Increasing enemy biodiversity strengthens herbivore suppression on two plant species

Concern over biodiversity loss, especially at higher trophic levels, has led to a surge in studies investigating how changes in natural enemy diversity affect community and ecosystem functioning. These studies have found that increasing enemy diversity can strengthen, weaken, and not affect prey suppression, demonstrating that multi-enemy effects on prey are context-dependent. Here we ask how one factor, plant species identity, influences multi-enemy effects on prey. We focused on two plant species of agricultural importance, potato (Solanum tuberosum), and collards (Brassica oleracea L.). These species share a common herbivorous pest, the green peach aphid (Myzus persicae), but vary in structural and chemical traits that affect aphid reproductive rates and which may also influence inter-enemy interactions. In a large-scale field experiment, overall prey exploitation varied dramatically among the plant species, with enemies reducing aphid populations by approximately 94% on potatoes and approximately 62% on collards. Increasing enemy diversity similarly strengthened aphid suppression on both plants, however, and there was no evidence that plant species identity significantly altered the relationship between enemy diversity and prey suppression. Microcosm experiments suggested that, on both collards and potatoes, intraspecific competition among natural enemies exceeded interspecific competition. Enemy species showed consistent and significant differences in where they foraged on the plants, and enemies in the low-diversity treatment tended to spend less time foraging than enemies in the high-diversity treatment. These data suggest that increasing enemy diversity may strengthen aphid suppression because interspecific differences in where enemies forage on the plant allow for greater resource partitioning. Further, these functional benefits of diversity appear to be robust to changes in plant species identity.