Biological Control in Theory and Practice

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Abstract
The conventional wisdom of biological control of insect pests, and its related ecological theory, is that successful natural enemies in long-lived exosystems (1) impose a low, stable pest equilibrium, and (2) share the following properties: (a) host-specific; (b) synchronous with the pest; (c) can increase in density rapidly when the pest does; (d) need only one pest individual to complete their life cycle; (e) have a high search rate for the pest; (f) aggregate at areas of high pest density, which is thought to stabilize the interaction. These features are more characteristic of parasitoids than predators. A stable pest equilibrium is neither a necessary nor a sufficient conditon for control. Satisfactory contol in model systems is compabible with both local extinction of the pest and polyphagy in the natural enemy. Only 1 of 9 real examples of successful control is convincingly a stable interaction; the remainder show either strong evidence for stability and local extinction of the pest or are consistent with this interpretation. Successful natural enemies have collectively violated all of features 1 and 2 above, and violations of features 1 and 2a, 2b, and 2d appear to have been central to success in several situations. Two strategies by which a natural enemy may control a pest in a nonequilibrium state, termed here lying-in-wait and search-and-destroy, are distinguished.