Hypotheses regarding the innate ability of natural enemies to parasitize or prey on nontarget species can be tested under microcosmic conditions in a laboratory or greenhouse. But the physiological host range revealed by such experiments may not represent the true (ecological) host range of the organism (Olckers et al., 1995; Watson, 1985). Single-choice experiments (one organism with one host) are poorly discriminatory and are likely to give false-positive results. Nonetheless, they are useful in eliminating concern about potential nontarget hosts. Multiple-choice experiments (one organism with two or more different hosts) reduce the chances of false-positives; nevertheless, the ecological host range expressed under field conditions is likely to be much more narrow than the physiological host range characterized by laboratory choice tests (Ridings et al., 1978; TeBeest, 1988; Watson, 1985). Any prediction of the ecological host range must consider both the experimentally determined physiological host range and the biological, ecological, and taxonomic host-range of any closely related organisms.
The prediction of ecological host range is particularly difficult with self-perpetuating organisms that function at the tertiary trophic level, such as predatory and parasitic arthropods. Although it can be determined in laboratory conditions whether a predatory or parasitic arthropod will utilize a range of hosts, such data cannot be freely extrapolated to field conditions. Complex spatial, temporal, and behavioral interactions commonly displayed by self-perpetuating macroorganisms ultimately determine whether an organism has even the opportunity to encounter a given host.
After release of the organisms into the field, appropriate monitoring will be necessary to check the reliability of the physiological host-range data. Monitoring over several years will identify direct and indirect effects on natural communities. Every introduction provides the opportunity to validate and/or modify protocols for estimating ecological specificity from microcosm studies. Climate, geography, floral phenology, and trophic interactions are key aspects that combine to define the relationship of an organism with coexisting organisms. Thus, ecological host range can be estimated with some accuracy, combining physiological host-range data with ecological evidence from other geographical areas where a similar system (ecological analogy) is in place, particularly if closely related biological-control organisms have previously been monitored there after release (National Audubon Society, 1994).
In general terms, a threshold level of a pest is required to sustain reproduction of a predatory or parasitic biological-control organism; and if high numbers of the biological-control organism suppress the capacity of the pest to reproduce, then negative feedback adversely affects the survival of the biological-control organism (National Audubon Society, 1994). As pest population levels decline, there is a decreased likelihood of contact between the biological-control organism and the pest, resulting in reduced reproduction of the biological-control organism. Although this traditional theory of predator-prey relationship remains