Organic fertilisation enhances generalist predators and suppresses aphid growth in the absence of specialist predators

Guillermo Aguilera,Laura Riggi,Kirsten Miller,Tomas Roslin,Riccardo Bommarco

doi:10.1111/1365-2664.13862

Guillermo Aguilera, Laura Riggi + Show 3 more

Open Access

https://doi.org/10.1111/1365-2664.13862

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Abstract

Abstract 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 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.

Highlights

Pest suppression by predatory arthropods, such as ground carabid beetles and spiders, provides a crucial basis for crop protection (Chaplin-Kramer & Kremen, 2012)
Three predator community treatments were introduced to each plot, one per cage (+A+P), on 18th of June (Appendix S2 in Supporting Information), simulating three predator community assembly patterns: Firstly, to simulate a landscape effect of generalist predators moving into the crop field from the surrounding landscape, we introduced 45 adult wolf spiders (Pardosa sp.; Roubinet et al, 2018)
We show that fertiliser type and the composition of highly mobile predators moving into crop fields from the surrounding landscape can drastically reduce herbivorous pest populations

Summary

| INTRODUCTION

Pest suppression by predatory arthropods, such as ground carabid beetles and spiders, provides a crucial basis for crop protection (Chaplin-Kramer & Kremen, 2012). In contrast, provide more directly accessible nutrients to the plant increasing plant nitrogen content, biomass and which can boost the herbivore growth rate (De Bruyn et al, 2002; Herencia et al, 2007; Mäder et al, 2002) Despite these indications of significant fertiliser effects on natural pest suppression, most research on this topic lacks the connection between above-ground communities and fertilisation management. We used 2 × 2 × 2 m mesocosms in an oat field, to examine interactive effects on natural pest suppression between experimentally simulated combinations of predator guilds (generalists and specialists) moving to the crop field from the surrounding landscape, and three fertilisation treatments (no fertilisation, organic manure and inorganic mineral fertilisation). We expected positive interactions between manure fertilisation and the inflow of predators from the surrounding landscape in suppressing aphid populations and in determining yield, by an increased top-down suppression

| MATERIALS AND METHODS

Findings

| DISCUSSION