Tritrophic Interactions and Reproductive Fitness of the Prairie Perennial Silphium laciniatum Gillette (Asteraceae)

Recent studies have revealed that natural enemies can inßuence reproductive success of plants by eliminating their herbivores, thereby reducing damage to photosynthetic or reproductive tissues. Some plant species apparently have evolved “indirect defenses” in response to such top-down selective pressures, producing volatile compounds that are used as cues by natural enemies searching for their herbivorous hosts. The research summarized in this article evaluates the potential for such top-down inßuences on plant Þtness in an endemic prairie system and the role of plant volatiles in location of hosts by parasitoids. The study system was comprised of the prairie perennial Silphium laciniatum L. (Asteraceae), the gall wasp Antistrophus rufusGillette (Hymenoptera: Cynipidae), and its parasitoid Eurytoma lutea Bugbee (Hymenoptera: Eurytomidae). In common garden experiments, we assessed the impact of gall wasp herbivory on growth and reproduction of S. laciniatum and the mediating inßuenceof theparasitoid using three treatments: plants cagedwith gallwasps, plants caged with gall wasps and parasitoids, and control plants caged without gall wasps. Despite technical difÞculties inexcludingwildgallwasps andparasitoids, plants cagedwithgallwaspsßowered later than control plants andhad reduced reproductive output, producing shorter ßowering stems and fewer and smaller seeds of lower viability. The parasitoid apparently “rescued” plant reproduction by killing gall wasp larvae, resulting in larger seeds thatweremore likely to germinate. Parasitoid females responded more strongly to volatiles produced by galled plants compared with ungalled plants in Þeld olfactometry bioassays. This seems to be the Þrst evidence in an endemic community of a plant species gaining a Þtness advantage by producing volatile compounds that attract natural enemies of herbivorous insects.