R. G. Van Driesche
University of Massachusetts, Amherst
University of California, Riverside
Biological control is a form of pest control that uses living organisms to suppress pest densities to lower levels. It is a form of ecologically based pest management that uses one kind of organism (the "natural enemies") to control another (the pest species). Types of natural enemies vary with the type of pest. For example, populations of pest insects such as scales are often suppressed by manipulating populations of parasitoids, which are insects that develop in or on the pest insects they attack and kill. Populations of plant-feeding mites, such as the common twospotted spider mite (Tetranychus urticae) are often limited by predators, especially mites in the family Phytoseiidae. Populations of weeds can be suppressed by specialized herbivorous insects that feed on them. Finally, many insect populations have pathogens (e.g., bacteria, viruses, or fungi) that infect them. Such pathogens, whether they occur naturally or are applied artificially as microbial pesticides, can locally and temporarily suppress a pest's numbers.
Biological control is thus about the relative numbers of pests and their natural enemies. Predators (or pathogens, parasitoids, herbivorous insects) increase in number over time and feed on the pest, whose population then declines, because of higher mortality or lower birthrates caused by predation by natural enemies. Biological control agents are living organisms that increase in number through reproduction in response to pests that are used for nutrition. Biological control, at least in some of its forms, has the potential to be permanent in its action, through the reproduction and spread of the natural enemies as they track target pest populations.
There are four broad ways in which people have manipulated natural enemies to enhance their action: natural enemy importation, augmentation, conservation, and application of microbial pesticides. Each of these approaches has its own rationale, history, and level of past successful use. Biological control has important advantages compared with other methods of pest control. Moreover, because pest control is relatively permanent and requires no further capital input, biological control through self-sustaining forms (natural enemy importation and conservation) is often cheaper than use of pesticides. Although there may be significant initial costs (especially for projects that import new species of natural enemies from the pest's country of origin), costs drop to low or even zero levels in later years, whereas the benefits of the pest control achieved continue to accrue for years. For other forms of biological control (natural enemy augmentation and use of microbial pesticides), control is not permanent and costs recur annually, as with pesticides. For the latter two approaches, biological control may be either more or less expensive than other approaches depending on details such as the cost of natural enemy production by commercial insectaries that sell beneficial organisms, and the efficacy of other control tactics. In all four forms, biological control has the advantage of being virtually harmless to people and vertebrates, whereas pesticides must be actively managed for safe use to mitigate harm to humans and other nontarget organisms.
Biological control as a scientific endeavor has a history of about 125 years of effective use (beginning in the 1880s), over which time new information, techniques, and technologies have increased humankind's ability to use biological control agents with increasingly greater understanding and effectiveness. Before this period of active use, there were several centuries during which ideas about predators, parasitoids, pathogens, and their links to pest populations evolved. These ideas had to be developed before biological control as an applied pest management activity could be conceptualized.
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