Submitted to: XI Symposium on Biological Control of Weeds
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 29, 2003
Publication Date: June 1, 2004
Citation: Smith, L. 2004. Avoiding and exploiting trophic cascading: its role in the selection of weed biological control agents. Virtual biological control of imaginary weeds. In: Cullen, J.M., Briese, D.T., Kriticos, D.J., Lonsdale, W.M., Morin, L. and Scott, J.K., editors. Proceedings of the XI Symposium on Biological Control of Weeds, April 27-May 2, 2003, Canberra, Australia. p.175-179. Interpretive Summary: Ecologists have long argued that the "world is green" because natural enemies, rather than host plants, limit the size of herbivore populations. Theoretically, in the absence of these higher trophic natural enemies, herbivore populations would increase until they overexploit their host plants, causing the populations to crash. Such drastic reductions of target weed populations are exactly what biological control practitioners are trying to accomplish. Historically, the pre-release evaluation of candidate biological control agents has focused primarily on finding agents that are host-specific, and secondarily on those that impact the target plant. Nevertheless, regardless of how much "impact" and individual natural enemy has on the target plant, successful classical biological control also depends on the production of large numbers of natural enemies.
Technical Abstract: Tech abstract: A biological control agent is likely to fail if its reproduction and survival is limited by factors such as incompatible climate, poor host plant suitability, or attack by higher trophic natural enemies. The first two factors are usually considered in biological control projects, but the latter is often overlooked. As a consequence, for example, two species of Coleophorid moths introduced to control Russian thistle (Salsola tragus) in the western United States, became widely established, but they are heavily attacked by predators and parasitoids and have not reduced the weed population. A tetranychid mite introduced to control gorse (Ulex europaeus) in the northwestern United States began heavily damaging the weed until predators appeared and reduced the mite populations. Pre-release evaluation of biological control agents can be improved by looking for natural enemies that are: 1) primarily attacked by specialists (which are not likely to occur in land of release); and 2) well defended from generalists.