Native versus Exotic Grasses: The Interaction between Generalist Insect Herbivores and Their Host Plant

Although the interaction between native and exotic plants and their insect herbivores has been examined extensively, experimental studies on plant responses to generalist insects as well as feeding preferences of generalist insects on native versus exotic plants have provided inconsistent results. This dissertation addresses this issue by incorporating recent methodological recommendations and by exploring interactions between generalist herbivores and native and exotic plants from both the plant and insect perspectives. Using native (Andropogon gerardii and Bouteloua curtipendula ) and exotic grasses (Miscanthus sinensis and Bothriochloa ischaemum) with generalist Melanoplus grasshoppers, I combined behavioral and molecular approaches to explore (1) plant resistance and tolerance to grasshopper herbivory, and (2) feeding preferences of Melanoplus grasshoppers for native and exotic plants. Overall, the results from this dissertation project have demonstrated lower resistance of exotic grasses to generalist grasshopper herbivory compared to native grasses; and similar level of plant tolerance to herbivory in native and exotic grasses. Melanoplus grasshoppers demonstrated strong feeding preferences for exotic plants in most of the behavioral experiments and under natural conditions. This suggests that exotic M. sinensis and B. ischaemum grasses with a lack of coevolutionary history with native generalist Melanoplus grasshoppers might have lower physical and chemical defenses than native grasses. Furthermore, generalist Melanoplus grasshoppers may provide biotic resistance to these exotic grasses should they invade natural areas. The results from this dissertation project have important applications for predicting the interaction between exotic plants and generalist herbivores in the introduced range, and if plant invasion has already occurred, for developing effective control plans.