| Different evolutionary pressures are hypothesized to lead to divergence in plant defenses based on trade-offs among growth/reproduction/defense and based on species traits such as life history and mating system. In order to understand how plasticity in induced responses can alter plant fitness and/or interactions with pollinators, I quantified growth, defense, and reproduction in a field experiment using the mixed-mating (outcrossing and selfing) species Datura stramonium (Solanaceae). I establish that induced responses protect plants from herbivory, but that these defenses can be costly in terms of vegetative and floral/reproductive allocation that can alter a plant's propensity to self-fertilize or outcross. In a phylogenetic analysis of defenses across the genus Physalis (Solanaceae), I show that defenses are evolutionarily labile and not constrained by trade-offs between constitutive and induced defenses. However, species traits (growth rates and life history) and ontogeny were found to impact plant defensive levels. Because the evolution of plant defenses depends on plant-insect interactions, I also examined the ways in which plant compounds affect multiple components of herbivore fitness. Costs to herbivores of feeding on toxic tissue may be counteracted by increased toxicity or resistance to natural enemies if these enemies have a sufficient fitness cost to herbivores. In a field survey, I observed 31-57% of Manduca sexta (Lepidoptera) to be fatally parasitized by Cotesia congregata (Hymenoptera) despite relatively faster larval development time during the instars preferred by parasitoids. Additionally, I found that secondary compounds do not alter M. sexta immune responses to artificial parasitoid eggs, suggesting any negative effects of host diet on parasitoid success are independent of a host's immune ability to respond to the parasitoid egg.;Finally, I found no differences in M. sexta fecundity in response to secondary compounds, despite negative effects on moth traits influencing other components of fitness, including survival, development time, and size. |
