Parasitic and predatory insects occur within a wide range of insect groups, and at times, can be relatively abundant. Some common representatives include predatory carabid, coccinellid, and staphylinid beetles; predatory bugs; lacewings; syrphid, chamaemyiid, and other predatory flies; ants; and parasitic wasps. Related to the beneficial insects are predatory mites and spiders. These different beneficials prey on and reduce phytophagous pest populations and, thus, promote higher standards of crop health and economic returns. They can be highly effective at little or no cost, serving as biotic insecticides in place of chemicals and providing long-term control without the target pests developing significant resistance to them, and with minimal or no harm to humans or the environment (Wilson and Huffaker, 1976).
The value or full value of the insect natural enemies is not always realized because the preferred agent(s) for the target pest(s) are not present, or their abundance or activity is limited by environmental factors, in particular, by human-implemented practices such as clean cultivation, pesticide application, etc. (Johnson and Wilson, 1995). Natural enemies do not act in isolation but within the framework of natural enemy communities comprising individual guilds (Ehler, 1994). These communities manifest definable structure in which species richness and host range are fundamental properties (Hawkins and Sheehan, 1994). Individual members often show marked differences in their utilization of successive life stages of their hosts (Mills, 1994) and manifest certain positions (trophic levels) in the feeding hierarchy (Powell et al., 1996). Also, more or less competitive interactions may occur among species participants (Rosenheim et al., 1995).
The significance of beneficials in agroecosystems is often taken for granted or overlooked, sometimes when they are most effective. At times their significance becomes apparent in their absence or when they have been reduced to ineffective levels allowing the pest to reach crop-injuring levels (Ridgway and Vinson, 1977). The value is also apparent when exotic pest species in a new area rapidly reach pest status, but later are suppressed by adapting indigenous natural enemies or newly released beneficials or both (Clausen, 1978; Nechols et al., 1995).
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