The Challenge of Attracting Pollinators While Evading Floral Herbivores: Patterns of Fragrance Emission in Cirsium arvense and Cirsium repandum (Asteraceae)

By increasing floral apparency to promote fertilization, plants risk attracting herbivores with the same signals that they use to lure pollinators. We hypothesized that fragrance is emitted in patterns that correspond to pollinators, with high emissions during periods of pollinator activity and low emissions otherwise, especially during periods of peak floral herbivore activity. Using a combination of analytical chemistry and field observations, we examined both the diel and ontogenetic patterns of fragrance in two Cirsium species, in relation to visitation patterns of pollinators and florivores. Emission rates were highest at reproductive maturity, when insect visitation by both pollinators and florivores was also highest. In Cirsium arvense, the diel pattern of fragrance emission matched patterns of pollinator activity and was lowest when florivores were active. In contrast, scent in Cirsium repandum peaked at midday rather than with insect activity; neither species had a diel pattern that followed ambient temperature. Fragrance emission from C. repandum was 25 times lower than from staminate C. arvense and may not be essential for pollinator attraction, at least from a distance. The scent dynamics we observed in C. arvense are consistent with the hypothesis that fragrance emissions correspond with pollinator activity and are low when florivores are active.