Floral signals and filters in a wasp- and a bee-pollinated Gomphocarpus species (Apocynaceae: Asclepiadeae)

Abstract

Floral traits may function as attractants for some flower visitors and also as mechanisms that filter out non-beneficial flower visitors. Our current understanding of floral filter mechanisms is limited as this particular aspect of floral function has received less attention than floral attractants. Here we report on a case of two closely related Gomphocarpus species, G. physocarpus and G. fruticosus, that are similar in their general floral shape and architecture but differ in their main pollinators, which are vespid wasps and honeybees, respectively. To identify the floral traits that best explain the observed pollinators we analysed and compared the floral scent composition, the flower colour, flower morphology, and nectar sugar composition of the two species. Furthermore, the response to floral scent was tested in behavioural experiments with Belanogaster and Polistes vespid wasps. The floral scent composition differed significantly between the plant species. Acetic acid was a main component in the wasp-pollinated G. physocarpus, which is highly unusual for a floral scent bouquet. Choice experiments suggest that the species-specific odour might be responsible for the selective attraction of either wasps or bees. Furthermore, wasp-pollinated G. physocarpus flowers offer relatively large amounts of nectar that is freely accessible to insect visitors, whereas in G. fruticosus the site of nectar accumulation is covered by a specific morphological structure that makes it difficult for short-tongued insects such as wasps to access the nectar. Altogether, pollinator specialisation in the two Gomphocarpus species seem to be due to a combination of floral filter mechanisms and olfactory cues that cause flower visitors to learn to differentiate between the two species.