Agricultural intensification and aphid-parasitoid foodwebs

Changes in agroecosystem management (e.g. landscape diversity, management intensity) affect the natural control of pests. The effects of agricultural change on this ecosystem service, however, are not universal and the mechanisms affecting it remain to be understood. As biological control is effectively the product of networks of interactions between pests and their natural enemies, food web analysis provides a versatile tool to address this gap of knowledge. The proposed project will utilize a molecular food web approach and examine, for the first time, how changes in plant fertilisation and landscape complexity affect quantitative aphid-parasitoid-hyperparasitoid food webs on a species-specific level to unravel how changes in food web interactions affect parasitoid aphid control. Based on the fieldderived data, cage experiments will be conducted to assess how parasitoid diversity and identity affect parasitoid interactions and pest control, complementing the field results. The work proposed here will take research on parasitoid aphid control one step further, as it will provide a clearer understanding of how plant fertilization affects whole aphid-parasitoid food webs in both simple and complex landscapes, allowing for further improvements in natural pest control.

Intensification of agriculture has led to changes in agroecosystems: for example, fertilizer input has been increased, while the landscape complexity has been decreased by creating larger and more uniform production areas. Still, it remains poorly understood how this intensification affects the functioning of agroecosystems and the delivery of regulative ecosystem services such as the biological control of aphids. This project examined how conventional NPK fertilization and landscape complexity, two important parameters of agricultural intensification, affect the interactions between cereal aphids, primary parasitoids and hyperparasitoids as well as the associated levels of biocontrol. As a first step, new molecular methodology was developed to allow examining the interactions between cereal aphids, primary parasitoids and hyperparasitoids on a species-specific level. Additionally, the system allowed identifying bacterial secondary endosymbionts in the aphids. These endosymbionts might confer protection against parasitoids and modulate aphid-parasitoid interactions. The molecular systems developed in this project provide an efficient, highly sensitive, and cost-effective tool for assessing cereal aphid-parasitoid-endosymbiont interactions. Within a large-scale field experiment comprising 13 fields, we found that plant fertilization negatively affected cereal aphid densities, parasitism rates and the specialization as well as generality of hyperparasitoids. However, these fertilization effects were modulated by landscape complexity including sometimes opposite effects in simple and complex landscapes. The same field experiment was used for an assessment of the interactions between the English grain aphid Sitobion avenae, and its secondary endosymbionts Hamiltonella defensa and Regiella insecticola as well as parasitoid communities in a real- world scenario, providing conclusive evidence that aphid-endosymbiont-parasitoid interactions are endosymbiont-specific. Moreover, we found that agricultural intensification modulates secondary endosymbiont occurrence, most likely via indirect mechanisms, which can have important consequences for the net effect of parasitoids on cereal aphid populations. Overall, our project shows that fertilization has pronounced bottom-up effects which cascade up to the fourth trophic level in food webs but that the net effect of this in-field measure and its direction can be modulated by the complexity of the surrounding landscape.