Classical biological control efforts are seldom instituted until a pest outbreak occurs, and, even then, there can be a delay because of the time involved to search for, import, quarantine, mass-rear, and release new agents. Although not generally followed, another approach is preventative biocontrol, the introduction of promising exotic biocontrol agents prior to the appearance of a forecasted or expected target pest. Except for strict monophages, oligophagous agents may be introduced for establishment on alternate prey/host species in association with the target agroeco-system (Stary et al., 1993). These then are present to attack the arriving pest, possibly before it is detected by humans. The approach provides a temporal advantage, and more or less limits population outbreak of the invading pest. Switching from a native host to a related introduced species can occur with striking results, and may include the indigenous beneficials (LaSalle, 1993). Understanding the host range of introduced exotics is key to achieving success in preventative biocontrol (Stary et al., 1993).
The host range of introduced beneficials should always be considered, i.e., control of more than one pest using the same regulatory agent. This requires an oligophagous agent. In principle, oligophagous agents can attack several pests in the same or different crops (Stary et al., 1993). Such situations contribute to the stability of the natural enemy interactions across cropping systems. However, such introductions are not without risks, e.g., host preference, poor alternation, or species-specific strains may eventually develop. The strategies, necessary attributes, and opinions for introducing biocontrol agents are widely discussed in the literature (DeBach, 1964; Stary, 1970a; Huffaker et al., 1971; van den Bosch and Messenger, 1973; Clausen, 1978; Croft, 1990; Ehler, 1990; Miller, 1993; Nechols et al., 1995).
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