Indirect nontarget eVects of host-speciWc biological control agents: Implications for biological control

Classical biological control of weeds currently operates under the assumption that biological control agents are safe (i.e., low risk) if they do not directly attack nontarget species. However, recent studies indicate that even highly host-speciWc biological control agents can impact nontarget species through indirect eVects. This Wnding has profound implications for biological control. To better understand the causes of these interactions and their implications, we evaluate recent case studies of indirect nontarget eVects of biological control agents in the context of theoretical work in community ecology. We Wnd that although particular indirect nontarget eVects are extremely diYcult to predict, all indirect nontarget eVects of host speciWc biological control agents derive from the nature and strength of the interaction between the biological control agent and the pest. Additionally, recent theoretical work suggests that the degree of impact of a biological control agent on nontarget species is proportional to the agent’s abundance, which will be highest for moderately successful control agents. Therefore, the key to safeguarding against indirect nontarget eVects of host-speciWc biological control agents is to ensure the biological control agents are not only host speciWc, but also eYcacious. Biological control agents that greatly reduce their target species while remaining host-speciWc will reduce their own populations through density-dependent feedbacks that minimize risks to nontarget species. Published by Elsevier Inc.