Biological Control in Theory and Practice
Biological control in practice is a tactical, empirical procedure. In classical biological control, typically various enemies from the pest's area of origin are released, together or in sequence, in the hope that one or more will prove successful. Pre-release screening concerns possible deleterious side effects or physiological tolerance, not the finer points of population dynamics. This has proved a quite successful procedure (Huffaker and Messenger 1976). By contrast, the literature concerning the general strategy of biological control (e.g., Huffaker and Messenger 1976) and possible relevance of ecological theory (e.g., Murdoch 1973; Hassell 1978) has emphasized aspects of pest-enemy popula- tion dynamics. The theory derives mainly from the Nicholson-Bailey model (e.g., Beddington et al. 1978; May and Hassell 1981), and it and the conventional wisdom in general agree broadly on several basic issues. (1) Successful biological control is caused by the enemy imposing a low, stable host equilibrium. (2) Success is most likely when the enemy has the following features: (a) it is host- specific; (b) it is synchronous with the pest; (c) it can increase in density rapidly when the host does; (d) it needs few pest individuals (usually only one) to complete its life cycle (and hence can persist when the pest is at its low equilib- rium density); and (e) it has a high searching ability. These features are more typical of parasitoids than predators. (3) General predators are considered poor candidates precisely because they are polyphagous, are not synchronized with the pest, and do not usually have a high potential for increase. (There is also the practical problem that general predators might eat beneficial insects.) Models of successful biological control, therefore, have been locally stable deterministic equations describing the dynamics of a single prey species and a single parasitoid or predator species.