Parasitic Plant‐Host Interactions: Plant Performance and Indirect Effects on Parasite‐Feeding Herbivores

While the role of parasites in shaping population and community dynamics has been increasingly appreciated, researchers have largely ignored the ecology of parasitic plants. Parasitic plants obtain water, sugar, nitrogen compounds, and a variety of secondary metabolites from their host plants. In a greenhouse study, I examined the direct interactions of a parasitic plant, Castilleja wightii (Scrophulariaceae), with three plant hosts and the indirect effects of this parasitism on the performance of a parasite—feeding herbivore. The three host species, Lupinus arboreus, Artemisia pycnocephala, and Eriophyllum staechadifolium, directly and differentially affected C. wightii flower production, although parasite biomass was not affected by host species. Surprisingly, parasite performance was weakest when attacking a leguminous host, although parasite total nitrogen was highest when growing on a legume. Parasites also exerted a strong direct effect on host performance: host dry mass was negatively correlated with parasite mass for all three host species tested. However, this effect differed among hosts: for E. staechadifolium, proportionately larger parasite mass was correlated with smaller overall biomass of the host and parasite combined, while for the other two hosts, overall host—parasite mass was not correlated with parasite mass, indicating that E. staechadifolium was more affected by parasite attack than were the other host species. Finally, host plant chemistry had strong indirect effects on the performance of an aphid, Nearctaphis kachena, that feeds on C. wightii. Aphids feeding on C. wightii that were parasitizing L. arboreus (alkaloid—producing, high nitrogen), A. pycnocephala (alkaloid—free, intermediate nitrogen), and E. staechadifolium (terpenoid—producing, low nitrogen) experienced 68.2, 65.3, and 49.5% survival, respectively. Further, aphids feeding on parasites using L. arboreus and A. pycnocephala produced 2.43 and 1.77 times more offspring on average than those feeding on parasites using E. staechadifolium. My results indicate that interactions between plant parasites and different host species can have strong direct effects on both host and parasite performance, as well as marked effects on the tritrophic interactions among plant hosts, parasitic plants, and their herbivores.