Predator richness predicts pest suppression within organic and conventional summer squash (Cucurbita pepo L. Cucurbitales: Cucurbitaceae)

Abstract

The biodiversity-ecosystem function (BEF) hypothesis predicts that increased richness within a trophic level supports greater resource capture. However, higher predator richness commonly found within organic compared to conventional agroecosystems does not always result in enhanced pest suppression. The striped cucumber beetle, Acalymma vittatum (Coleoptera: Chrysomelidae), causes damage via direct feeding and pathogen transmission to crops in the family Cucurbitaceae. The aims of this study were to: 1) identify the predators that contribute to A. vittatum suppression using molecular gut content analysis, 2) determine if the richness of predators testing positive for A. vittatum DNA was negatively correlated with prey abundance, and 3) examine how organic versus conventional cucurbit production influenced these relationships. Using a species-specific primer for A. vittatum, we found that a diverse community of predators feed on this pest, including Carabidae, Coccinellidae, Formicidae, Henicopidae, Opiliones, Lycosidae, and Staphylinidae. The richness of predators testing positive for A. vittatum DNA within a cucurbit field was negatively correlated with pest abundance, supporting the BEF hypothesis. We found no effect of farm management on the proportion or richness of predators testing positive for pest DNA, despite a 43.4% greater pitfall trap catch of predatory taxa within organic fields. This illustrates that using traps to measure predator richness and abundance may not accurately predict the strength of biological control service provided by a guild of natural enemies. Application of molecular gut content analysis proves a useful tool in advancing both the theory and practice of conservation biological control.