Exploring the Relationship among Predator Diversity, Intraguild Predation, and Effective Biological Control

In conservation biological control, we seek to make agricultural systems more hospitable to natural enemies, in an attempt to increase enemy abundance and diversity. However, it is unclear whether the effectiveness of biological control actually increases with growing natural enemy diversity, in communities including many species of generalist predators. Studies have shown that suppression of herbivores may be either enhanced or disrupted by adding predator species to a community, but these studies do not distinguish between the effects of predator diversity and the effects of predator abundance, identity, and composition. Here, we first demonstrate that a diverse community of natural enemies, dominated by generalist predators, attacks the green peach aphid, Myzus persicae, in potato fields in Washington State. Second, in a large-scale field experiment, we experimentally isolate the effect of predator diversity on aphid biological control. We show that increasing predator diversity does not affect prey exploitation; overall there is no strong, net complementarity or interference among predators that alters the strength of aphid suppression. However, our experiment revealed strong effects of predator species identity, because predators varied dramatically in their per capita consumption rates. Because of these strong species identity effects, green peach aphid biological control will improve with growing predator diversity, because particularly effective aphid predators will be more likely to be included within diverse communities. However, our results do not suggest any benefits to biological control of natural enemy diversity per se.