and parasitoids has been on developing natural control agents that are resistant to pesticides so that they might function within a system dominated by broad-spectrum pesticides. In the future, both classical breeding techniques and genetic engineering could be used to increase the efficiency of biological-control agents in specific agricultural systems. Characteristics that could be altered include heat and cold tolerance, host range, and migratory cues.
Certain fungi trap and feed on plant-parasitic nematodes. These and fungi that parasitize other fungi, such as Trichoderma spp. and Gliocladium spp., are useful for biological control of soil-borne diseases in greenhouses (Lumsden et al., 1993). More can and should be done to determine the importance of parasitism in the biological-control activity of these fungi and to characterize the molecular basis of parasitism. As with other biological-control agents, more information is needed about their inherent genetic variability as well as information about which characteristics of the biological-control organism are important targets for strain improvement.
An important concern of the general public about the release of non-indigenous predators and parasitoids is their potential to harm indigenous nonpest species. To allay these concerns requires a better understanding of the genetics of host-range specificity, which will provide insights into how organisms can be genetically altered to limit their current and potential host range and thus enhance their safety as control organisms.
Some progress has been made in understanding the genetic basis of host selection by microbial plant pathogens—an important step toward understanding the molecular basis of host specialization by plant pathogens. In fungal, bacterial, and viral plant pathogens, genes that determine the spectrum of plant cultivars that pathogens can infect have been identified. Staskawicz and colleagues (1984) changed the host specificity of a selected pathogen by deleting or inactivating a single gene. The host range of a plant pathogen was narrowed genetically by Mellano and Cooksey (1988).
The genetic basis for host selection and host specificity in arthropod predators and parasitoids is less understood than it is for plant pathogens, but progress is being made in this area. It is known, for instance, that host specificity of an arthropod parasitoid is determined by a virus that the parasitoid transmits to its host when it inserts one of its eggs into the host. It is also known that within a given host range, arthropod parasitoids can adjust their host preferences by learning cues that lead to a suitable host. Although predators with a broad host range have preferences for specific prey, the genetic factors that govern preference are not understood. Research in the area of host selection could have long-term benefits in improving the efficiency and safety of these biological-control organisms.