Organic vs. conventional farming dichotomy: Does it make sense for natural enemies?

Summary

As an alternative to conventional farming, organic farming is considered a promising type of production to meet the challenges of modern agriculture. In particular, organic farming is assumed to favour the biological control of pests by their natural enemies and, therefore, is considered a possible way to reduce the use of pesticides. Effects of organic vs. conventional farming on insects natural enemies have been compared, but the results remain uncertain, probably because the diversity of crop management strategies is rarely considered. In this study, we assessed whether or not the diversity of farming practices implemented in organic and conventional farming affects natural enemies of aphids (ladybirds, carabid beetles and parasitoids) in winter wheat. Entomological surveys were carried out in 20 pairs of organic and conventional fields. We interviewed the farmers to identify variables that describe farming practices and identified the most important practices using a ranking approach based on random forests. Abundances of aphids' natural enemies were tested in relation to different levels of description of farming practices (from organic vs. conventional farming to individual practices). We found a large diversity of farming practices, which were evenly distributed along a gradient from organic to conventional farming. Abundances of aphids' natural enemies were greater in organic fields, but the three species groups had different responses to the diversity of farming practices. Ladybirds were influenced by tillage frequency, number of wheat varieties and pesticides, and carabid beetles by tillage frequency, whereas parasitoids were not affected by any practice. Even though the organic vs. conventional farming dichotomy was meaningful to explain aphids' natural enemies abundances, the consideration of more detailed practices improved our understanding of their response to crop management strategies. Our results help identify the level at which agro-ecosystem actors must intervene to promote effective biological control