Nitrogen supply in conventional versus organic farming systems: effects on the performance of cereal aphids

Abstract

Cereal aphids (Homoptera: Aphididae) are a periodical threat to winter wheat (Triticum aestivum L.) production worldwide, and outbreaks have been mainly related to increasing inputs of mineral fertilizers. The hypothesis of lower aphid abundance under organic fertilizer treatments, however, has not been tested at the species-specific level. In a 4-year study, we examined cereal aphid populations and plant parameters (dry biomass and nitrogen content) in winter wheat fields under low (legume-based), intermediate (legume-based plus organic liquid manure), and high (mineral) nitrogen intensities; low and intermediate intensities are characteristic of organic fields, while high intensities are inherent in conventional farming systems. Aphid species differed markedly in their response to fertilizer treatment. Unexpectedly, legume-based organic fields were related to higher abundances of the ear-colonizing species Sitobion avenae (F.), whereas manure-fertilized organic fields and conventional fields had significantly higher abundances of Metopolophium dirhodum (Wlk.) and Rhopalosiphum padi (L.). Nitrogen concentration of winter wheat increased with fertilizer intensity, and total aphid abundance (all species) was positively correlated with nitrogen content of grains in manure-fertilized organic fields and conventional fields, but was not correlated in legume-based organic fields dominated by S. avenae (89–96 % of the aphid community). Collectively, we demonstrate strong bottom-up effects that result in an improved performance of S. avenae in low-input systems. As total aphid abundance (all species) simultaneously decreased in legume-based organic fields, crop managers risk to underestimate pest damage by ignoring the impact of a single species, S. avenae, which has the greatest potential to reduce crop yield.