Influence of relative abundance and taxonomic identity on the effectiveness of generalist predators as biological control agents

A central yet relatively untested assumption of conservation biological control is that an assemblage of naturally occurring natural enemies is more effective at controlling pests than any individual species within the assemblage. However, often ignored in this assumption is that natural enemies typically vary in relative abundance, such that one or a few species are highly abundant while most are relatively scarce. Little is known of the combined roles of relative abundance and taxonomic identity in the mortality imposed by assemblages of natural enemies on pest species. We investigated the influence of relative abundance and taxonomic identity among three generalist arthropod predators found in collards (Brassica oleracea var. acephala) on the mortality of the imported cabbageworm, Pieris rapae. We altered the relative abundance of the generalist predators in experimental mesocosms and determined the mortality of 1st instar cabbageworms. The impact of relative abundance on cabbageworm mortality was mediated by the taxonomic identity of the highly abundant predator. Further, the level of mortality imposed by highly abundant predators was in some cases influenced by the occurrence of intraguild predation involving less abundant predators. Our results suggest that the success of management strategies involving the preservation of highly abundant predators in managed systems via conservation biological control tactics may be dependent on the identity of both the highly abundant and scarce natural enemies.