Herbivory of native and exotic North-American prairie grasses by nymph Melanoplus grasshoppers

Abstract

Although the general interaction between native and exotic plants and specialist insect herbivores has received considerable attention in ecological studies, plant responses to herbivory by generalist insects, as well as feeding preferences of generalist insects, are still poorly understood. Experimental studies comparing leaf damage of native and exotic plants to generalist insects have provided inconsistent results due to irregularities in methodology such as using only one type of experiment or using non-standard measurements. Our study addresses these issues by examining leaf damage caused by generalist Melanoplus grasshoppers in morphologically and physiologically similar native (Andropogon gerardii and Bouteloua curtipendula) and exotic grasses (Miscanthus sinensis and Bothriochloa ischaemum) in Ohio and Maryland. In a 2-year study, we explored whether exotic grasses sustain less damage than native grasses, and consequently, whether generalist grasshoppers may potentially pose biotic resistance to exotic grasses. Using a combination of choice and no-choice feeding experiments with intact plants and with clipped leaves under different (field and greenhouse) conditions, we found that overall both herbivory and grasshopper performance (body length) were greater on the exotic grasses compared to native grasses. Our results suggest that exotic grasses which do not share a coevolutionary history with native generalist Melanoplus grasshoppers might have lower physical and chemical defenses than native plants. Consequently, Melanoplus grasshoppers may provide biotic resistance to these exotic grasses should these plants invade natural areas at the study regions. These results have important applications for predicting the interaction between exotic plants and generalist herbivores in the introduced range, which is critical for understanding factors facilitating plant invasions.