Invasive predator disrupts link between predator evenness and herbivore suppression.
Biodiversity provided by non-crop plants has long been thought to strengthen conservation biological control by providing food and habitat for natural enemies. More recently, greater evenness among natural enemies has also been suggested to promote strong top-down control, but habitat characteristics and management strategies that promote or disrupt evenness among predators remain unclear. We examined effects of non-crop plants, and the activity density of the invasive intraguild predator Pterostichus melanarius (Coleoptera: Carabidae), on evenness among predators of the crucifer flea beetle (Phyllotreta cruciferae; Coleoptera: Chrysomelidae). We sampled ground-active predators, flea beetles, and in-row coverage of non-crop vegetation on 52 Brassica fields across three US states, and used structural equation models to evaluate direct and indirect, predator-mediated effects of non-crop plant cover on pest abundance. We found that non-crop plant cover and high evenness among predators associated with higher predator activity and indirectly with lower herbivore densities, consistent with stronger biocontrol. However, non-crop plants attracted the invader P. melanarius. In turn, P. melanarius linked with lower evenness among other ground-active predators, while showing no direct effect on flea beetles, suggesting potential disruptive interference by P. melanarius. While non-crop plants are widely known to attract diverse predators, it may be difficult for growers to manipulate plant communities that increase enemy evenness and promote natural pest control if intraguild predators commonly benefit from these resources as well. More generally, our results suggest a need for closer examination of role of predator-predator interactions in relationships between plant diversity and biological control across agroecosystems.