Interactive effects of pathogens, plant defenses and predators on herbivore performance and population dynamic

Variation in plant defenses and predation has the potential to cause profound changes in herbivore performance and population dynamics. Evaluating the complex interactive effects of plant defenses and predators that lead to the exponential or density-dependent growth of herbivorous insects is critical for understanding their population dynamics. For my doctoral research I (i) investigated the effects of pathogen-induced plant defenses on herbivores, (ii) measured the effects of constitutive and induced plant defenses on the strength of herbivore density-dependent growth and (iii) evaluated the influence of plant resistance on predator-prey interactions. Chapter 1 evaluates how different strains of Potato virus Y differentially affect plant defensive pathways, showing that strains that strongly induce the salicylic acid pathway increase susceptibility to chewing herbivores, while not affecting the performance of its vector, a phloem feeding aphid. Chapter 2 demonstrates that plants with low constitutive levels of defense cause strong density-dependent population growth in aphids, whereas populations on plants with high levels of constitutive defense show density-independent growth. Building on chapter 2, chapters 3 and 4 focus on the critical role of plant defenses in predator-prey interactions. Chapter 3 shows that plant resistance affects not only the consumptive, but also the nonconsumptive effects of predators on prey. I demonstrate that aphid prey could not exhibit predator-induced behavioral responses on plants with high levels of resistance. Chapter 4 explores the underlying mechanisms by which variation in plant defenses affect insect population: directly, through changes in herbivore population growth, or indirectly, by modifying predator impacts on prey populations. Chapter 4 shows that predators only cause prey density-dependent population growth when the herbivorous prey are feeding on low-resistance plants, which are still able to induce high levels of herbivore-associated cues. Collectively, these results highlight the strong effect that plant defenses have on herbivore population dynamics. This work has important implications; it suggests that induced plant responses to prey should be accounted for in biological control assessments, as these responses determine the efficiency of predators on target prey.