Hemiparasites mediate interactions between their host plants and herbivores
Hemiparasitic plants attach to the roots of neighboring plants and extract nutrients, water, carbon, and secondary chemicals. Since they also interact with herbivores, they may mediate indirect interactions between their hosts and herbivores. Specifically, if host plants differ in their effects on a hemiparasite’s traits (e.g., size, phenology, nutrition) they will influence its suitability as a food source. This pattern has been confirmed by a handful of studies, but remains poorly understood. We tested the effects of six host species and a no-host control on Castilleja levisecta (golden paintbrush) and early-instar larvae of specialist herbivore Euphydryas editha (Edith’s checkerspot), which feed on Castilleja and a few other taxa. Castilleja spp. produce iridoid glycosides, secondary chemicals that are sequestered by E. editha larvae as they feed and are used as a defense against predators. In a greenhouse, we tested effects of each host on hemiparasites and their herbivores in caged enclosures. For host species that were also potential food plants for checkerspot larvae, we used screen barriers to restrict larval feeding to Castilleja.We measured survival, mass, and secondary chemistry of both the hemiparasite and herbivore, as well as height, stem number, and leaf nitrogen for the hemiparasite.
Host plant identity indirectly affected survival and mass of larvae feeding on C. levisecta, indicating a strong hemiparasite-mediated indirect effect can occur in this system. For example, mean mass of larvae feeding on C. levisecta that parasitized Achillea millefolium or Plantago lanceolata was double that of larvae feeding on C. levisecta associated with Deschampsia caespitosa, Lupinus lepidus, or the no-host control. Host identity directly affected mass, stem length, and leaf nitrogen concentrations in C. levisecta. These direct effects of host identity on the hemiparasite were usually also reflected by performance of the larvae; that is, hosts with positive effects on Castilleja generally also tended to positively affect survival and mass of larvae. Preliminary analyses suggest larval survival and mass were positively associated with biomass and leaf nitrogen concentrations in the hemiparasite, although we have not yet disentangled the relative importance of these effects. Preliminary data also suggest that host identity indirectly affects iridoid glycoside sequestration by the the larvae (iridoid glycoside concentrations in Castilleja are still being analyzed). We conclude that hemiparasites can mediate strong indirect interactions between their host plants and herbivores, and that these effects can be traced through specific host-derived traits of the hemiparasite.