Contrasting Responses of Soybean Aphids, Primary Parasitoids, and Hyperparasitoids to Forest Fragments and Agricultural Landscape Structure

Primary parasitoid species, usually Hymenopteran wasp species, contribute to pest regulation services in agroecosystems by parasitizing crop pests and reducing their abundance. However, this positive effect can be limited if primary parasitoids themselves are parasitized by secondary parasitoids, also known as hyperparasitoids. These trophic dynamics that influence pest regulation take place within the context of changes to agricultural landscape structure including loss of natural habitat and landscape simplification. Therefore, there is great interest in understanding how landscape structure influences pest, primary parasitoid, and hyperparasitoid dynamics. We investigated how the structure of agricultural landscapes in Southern Quebec affects primary and hyperparasitism rates of soybean aphid (Aphis glycines). We found that pests, primary parasitoids, and hyperparasitoids responded in contrasting ways to landscape structure. While aphid abundances and hyperparasitoid rates increased with increasing distance-from-forest in soybean fields and as the proportion of forest in the surrounding landscape decreased, primary parasitoid rates showed the opposite trends. Relationships with forest fragment size and isolation were complex, with contrasting positive, negative, and neutral effects across the three groups. Finally, wider fields consistently had lower aphid abundances, primary parasitoid rates, and hyperparasitoid rates. Our results highlight the complexity of the trophic dynamics that underlie pest regulation and how changes to landscape structure can lead to conflicting and contradictory effects on pest regulation.