Organic fertilization suppresses aphid growth via carabids in the absence of specialist predators

Biological control by natural enemies is a valuable ecosystem service. The predator community in a crop field is a combination of predators dwelling in the field and those moving into it from the surrounding landscape. The former is mainly affected by field management, the latter more by the composition of the surrounding landscape. Yet, separate and combined effects of local and landscape management on pest suppression have seldom been investigated. We set-up mesocosms within an existing long-term agricultural field experiment to investigate the effects of local management of organic manure or inorganic mineral fertilisation and simulated the spillover from the surrounding landscape of different predator types: no predators, generalist predators (wolf spiders) and specialist predators (ladybirds). We examined whether aphid density was driven by top-down or bottom-up processes under different fertilisation treatments, and how the magnitude of pest suppression was affected by predator community composition. We found positive synergistic effects between manure fertilisation and predator spillover on the suppression of aphid growth. Top-down suppression of aphids was more effective under manure fertilisation and in presence of specialist predators (ladybirds). Bottom-up effects on the plant biomass growth dominated in inorganically fertilised plots. Organic and inorganic fertilisation gave the same seed yield, but through different mechanisms. The abundance of locally emerging predators in the manure treatment increased top-down pest suppression yielding plant biomass levels comparable with inorganically fertilised plants, being the latter driven by bottom-up effects. Synthesis and applications. Organic fertilisation enhanced local emergence of predators increasing top-down pest suppression. In contrast, local predator communities were unable to suppress aphid populations in inorganic and no-fertilisation treatments. Here, predator inflow from outside the crop field was essential for lowering aphid population growth. Managing landscapes to promote mobile predators emerges as particularly important for crop fields without manure amendments. We advise the active promotion of both local predators in the crop field and mobile predators in the landscape to secure the conservation of biological insect pest suppression.