Combined ivermectin and spinosad effects on sepsid flies

Human-generated pollutants can act in combination to affect the dynamics of non-target beneficial insect populations. Here we simulated a natural scenario by exposing five closely related species of temperate sepsid dung flies (Diptera: Sepsidae) to realistically low concentrations of the parasiticide ivermectin during their larval growth stage, and subsequently to the widely used insecticide spinosad during their adult stage. We evaluated the effects of combined pesticides on several life-history traits associated with organismal fitness. The clearest effect of the combined contaminants across all species was a delay of the egg laying onset, although the most severe effects were observed as dramatic reductions in offspring number in various treatment combinations. As the strongest negative effects of both contaminants in combination were not necessarily at the highest pesticide concentrations, explanations for these non-monotonic dose-responses, including hormesis, are provided. Our results suggest that multiple pesticides released in the environment can interact in different ways to affect insects in agricultural landscapes, with consequences for population dynamics and ecosystem functioning.