Enhanced diversity of aquatic macroinvertebrate predators and biological pest control but reduced crop establishment in organic rice farming

Abstract

Organic farming has been adopted as the main alternative to reduce the environmental impact of intensive agriculture by banning the use of synthetic inputs and bolstering key ecological functions such as biological pest control. Yet the potential cascading effects of farming management on the communities of natural enemies, the provisioning of pest control, and the ultimate impact on crop performance remain poorly explored. This is especially true for semi-aquatic crops such as rice (Oryza sativa, L) that largely depend on aquatic macroinvertebrate predators for biological pest control. Here we performed two consecutive landscape-scale field experiments by combining monthly surveys of macroinvertebrate aquatic predators with both pest predation and seed rice germination trials to assess differences between conventional and organic rice farming in all the above-mentioned issues. Specifically, we selected 6 organic and 6 conventionally managed rice paddies and evaluated differences in i) the diversity of aquatic macroinvertebrate predators (taxa richness and evenness), ii) the infestation level of a seed pest (chironomids larvae), iii) the efficiency of biological pest control provisioning and iv) the impact on germination of rice seeds. Communities of macroinvertebrate aquatic predators were more diverse (taxa richness) and abundant in organic than in conventional fields, yet community evenness was higher in conventionally-managed fields. Accordingly, we found that macroinvertebrate-mediated pest control was markedly more efficient in organic farms during the establishment crop stage. Pest infestation, however, remained also higher in organic farming, most likely because biological pest control in organic fields is not able to outcompete chemical control provided in conventional fields. Finally, the increased infestation level observed in organic fields ultimately resulted in reduced germination rates of rice seeds, thus limiting crop performance. Our results highlight the potential positive effects of pesticide banning in organic rice farming on aquatic macroinvertebrate communities and the consequently improvement of biological pest control provisioning. Yet conditions enhancing biodiversity may also favor pest infestation and ultimately increase crop damage. Management recommendations should therefore account for potential services and disservices associated to organic farming in order to avoid negative yield outcomes for rice farmers.