The effect of pollinators and herbivores on selection for floral signals: a case study in Brassica rapa

Abstract

Through their preferences for floral cues, pollinators and herbivores can mediate selection on a variety of plant traits. Selection by mutualists and antagonists may not be independent from each other, however, as the selection on a trait through one interaction can depend on the presence or intensity of another interaction. Bumble bees (Bombus terrestris) and cabbage butterflies (Pieris brassicae) are both pollinators of Brassica rapa, but cabbage butterflies also use B. rapa as a host plant for their caterpillars. In a cage experiment we exposed B. rapa plants to (a) bumble bees, (b) cabbage butterflies, and (c) bumble bees and cabbage butterflies together and analysed the resulting patterns of selection. We found an association between flower visitation and corolla size as well as the scent compound phenylacetaldehyde for both bumble bees and cabbage butterflies. Additionally, oviposition by butterflies was associated with the same two floral signals. Whereas corolla size was under positive selection in all three pollinator treatments, selection on phenylacetaldehyde was found only in the “pure” bumble bee treatment. Additionally, in plants exposed to both insect species negative correlational selection on corolla size and phenylacetaldehyde was found, as well as nonadditive selection on phenylacetaldehyde. These results demonstrate a strong overlap in the preferences for floral traits in pollinators and herbivores causing a conflict between the attraction of mutualists and the avoidance of antagonists. Further, our study shows that interactive effects on fitness by mutualists and antagonists can contribute to complex selection patterns.