The introduction of exotic natural enemies to control exotic pests is the primary approach in classical biological control (DeBach, 1964). Exotic pests, usually inadvertently introduced and without their natural enemies, commonly result in rapid increases and population outbreaks. The lack of natural enemies provides the impetus for purposeful introductions of potentially promising natural enemies. Such enemies are usually sought in the homeland of the target pest, but sometimes are obtained from areas outside the original home of the pest, where secondary adaptation by indigenous species has occurred. Consideration of world populations broadens the scope for selection of useful species and biotypes, and increases the prospect for success (Stary, 1970a; Huffaker et al., 1971; Greathead, 1986; Ehler, 1990; 1992).
New beneficial introductions add to the biodiversity of pest-infested areas, but also they can impact the indigenous biota and, in some instances, stand as biotic contaminants (Samways, 1993) in terms of host displacement and nontarget prey interspecies competition. Ideally, environmental impact determinations or projections on prey specificity should be studied prior to a release (Stary, 1993). With the exception of strict monophagous agents, some adaptation to indigenous biota is expected. Endemic ecosystems may be disrupted or partially disrupted due to various reasons and thus are more easily attacked by exotic invaders (Stary, 1994). An introduced biocontrol agent may not be necessarily harmful to indigenous communities where the spectrum of natural enemies includes numerous broadly oligophagous species (Stary et al., 1988) or where invasive species are suppressed (Samways, 1993).
New pests in an area can be attacked and suppressed to varying degrees by indigenous natural enemies. Here, there is no increase in species richness of natural enemies, simply expanded adaptation and enlargement of the prey/host spectrum. However, where new strains or biotypes occur, differences in intraspecific competition may result.
Once introduced species become established, they become part of the natural enemy spectrum for the target pest and associated area, and thus subject to all of the management approaches applied, such as conservation, augmentation, etc. New agents, however, need to be classified as generally more vulnerable to environmental modifications depending upon the degree of adaptation in their new environment.
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