Effects of fertilization type on biocontrol of pests

Biological control of pests in arable land is an important ecosystem service provided by natural enemies such as predators and parasitoids. Plant fertilization has shown to influence both pests and natural enemies and to induce strong bottom-up effects, leading to trophic cascades up to the fourth trophic level. There is increasing evidence that these bottom-up effects differ between fertilization types. Moreover, the type of fertilization may affect the efficacy of pest control by natural enemies. However, hardly anything is known on how fertilization type effects pest-natural enemy interactions and the efficacy of biocontrol. The current project addresses this gap of knowledge by experimentally comparing how different fertilization types affect invertebrate assemblages in the field. As biological control is effectively the product of networks of interactions between the pest, its natural enemies and the non-pest food sources utilized by the latter, we will employ a food web approach to obtain a better mechanistic understanding of how fertilization type influences species interactions and biocontrol. Aphids and cereal leaf beetles are major pests in cereals, causing significant economic losses. Here we will employ a molecular food web approach to measure the frequency of trophic interactions between these cereal pests and their parasitoids as well as between the pests, parasitoids, non-pest prey and ground- and foliage-dwelling predators under different fertilization regimes. Plots with organic and inorganic fertilization as well as unfertilized controls will be established in replicated wheat fields. The fertilisation effect on invertebrate densities and community composition as well as on food web interactions and the levels of pest control will be measured. There are three aims to this three-year research program: 1) to generate and compare, based on the molecularly-derived data, aphid/leaf beetle-parasitoid-predator-non-pest prey food webs for organic, inorganic and control fertilization regimes at herbivore invasion and at herbivore establishment, 2) to determine the level of intraguild trophic interactions as well as the consumption frequency of pest- and non- pest prey and to compare the frequency of these trophic linkages between fertilization regimes, and 3) to measure pest control in the different fertilization regimes to unravel how biocontrol efficacy is affected by food web interactions and how this is modulated by the fertilization regime. This work will provide novel insights into how the type of fertilization, one of the major bottom-up measures in agricultural systems, effects trophic interactions and herbivore control in complex species assemblages. It will provide a much needed scientific basis for a better mechanistic understanding of biological control under inorganic and organic fertilization regimes, with important implications for both pure and applied ecology.

The biological control of pests is an important ecosystem service in arable land which is delivered by natural enemies such as parasitoids and predatory arthropods. Both pests and natural enemies are affected by the fertilization of plants and there is increasing evidence that the type of fertilizer affects these organisms and consequently also the efficacy of biological pest control. This project aimed at experimentally assessing how different types of fertilizers (organic, inorganic, unfertilized) affect the densities and community composition of invertebrates in cereal crops as well as their feeding interactions. As biological control is the product of interactions between pests, natural enemies and alternative prey, a molecular food web approach was employed to obtain a better mechanistic understanding of how fertilization impacts the interactions between the species and the level of biological control. We found that the natural enemies responded quickly in response to different types of fertilizers by changing their feeding behaviour, inducing a dynamic network of interactions between pests and natural enemies. Interestingly, we could show, for the first time, that these changes were evident before the abundances of the species were affected. When comparing experimental plots with low and high biological control levels we also found that the efficacy of biological control was boosted when the natural enemies could utilize a broad spectrum of alternative prey which decreased the competition between predators. This was the case for those plots where organic fertilizers were provided which, in turn, are an important food source for detritivores such as earthworms and collembolans, a preferred alternative prey of predators. Contrariwise, inorganic and no fertilization lead to a decrease in biological control and made it less resilient due to the non-existing positive effect of the detritivorous alternative prey. Our findings have important implications for the sustainable management of arable land as they show that organic fertilization stabilizes and increases the pest control function of natural enemies in cereal crops and which make this ecosystem service more resilient.