Organic fertilizer amendment promotes wheat resistance to herbivory and biocontrol services via bottom-up effects in agroecosystems

Excessive mineral fertilizer input results in little extra yield but exacerbates insect herbivory and affects environmental health and ecosystem services. The use of organic fertilizer is considered to have promise for mitigating those impacts. How organic fertilizer amendment modifies crop resistance to insect herbivory and modulates biocontrol services on a landscape scale has not been well studied. We conducted a series of field experiments on a large spatial scale with three fertilization regimes (mineral fertilizer, mineral fertilizer amended with organic matter, and no fertilizer control) in Shandong Province, northern China. Soil nutrient content, wheat plant metabolism, cereal aphid abundance, parasitism rate, and wheat yield were quantified. Maize straw amendment combined with mineral fertilization significantly reduced cereal aphid abundance and promoted parasitism during the peak aphid period, compared with mineral fertilizer alone and the no fertilizer control. Modeling simulations showed increased biological pest control when a larger proportion of fields were additionally treated with maize straw amendment. Foliar chemical analyses revealed that the types and content of plant free amino acids, rather than plant defensive compounds, most likely accounted for the variation in aphid abundance and biological control efficiency. A mineral fertilization regime plus plant straw amendment may promote wheat resistance to herbivory and benefit biocontrol via bottom-up effects. Heterogeneity in fertilizer regimes between fields may be the key ecological force shaping pest control at a landscape scale.