Promoting generalist predators of crop pests in alley cropping agroforestry fields: farming system matters

Summary

Developing agroecological practices that enhance biological control of crop pests is a major issue for the tran-sition of agriculture towards sustainable and biodiversity-friendly systems. Agroecological infrastructures (AEI) are devoted to the support of ecosystem service providers, although they have mixed effects on natural enemies of crop pests. In temperate regions, alley cropping agroforestry involves within-field AEI, in the form of tree rows and associated understory vegetation strips. The objective of this study was to assess the potential of generalist predators (carabid beetles and cursorial spiders) to control weed seeds and invertebrate pests in alley cropping vs pure crop systems, under two contrasting farming systems (conventional vs organic). Predator surveys were carried out in May and June 2017 in South-Western France over 12 winter cereal fields. Our study revealed that the effect of alley cropping was modulated by the farming system. Under conventional farming, alley cropping had a negative effect on the activity-density and species richness of generalist predators, especially regarding carnivorous carabids whose activity-density was reduced by nearly 50%. Under organic farming, alley cropping enhanced both the activity-density and complementarity of generalist predators, with a two-fold increase in the activity-density of seed-feeding carabids (predominantly granivorous and omnivorous), potentially promoting weed seed and invertebrate pest control. Our results suggest that the effectiveness of AEI in promoting natural enemies depends on the farming system at the field scale, which affects resource availability and determines spillover intensity between habitats. AEI are often perceived as sources of natural enemies. However, sink and retention effects (i.e. reduced or delayed spillover into crop fields due to higher attractiveness of AEI) should not be overlooked as they can explain the mixed impacts of AEI on natural enemy communities.