Host species affects herbivory, pollination, and reproduction in experiments with parasitic Castilleja

The relative performance of a parasitic plant on different host species will depend on both direct and indirect effects of hosts on parasite interactions with mutualists and antagonists. Host species could affect parasite interactions with both herbivores and pollinators due to the uptake of defensive compounds and nutrients. However, the effects of different host species on parasitic plants have not been experimentally tested in the field. I determined the effect of two native host species, an alkaloid-producing, nitrogen-fixing lupine and non-alkaloid, non-nitrogen-fixing grass, on herbivory, pollination, and reproduction of the hemiparasitic plant Indian paintbrush (Castilleja indivisa). Within this experiment, I manipulated herbivory and pollination to determine their effects on Indian paintbrush reproduction. Indian paintbrush parasitizing lupines produced three times as many seeds and were more attractive to pollinators than Indian paintbrush parasitizing grass. However, there was no effect of host species on early season or floral herbivory. In addition, MANOVA revealed that host species influenced the response of Indian paintbrush female reproduction to experimentally manipulated herbivory and pollination treatments. Thus, the effect of hosts on parasites is mediated by interactions with herbivores and pollinators, and both direct and indirect effects may shape the selective pressures mediating interactions between hosts and parasites.