Ecological concerns of using biological control agents
In the more than 100 years of biological control, hundreds of species of exotic natural enemies have been imported, mass-reared and released as biological control agents, resulting in successful control of many species of pests and considerable reductions in pesticide use (e.g. Greathead, 1995; van Lenteren, 2000a; 2003; Gurr and Wratten, 2000). Negative environmental effects of these releases have rarely been reported (Hokkanen and Lynch, 1995; Follett et al., 2000; Louda et al., 2003; Lynch and Thomas, 2000; Lynch et al., 2001). An important difference between biological control and the use of chemical pesticides is that natural enemies are often self-perpetuating and self-dispersing, and, as a result, biological control is regularly irreversible, although this is not always the case in inundative types of biological control. Inundative biological control is the release of large numbers of mass-produced biological control agents to reduce a pest population without necessarily achieving continuing impact or establishment; classical biological control is the intentional introduction and permanent establishment of an exotic biological agent for long-term pest management. It is exactly the self-perpetuation, self-dispersal and irreversibility that is so highly valued in properly executed classical biological control programs: it makes them sustainable and highly economic compared with any other control method (Bellows and Fisher, 1999; van Lenteren, 2001). The current popularity of biological control may, however, result in problems, as an increasing number of activities will be executed by persons not trained in identification, evaluation, and release of biological control agents (Howarth, 2000).