Differences in the flower visitor behaviour on female and hermaphroditic flowers of Cimicifuga simplex

The behavioural patterns of flower-visiting insects are influenced by floral display size, floral reward, etc. We tested whether insects of various taxa visiting Cimicifuga simplex would change their behaviour between hermaphroditic and female flowers. Male-phase flowers of hermaphroditic ramets provide flower visitors with nectar and pollen as rewards, whereas female flowers of female ramets provide flower visitors with only nectar. The appearance of the both sexes in C. simplex is different with hermaphrodite being relatively large and female ramet being relatively small. Therefore, to investigate how sexual differences in floral rewards and conspicuousness affect flower visitor behaviour, we compared flower visitor behaviour between male-phase hermaphroditic ramets and female ramets. C. simplex was visited by the bumblebee Bombus beaticola beaticola (Hymenoptera), Vespula flaviceps (Hymenoptera), Anthomyiidae spp. (Diptera), Episyrphus balteatus (Diptera), and Pidonia aegrota (Coleoptera). The visitation frequency of all visitors, especially of bumblebees and dipteran insects, to male-phase hermaphroditic ramets was higher than that to female ramets. The number of flowers contacted per single visit to an inflorescence was significantly higher for hermaphroditic ramets than for female ramets in bumblebees, but no difference was found in other insect groups. These results suggest that bumblebees visit the low-reward female ramets less often than male-phase hermaphroditic ramets, and they stay on female ramets for a shorter time. In contrast, insects other than bumblebees seem to decide which flowers to visit according to how conspicuous the flowers are, but once they visit a ramet they continue to visit flowers on the ramet even if no pollen rewards there are.