The composition of soil-dwelling pathogen communities mediates effects on wireworm herbivores and wheat productivity

Abstract

Greater natural enemy diversity generally increases prey mortality. Diversity can be beneficial when natural enemy species occupy distinct niches (complementarity effects) or when diverse communities contain the most impactful species by chance (identity effects). Most research assessing effects of natural enemy diversity focuses on aboveground predators, however, and few studies have assessed whether increased entomopathogen diversity in soil affects belowground herbivores. Here, we assessed effects of entomopathogen richness on herbivore suppression and plant productivity in a system consisting of entomopathogens, wireworm herbivores, and wheat plants. Specifically, in field experiments we varied the richness of four entomopathogen species (two nematodes, two fungi) that attack herbivorous Limonius californicus larvae belowground. We show that the presence of entomopathogens increased wireworm mortality and subsequently increased plant productivity, but a single entomopathogen species, Metarhizium brunneum, had stronger effects than other species (Beauveria bassiana, Heterorhabditis bacteriophora, and Steinernema carpocapsae). Moreover, entomopathogen communities with M. brunneum consistently produced the strongest effects on prey mortality and plant productivity. Our results suggest that diverse entomopathogen communities increased prey infection more than any single species, indicating complementarity occurred. However, increased plant productivity appeared to be driven by species identify effects. Our study shows that the composition of entomopathogen communities had stronger effects on a pest species than species richness, suggesting that careful selection of entomopathogen species for biological control may be more impactful than promoting entomopathogen diversity.