Multiple agents in biological control: improving the odds?

The current interest in risks associated with classical biological control led us to review the literature to determine whether the introduction of multiple biological control agents has been more effective than the introduction of a single control agent. We analyzed 59 projects against weeds and 108 projects against insect pests. Establishment of control agents was significantly higher in single-agent projects than in multiple-agent projects against insect pests, but not in projects against weeds. The success of biological control against weeds increased with the number of agents released. However, no relationship was found between the number of agents released and biological control success for insect pests. These results suggest that negative interactions may play a significant role amongst biological control agents of insects. Thus, multiple releases may be effective against weeds only. In over 50% of the successful multiple-agent projects against weeds and insect pests, a single agent was shown to be responsible for success. This result indicates that, in a majority of biological control projects, multiple agents are not released for a cumulative control effect but to increase the likelihood that the right control species is released (lottery model). Considering the possibility of negative interactions among biological control agents and taking into account the risks associated with natural enemy introductions, we recommend restraint in the introduction of multiple agents for biological control. 2002 Elsevier Science (USA). All rights reserved.