Recent studies have revealed that natural enemies can influence 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 influences on plant fitness 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 rufus Gillette (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 influence of the parasitoid using three treatments: plants caged with gall wasps, plants caged with gall wasps and parasitoids, and control plants caged without gall wasps. Despite technical difficulties in excluding wild gall wasps and parasitoids, plants caged with gall wasps flowered later than control plants and had reduced reproductive output, producing shorter flowering stems and fewer and smaller seeds of lower viability. The parasitoid apparently “rescued” plant reproduction by killing gall wasp larvae, resulting in larger seeds that were more likely to germinate. Parasitoid females responded more strongly to volatiles produced by galled plants compared with ungalled plants in field olfactometry bioassays. This seems to be the first evidence in an endemic community of a plant species gaining a fitness advantage by producing volatile compounds that attract natural enemies of herbivorous insects.
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