A generalist and a specialist herbivore are differentially affected by inbreeding and trichomes in Mimulus guttatus

Inbreeding in plants is well known to affect plant–herbivore interactions, but these effects can depend on herbivore host breadth. If the phenotypic effects of inbreeding are analogous to those of environmental stress, the plant stress hypothesis predicts that generalist herbivores will perform better on inbred relative to outbred plants because inbreeding is expected to lower chemical defenses. The plant vigor hypothesis predicts that specialist herbivores will perform better on outbred relative to inbred plants because specialists are more tolerant of plant defenses and benefit from the higher nutritional quality of outbred plants. However, these plant defense theories do not account for structural defenses. Using a factorial design, we examined effects of inbreeding and trichome density in Mimulus guttatus on performance of the generalist, Helicoverpa zea, and the specialist, Junonia coenia. We also examined preferences of each herbivore for leaf disks and olfactory cues from inbred vs. outbred plants. The generalist H. zea developed most efficiently on inbred plants. The specialist J. coenia exhibited the greatest mass gain and pupal mass and developed fastest on outbred plants. Trichomes negatively affected performance of both species. Helicoverpa zea did not exhibit strong preferences for inbred vs. outbred plants, but J. coenia consistently preferred outbred plants. Our results support predictions on the effects of inbreeding on herbivore performance, but trichomes reduced performance of both herbivores. We conclude that the ability of the plant stress and plant vigor hypotheses to predict inbreeding effects on plant–herbivore interactions may depend on which plant traits are affected by inbreeding.