Bombus spp.) and honeybees, Apis mellifera, both use odour cues deposited on flowers by previous visitors to improve their foraging efficiency. Short-lived repellent scents are used to avoid probing flowers that have recently been depleted of nectar and/or pollen, and longer-term attractant scents to indicate particularly rewarding flowers. Previous research has indicated that bumblebees avoid flowers recently visited by themselves, conspecifics and congeners, while honeybees avoid flowers visited by themselves or conspecifics only. We found that both bumblebees and honeybees also avoided flowers previously visited by each other when foraging on Melilotus officinalis, that is, bumblebees avoided flowers recently visited by honeybees and vice versa. Twenty-four hours after a visit, this effect had worn off. Honeybees visited flowers that had been visited 24 h previously more often than flowers that had never been visited. The same was not true for bumblebees, suggesting that foraging honeybees were also using long-term attractant scent marks, whilst bumblebees were not. Flowers previously visited by conspecifics were repellent to bumblebees and honeybees for ca. 40 min. During this time, nectar replenished in flowers. Honeybees were previously thought to use a volatile chemical (2-heptanone) as a repellent forage-marking scent. We suggest that they may be using a less volatile chemical odour to detect whether flowers have recently been visited, possibly in addition to 2-heptanone.