Induced defence by a root hemiparasite increases host plant resistance against future infection.

European grassland plants are frequently attacked by root hemiparasites. However, little is known about host defence responses to parasitism. We investigated whether prior parasitization by a root hemiparasite makes hosts more susceptible to parasitism or, on the contrary, stimulates host defence against a future attack by hemiparasites. We grew three host species (Lolium perenne, Trifolium repens and Sanguisorba minor) in phase 1 for 3 months with the hemiparasite Rhinanthus alectorolophus, removed the parasite and grew the same host individuals in phase 2 with the hemiparasites R. alectorolophus or Melampyrum arvense. Previous infection by a parasite reduced the survival of the seedlings of Rhinanthus and Melampyrum with all host species but increased the biomass of the surviving parasites. A previous infection reduced the biomass of the hosts in most treatment combinations, but variation in initial host biomass at the start of phase 2 only partly explained this effect. Some of these interactions were specific to particular parasite-host species combinations. The results indicate that infection by root hemiparasites induces in the hosts defence mechanisms against future infection by the parasites (increased pre-attachment resistance), but parasite individuals that overcome this defence may then also be particularly good at exploiting the hosts (no increased post-attachment resistance). Thus, infection by root hemiparasites may activate host defence pathways that can influence future interactions with herbivores and pathogens and thus community dynamics.