Understanding the relationship between plant diversity and herbivore abundance remains a key challenge for basic and applied ecology. We measured the density of an herbivorous pest, the potato leafhopper (Empoasca fabae), and its primary predator, Nabis spp., in alfalfa monocultures and in alfalfa polycultures (alfalfa mixed with orchard grass, a non-host plant). We also manipulated predators (Nabis present, Nabis absent) and plants (monoculture, polyculture) in a 2 x 2 factorial experiment to examine how the suppression of leafhoppers by Nabis may be influenced by increasing plant diversity.

Results/Conclusions Compared with monoculture fields, polyculture fields had fewer leafhoppers per alfalfa plant, but there was no significant difference between fields in the density of Nabis. In the experiment, leafhopper survivorship was reduced when Nabis was present, and the effect of Nabis on leafhopper survivorship was greater in polyculture than in monoculture (predator x plant interaction, P < 0.05). This result suggests that Nabis foraging efficiency is greater in polyculture and/or that antipredator behavior by the potato leafhopper is more costly in polyculture. In the upcoming field season, we will test these hypotheses in an effort to determine if Nabis contributes to the reduction in leafhopper density that we have observed in polyculture fields. Further progress in understanding the mechanisms by which plant diversity reduces herbivore abundance may lead to improvements in pest management, and could reduce the need for pesticides that threaten environmental and human health.