Consumers are strongly influenced by plant phenotypic variation. Such variation may have a genetic or environmental basis, and occurs when plant genotypes or species vary in traits or when patches of co-occurring plants vary in the number of genotypes or species. However, these sources of plant variation have usually been studied separately, their underlying mechanisms are poorly understood, and the evolutionary consequences are largely unknown. This dissertation aims to fill these gaps in research by evaluating: (i) the mechanisms by which environmental- and genetic-driven trait variation in the herb Ruellia nudiflora influence seed-eating caterpillars and wasps parasitizing the latter (Chapter 1), (ii) the evolutionary consequences of R. nudiflora trait variation on insects and of insect effects on the plant (Chapter 2), (iii) the ecological and evolutionary consequences of R. nudiflora genotypic diversity effects on these interactions (Chapter 3), and (iii) the dual effects of Swietenia macrophylla (mahogany) genotypic and neighbor tree species diversity on insect herbivores (Chapter 4), and herbivore-enemy interactions (Chapter 5).

By manipulating soil fertility and R. nudiflora genotype identity (progeny of same mother plant) in a common garden, I found that genotype effects were stronger than fertility effects and operated via different mechanisms (Chapter 1). Further analyses revealed that caterpillar selection on plants was dampened by parasitoids, and that increasing soil fertility weakened caterpillar and parasitoid selection on the plant (Chapter 2). A second experiment manipulating genotype number in patches of R. nudiflora revealed that genotypic diversity influenced plant-caterpillar interactions, and that such effects altered caterpillar selection on plant traits; however, parasitoids eliminated this feedback (Chapter 3). Finally, in a large-scale field experiment I found that tree species diversity (but not mahogany genotypic diversity) influenced herbivore abundance (Chapter 4). However, surrounding tree species diversity influenced specialist herbivores (feeding only on mahogany) but not generalists. Additionally, neither source of diversity influenced interactions between parasitic wasps and leaf-mining caterpillars (Chapter 5). Collectively, these results underscore the predictive value of determining the mechanisms and sources of plant phenotypic variation influencing consumers (environmental vs. genetic, genotypic vs. species diversity), and present evidence for novel feedbacks between plants and consumers.