Direct and indirect effects of the spatial context on the natural biocontrol of an invasive crop pest

Natural biocontrol of pests can be influenced by both direct effects (spatial context, natural enemies) and indirect effects (spatial effects on natural enemies). The objectives of this study were to: 1) Evaluate the relative influence of natural enemies and spatial context on the natural biocontrol of the soybean aphid (direct effects), and 2) Determine the effect of the spatial context on the guild of natural enemies best associated with the natural biocontrol (indirect effects). Regarding the first objective, our results showed that, in 2011 (high infestation), the soybean aphid natural biocontrol was mainly explained by a shared effect of natural enemies (entomopathogenic fungi, predators, functional diversity) and spatial context (presence of buckthorn, proportion of soybean, crop richness), accounting for 50% of the variation. The functional diversity was the only natural enemy variable positively associated with the natural biocontrol (negative effect on the cumulative aphid-days). In 2012, (moderate infestation), the natural biocontrol was mainly explained by the natural enemies (species richness, 23.5% of variation) followed by a shared effect of natural enemies and landscape context (15.5%). Regarding the second objective, we found the natural enemies positively influenced by the proportion of woodlands, and negatively influenced by the proportion of soybean in the landscape. Our results suggest that the integrated pest management of the soybean aphid should include management of habitats that conserve the natural enemies, especially those promoting functional diversity. For instance, a better natural biocontrol could be achieved by reducing the proportion of soybean, increasing crop richness and conserving woodlands.