Cereal aphid control by generalist predators in presence of belowground alternative prey: Complementary predation as affected by prey density

Abstract

Summary Generalist predators are important antagonists of pest species in agroecosystems. Increasing populations of alternative prey through detrital subsidies is one way to foster these predators. However, alternative prey may also distract generalist predators from their prey thereby diminishing the efficiency of biological control. To develop reliable predictions for biological control, it is essential to evaluate the relative importance of generalist predators, pests, alternative prey and their respective interactions. We investigated the effects of an assemblage of generalist predators on the grain aphid Sitobion avenae in winter wheat. Treatments with 10, 100 and 1000 aphids were established inside 2 m 2 sized caged plots with control and reduced density of predators. Three weeks after the experiment started, samples were taken to estimate the size of aphid populations and those of alternative prey. Three prey taxa were significantly reduced by generalist predators: the grain aphid S. avenae (Aphidina), the click beetle Adrastus pallens (Elateridae) and the springtail Isotoma viridis (Collembola). Springtails were decreased by generalist predators independent of aphid densities, indicating complementary predation of springtails and aphids. At high aphid densities, grain aphid population peaks were decreased to the threshold level of economic damage, demonstrating efficient aphid suppression by the predator community. Click beetle numbers declined only at low and medium aphid densities. The results suggest that generalist predators preferentially fed on click beetles at low and medium aphid densities and switched to aphids at high aphid densities. Early-season predators likely had the greatest influence on aphid suppression. Our results indicate that alternative prey from the belowground system forms a substantial food resource for generalist predators, suggesting that the belowground subsystem modulates predator–prey interactions above the ground.