Do natural enemies really make a difference? Field scale impacts of parasitoid wasps and hoverfly larvae on cereal aphid populations

Abstract

Naturally occurring predators and parasitoids are known to reduce the abundance of pest invertebrates in arable crops, yet current treatment thresholds do not account for such a contribution to pest management. In the present study, we provide evidence for the presence of natural enemies correlating with a subsequent reduction in pest population growth. The abundance of cereal aphid pests and two key aphidophagous natural enemies, parasitoid wasps (Aphidiinae) and hoverfly larvae (Syrphinae), was assessed at field boundaries and interiors in southeast England. The highest rate of aphid population growth was associated with locations where no natural enemies were found. The presence of either Aphidiinae wasps or predatory Syrphinae larvae was associated with a reduction in the rate of aphid population growth, irrespective of location within the field, and overall aphid population growth was negatively correlated with increasing natural enemy abundance. The results of the present study indicate that natural enemies contribute significantly to pest control, and provide further evidence supporting the use of management strategies for promoting natural enemies in agro-ecosystems. Aphid predators and parasitoids make an important contribution to aphid pest control within cereal fields, and thresholds for insecticide application should account for this to avoid unnecessary treatments.