Nontarget Impacts Of Parasitoids And Predators

The past two decades have brought increasing concerns and discussion over the impact of invasive, exotic organisms on native flora and fauna throughout the world (U.S. Congress OTA, 1993). Parasitoids and predators used for biological control have been included in these concerns and discussions. Therefore, we present a brief summary of the current status of studies and debate regarding potential nontarget impact of parasitoids and predators.

The use of genetically altered organisms in pest management programs has always drawn intense debate regarding safety and unintended nontarget impacts (e.g., de Selincourt, 1994; Gould, 1994; Barrett et al., 1996; Bengtsson and Ort, 1997; Hoy et al., 1998). In contrast, biological control employing naturally occurring organisms has historically been considered an environmentally safe and effective means of managing insect pests (Doutt, 1972; DeBach, 1974; Caltagirone, 1981). Recently, however, the potential impact of naturally occurring parasitoids and predators on nontarget organisms has also come under scrutiny from a variety of sources (Howarth, 1983, 1991; Simberloff, 1992; Lockwood, 1993 a,b; Simberloff and Stiling, 1996, 1998; Lockwood, 1997).

These concerns have prompted a much-needed discussion regarding the potential for nontarget effects and approaches to addressing the issue (Carruthers and Onsager, 1993; Simberloff and Stiling, 1996, 1998; Duan and Messing, 1997; Van Driesche and Hoddle, 1997; Follett, 1999; Frank, 1998). Most of the discussion has focused on the risks posed by the importation and release of exotic natural enemies against exotic pests, i.e., classical biological control (see papers in Follett, 1999). Far less attention has been paid to potential for nontarget effects from augmentative releases of natural enemies (Orr et al., 1999), and conservation biological control has apparently not raised any issues related to nontarget impacts.

Simberloff and Stiling (1996) summarize the controversy associated with importation biological control and highlight potential risks such as predation or parasitism of nontarget species, competition with native species, community and ecosystem effects, and unexpected effects such as loss of species dependent on the target species of biological control efforts. Simberloff and Stiling (1996) argue that the few documented cases of nontarget impacts, compared with the number of natural enemy introductions, may be more the result of a lack of monitoring and documentation than a lack of actual impacts. These authors also suggest that current regulations and protocols are inadequate, and should do more to assess potential effects on noneconomic species and ecosystems, as well as compare the effects of target pests with the potential nontarget impacts of natural enemies prior to release.

Despite these concerns, the management of exotic pests with importation biological control is being advocated by some as a tool to assist conservation of natural areas (Frank and Thomas, 1994; U.S. Congress OTA, 1995; Van Driesche, 1994). The invasion of alien species appears likely to continue, and perhaps worsen, as a result of international trade and travel (Sailer, 1978; Frank and McCoy, 1992). For many of these alien species that develop into pest problems, importation biological control practiced in a scientifically sound manner may be the only economically viable, long-term solution.

Orr et al. (1999) discuss the potential for nontarget impacts of augmentative releases of arthropod natural enemies, especially Trichogramma species. In general, it appears that the measured potential for nontarget impacts from augmentative releases is strongly a function of how measurements are taken. As studies progress from simple no-choice tests in laboratories to more biologically realistic cage and field trials, the observed potential impact of parasitoids on nontarget organisms typically declines (e.g., Duan and Messing, 1997; Orr et al., 1999). This suggests that, at least in the cases studied, the actual nontarget impacts of augmentative releases in field conditions are probably negligible. Risks from augmented arthropods would primarily be of concern if released organisms were non-native. However, with native organisms there might also be potential for subtler impacts from "genetic pollution," i.e., one race, strain, biotype, ecotype, etc. being introduced into an area it previously did not occupy. The biological soundness of mixing and resulting hybridization of different populations as a result of movement and release has been questioned (e.g., Pinto et al., 1992).

The controversy over potential nontarget impacts of biological control is far from resolved. It has, however, prompted some biological control researchers to collect more data relevant to potential nontarget impacts as part of their work plans. Sufficient data collection of this type should allow resolution of these conflicts, at least on a case-by-case basis, and ensure that parasitoids and predators can continue to contribute to insect pest management in as safe a manner as possible.

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