| Agricultural pest suppression is a valuable ecosystem service that may be enhanced by the conservation of natural enemy species. However, it is unclear how conserving natural enemy diversity per se affects the biological control of pests, because few studies have attempted to distinguish between the effects of enemy abundance, identity, composition, and diversity. In chapter 1, I discuss how an experimental approach traditionally used in plant studies can be adapted to isolate the effects of enemy diversity on the suppression of herbivores. In chapter 2, I apply this experimental approach to examine how increasing enemy diversity within the potato agroecosystem affects the biological control of the green peach aphid, Myzus persicae . In this study, I find that increasing enemy diversity has no overall effect on the strength of aphid suppression. In chapter 3, I focus on a different assemblage of enemy species, one in which interference may be particularly likely, and examine how both the diversity and composition of the enemy community affect the strength of biological control. I find that increasing enemy diversity can strengthen, weaken, or not affect the suppression of herbivores, depending on the particular composition of enemy species that coexist in the community. In chapter 4, I test the hypothesis that multi-enemy effects on prey suppression may also vary with the identity of the plant species upon which the higher trophic levels interact. Here I focus on a single composition of enemy species that occurs in both potato and collard agroecosystems. I find that increasing diversity within this enemy assemblage leads to stronger aphid suppression on both plant species. However, because aphids reproduced at a much higher rate on collards than on potatoes, increasing enemy diversity reduced aphid populations by ∼200 aphids per plant on collards but only by ∼6 aphids per plant on potatoes. This indicates that the value of conserving enemy diversity will be far greater on collards than on potatoes. Taken together, this body research shows that a general relationship between enemy diversity and biological control may not exist, and that the value of conserving enemy diversity per se will likely vary among agroecosystems. |
