valid, other components such as foraging behavior also contribute to these ecological interactions (National Research Council, 1986b). Current knowledge of host-prey relationships is useful as a starting point in assessing risk of nontarget effects on macroorganisms. However, the continuing contributions of ecologists, biologists, entomologists, weed scientists, botanists, zoologists, and other scientists will be necessary for a comprehensive evaluation of these complex ecological systems.
Because of the enormous diversity of microbial pathogens that might be used as pest controls, there is incomplete understanding of their potential adverse effects on nonpathogenic microorganisms in natural systems. Because microorganisms occupy numerous soil or plant habitats, laboratory experiments can provide data only on selected populations, and evaluation of nontarget effects from these studies may not be applicable to agricultural ecosystems (Hollander, 1991; Tiedje et al., 1989). Though the species composition of any particular microbial ecosystem may not be known, there is considerable knowledge about some of the functional roles and mechanisms of microbes found there; this information can be used to identify potential effects on nontarget species (Cook, 1991; National Research Council, 1989a; Tiedje et al., 1989).
Certain microorganisms, such as B. thuringiensis, suppress plant pests by producing toxins or antibiotics, and risk evaluation of these biological-control organisms is focused on the persistence of the toxin and its effects on nontarget species. Experience has demonstrated a basis for concern about potential effects of -endotoxin (Bt) on nontarget arthropods within the class against which the biological-control product is active, particularly in the case of certain endangered species of butterflies. In these cases, the habitats of concern have been identified and product use has been prohibited there (Hutton, 1992). This approach is especially appropriate when a gene encoding a toxin is introduced into an organism that occupies a habitat where the toxin would not otherwise be found. The encapsulated Bt protein toxin is nontoxic to vertebrates and has been demonstrated to be of minimal risk to other nontarget organisms (Panetta, 1993); it has established an excellent precedent, but similar benign properties must be established for other microbial toxins that may be useful in EBPM. Molecular techniques can provide scientists with the tools to make precise alterations in toxin-encoding genes, thereby improving product performance and increasing the ability to focus the effect of the control on the target organism.
The long history of plant breeding suggests that resistant plant cultivars rarely cause significant effects on nontarget organisms. Plant resistance factors range from chemical to physical and can be specific to a single pest species or have broad effects on an array of organisms as different as arthropods and pathogens.
The potential risks to nontarget organisms resulting from new or elevated levels of toxins in resistant plant cultivars are not well known. If a cultivar resistance is toxicity based, then there is potential for native herbivores such as caterpillars or honey bees to be harmed, though these occurrences are not well