introduced pests and often provides large economic or environmental advantages over alternative methods. An example given in the paper is control of the alfalfa weevil: introduction and widespread releases of 11 species of parasitic hymenoptera have yielded substantial control of this major pest with no known negative side effects and with an estimated benefit-to-cost ratio of 87:1.
Risks of CBC programs have three different sources: the organism itself (e.g., parasitism or predation on nontarget species), associated organisms (e.g., pests of the introduced beneficial organism), and unrelated passenger organisms arriving with shipments of the introduced organism. Some adverse effects of all three types have been documented (Pimentel et al., 1984, Howarth, 1991), including local extinctions of nontarget species, especially in island situations. Although there is little documentation of notable adverse impacts of CBC programs in the United States, more precise prediction of benefits and risks would be desirable. Unfortunately, accurate prediction of both positive and negative impacts (target and nontarget effects) of CBC programs has not been achieved. The lack of predictive ability leaves CBC risk assessments in the realm of informed scientific judgment-based on limited published data.
In addition to requirements of various federal laws, guidelines have been developed to improve safety in CBC. Agricultural Research Service protocols (now under revision) require federal permits for importation and movement of organisms, quarantine, authoritative identifications, environmental and safety evaluations, documentation of movements and releases, and retention of voucher specimens. Current policy requires an environmental assessment (EA) to accompany applications for permits for field release of exotic organisms. Although the components of an EA depend on the specific situation, the documentation required is fairly extensive. At any step in the process, a proposed introduction can be deemed inappropriate and the project terminated.
(Led by J. T. Carlton, Williams College, and D. Policansky, National Research Council)
Classical biological control is only one kind of introduction of nonnative species. Others include range expansions (either natural or mediat-