Synergistic pesticide effects on complex arthropod trophic interactions in arable systems

Arable systems have a high dependence on diverse natural biota to support pest control, soil bioturbation, and nutrient recycling. However, pesticide regulatory assessments fail to consider complex trophic interactions, underestimating real-world risks. This problem may be exasperated when mixtures of pesticides interact synergistically. Using model arable communities of pests (aphids and weevils), predators (e.g., beetles, spiders, and mites), and soil biota (including mites and earthworms), we assess the impacts of synergistic interactions between a pyrethroid insecticide (cypermethrin) and fungicides (Azoxystrobin and Prochloraz). The impacts of these mixtures on biological control, the biomass of major invertebrate trophic groups, and soil ecosystem processes were additive. However, the architecture of predatory arthropod trophic interactions was simplified in response to synergisms between the pyrethroid and fungicides. Synergisms may act through multiple potential mechanisms to impact food web structure. This may include not only toxicity but also detoxification of pyrethroids within the soil, as well as indirect effects mediated through the loss of critical fungal food resources for detritivores. These findings emphasize the need to consider complex community structure endpoints when assessing system risk to safeguarded agri-ecosystems.