Aggregation of organically managed fields promotes aphid parasitism in cereal crops under Mediterranean conditions

Context: Maintaining a balance between semi-natural habitats and arable land is not always feasible for farmers. The promotion of biological control agents can be addressed through management at farm or field level, and/or by deploying lower intensity, biodiversity-friendly practices which can act either directly or indirectly through their effect of the plant community. Objectives: We studied the effects on cereal aphids and their parasitoids of agricultural management at field and landscape levels. We tested the effect of organic and conventional farming, and of the within field characteristics, on the cereal aphid-parasitoid community, across a gradient of organic farming aggregation and of percentage of arable land. Methods: In spring 2015, we sampled aphid populations in 30 cereal fields in five agricultural areas in Catalonia (Spain) with contrasting levels of organic farming aggregation. In each field, we also assessed weed and crop cover. As landscape variables, we calculated the Percentage of Agricultural Land (PAL) and the Percentage of Organically Managed Land (POML) in a 500-m buffer around each field. We sampled cereal tillers 3 m from the field edges and collected all aphids detected. In addition, we reared mummies (parasitized aphids) until they hatched. Results: Our results show that management at landscape level has significant effects on parasitism rates: a higher proportion of surrounding fields under organic management increased the amount of parasitism, as did less agricultural land cover. On the other hand, aphid populations were mainly affected by two in-field factors, namely, crop density and crop variety. Differences in weed communities did not seem to have any effects on either aphids or parasitoids. Conclusions: Rather than concentrating on the individual management of fields, a coordinated implementation of organic farming at landscape level would seem to be a much better strategy for improving the biological control of aphids.