Plant diversity increases herbivore movement and vulnerability to predation

Abstract

Understanding how changes in plant diversity affect agroecosystem functioning remains a key challenge. We examined how intercropping alfalfa, Medicago sativa, with orchardgrass, Dactylis glomerata, affects the potato leafhopper, Empoasca fabae, its host plant (alfalfa), and the efficiency of a leafhopper predator, Nabis americoferus. In a field experiment, intercropping reduced the reproductive efficiency of the leafhopper. Nabis was more effective at reducing leafhopper abundance, and protecting alfalfa from hopperburn, in the polyculture than in the monoculture of alfalfa. In a series of laboratory experiments, we investigated mechanisms by which intercropping could enhance the efficiency of Nabis. Intercropping resulted in changes in vegetation structure and the spatial distribution of leafhoppers, but there was little evidence that these factors influenced the efficiency of Nabis. Instead, orchardgrass, a nonhost for leafhoppers, increased leafhopper movement, and Nabis captured leafhoppers more efficiently when the herbivores were more mobile. These results indicate that intercropping with nonhost plants promotes leafhopper movement and vulnerability to predation, and reveal a novel mechanism by which plant diversity can reduce herbivory.