restricted access VII.1 Biological Control: Theory and Practice
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VII.1 Biological Control: Theory and Practice William Murdoch OUTLINE 1. Settings and protagonists 2. Locally persistent pest–enemy interactions 3. Locally nonpersistent systems 4. Ecological theory and biological control Biological control—defined here as the suppression of insect pests by other insects that attack them—has been pursued by entomologists for more than a century, in part because it is typically cheap and can yield very large economic returns on investment. Over this period, the empirical record is one of often spectacular successes mixed with rather more failures. It is also a history of trial and error. In an extreme example, entomologists introduced about 50 species of enemy insects before achieving great success in controlling California red scale on citrus, a case discussed below. Such applied population dynamics has naturally attracted the interest of ecologists who, together with many of the entomologists themselves, have worked to develop theory that would explain the essential features underlying success. This theory and its connection to real biological control comprise the subjects of this chapter. The theory’s domain, however, is much broader—it is the dynamics of interacting resource populations and the consumer populations that attack them. GLOSSARY consumer–resource interactions. These include interactions between populations of predators and prey, parasitoids and hosts, indeed any interaction in which one species depends on another for sustenance . These terms are used interchangeably here. density-dependent processes. These cause the per-head rate of increase of the population to decrease when the population’s density increases. equilibrium. Density at which the population will remain , once it is reached, if the population is not perturbed. parasitoid. An insect that parasitizes another insect (the host species) by laying egg(s) in or on the host, which is eaten by the immature parasitoid. scale insects. Plant-sucking bugs that stay attached to the plant for almost their entire life history. stable equilibrium. An equilibrium is stable when the population tends to return to it after the population is perturbed. unstable equilibrium. The population moves away from the equilibrium following a perturbation. The result may be cycles in abundance, extinction , or chaos, in which the densities are always bounded, but there are no repeated sequences of abundance. 1. SETTINGS AND PROTAGONISTS It is useful to distinguish two settings in which biological control occurs. First, most successful biological control has occurred in relatively long-lived agricultural systems such as orchards and, less successfully, forests. It is known for at least some cases that the pest and enemy populations persist together at a small spatial scale such as a single orchard. This is the kind of dynamics for which the bulk of ecological consumer– resource theory has been developed. In the majority of successes in this class, the pest is an alien species as, almost always, is its successful enemy . Typically, the entomologist faced with an introduced pest travels to its place of origin, searches for enemy species in that environment and, after some preliminary laboratory work, releases the enemy, which usually confines its attacks to the pest species—it is effectively a specialist in the introduced agricultural ecosystem. Again, most consumer–resource theory is developed for specialist consumers. The second class of control occurs in temporary crops, an environment where success is more elusive, at least in part because enemy species (and the pest) usually do not persist locally. Pest species in these situations may be native or introduced. Less is known about control here; it is likely to involve a number of enemy species, some of which may be introduced and some of whom are likely to be generalist feeders. There is also less relevant theory available. N. J. Mills has shown that the greatest number (and proportion) of successful cases of biological control are against plant-sucking bugs (Homoptera), for example, scale insects and whiteflies. Caterpillars of moths and butterflies (Lepidoptera) constitute the second largest group, although the number of successes and especially the success rate are lower. The majority of successful control agents are parasitoids, although predatory insects have also been successful. 2. LOCALLY PERSISTENT PEST–ENEMY INTERACTIONS The central assumption in classical biological control, in which an introduced pest is suppressed by an introduced specialist enemy species, is that the natural enemy controls the pest by maintaining it locally at a low stable equilibrium density. This is because most theory for population dynamics is equilibrium centered, and such a framework provides the most straightforward way to account for long-term persistence of...