Evolutionary ecology of plant resistance traits across the herbivore diet spectrum: A test in the model plant Mimulus guttatus

Hypothesis: Based on the phytochemical coevolution hypothesis and related host specialization hypotheses, we predicted that chemical resistance traits would have greater detrimental effects on generalist herbivore performance than on specialist herbivore performance. In an extrapolation of the phytochemical coevolution hypothesis, we predicted that physical resistance traits may be more detrimental to specialist herbivores that do not regularly encounter a particular physical resistance trait, than to generalists. Organisms: Mimulus guttatus (syn: Erythranthe guttata) plants, derived from natural populations chosen for variation in resistance traits, were fed to natural lepidopteran herbivores from the specialist to generalist diet spectra: Euphydryas chalcedona, Junonia coenia, Grammia incorrupta, Trichoplusia ni, and Spodoptera exigua. We also included the generalist slug species Deroceras laeve. Methods: We used a series of performance or preference trials with a spectrum of specialist to generalist herbivores that feed on the model plant Mimulus guttatus. We analysed how specific constitutive resistance traits influenced the performance or preference of the organisms. Results: We found more specialized herbivore performance tended to have positive associations with some phytochemicals while having a negative association with physical traits (i.e. trichome density). Performance of the generalist herbivores, with a few exceptions, had a negative relationship with phytochemical traits and neutral relationships with trichome density. Conclusions: The patterns observed generally fit the predictions of the phytochemical coevolution theory and the related host specialization hypotheses, although there was one exception for each generalist species. Some of the results could also be explained by the arms race, pairwise coevolution, and/or synergism hypotheses of phytochemical evolution.