Different responses of pollinating bees to size variation and sexual phases in flowers of Campanula

To examine the response of pollinating bees to size and sexual phases of flowers, we constructed an artificial population of Campanula having large flower variation and presented it to potentially pollinating bees in nurseries. The pollinating bee groups (halictid, megachilid and bumble bees) responded differentially to both the flower size and to the sexual phases of the flowers. Whereas visitation rate of megachilid bees increased with the flower size, those of halictid bees and bumble bees did not show particular trends; for example, bumble bees visited almost all of the flowers consistently. Visitation frequencies to male-and female-phased flowers were significantly different between megachilids at Tokyo and halictids. This study indicates that pollinator attraction could not solely explain the evolution of the flower size in Campanula, and that other factors such as pollen transfer efficiency, should be considered.