Parasitoid Wasp Diversity in Apple Orchards Along a Pest Management Gradient

Abstract

Various studies have found higher biodiversity, particularly of arthropod natural enemies, on organic than on conventional farms. However, using broad management categories to compare farm diversity is complicated by farms using intermediate practices such as Integrated Pest Management (IPM). We used a pest-management toxicity index to compare Hymenopteran parasitoid species richness, abundance, and community composition in six apple orchards in southeastern Michigan, USA, along a gradient of management practices: organic, varying levels of IPM, and conventional. We conducted monthly vacuum-sampling of wasps in each orchard during 2009, and quantified pest-management practices based on pesticide application records. We then assigned a toxicity score to each orchard using a modified pesticide toxicity index and arranged the orchards along a toxicity gradient. One conventional orchard had a lower toxicity score than two orchards using IPM. Additionally, toxicity scores varied from month to month within each orchard. We found higher total wasp species richness in the organic orchard; however, richness varied by month and in August a conventional orchard had the highest species richness. Total wasp abundance was highest in orchards at opposite ends of the toxicity gradient, but patterns of wasp abundance also varied by month and the conventional orchard had the highest mean abundance in August. Monthly toxicity scores did not correlate with wasp species richness, but did with wasp abundance. Broad pest management categories were not consistent predictors of monthly variation in parasitoid wasp diversity. In contrast, monthly toxicity scores helped explain patterns of wasp abundance, though not species richness. With further refinement, a pest-management index could improve our understanding of monthly variation in orchard natural enemy biodiversity, and serve as a tool for farmers looking to increase biological control of orchard pests by parasitoid wasps.