insects will “hitch-hike” to fixation. Strategies have been developed that involve use of strains with Wolbachia and a virus that carries a gene for disease refractoriness (Sinkins et al. 1997). Other strategies involve isolation of the Wolbachia genes that code for cytoplasmic incompatibility; these genes are used to transform insect strains directly (Sinkins et al. 1997).
In addition to its use to develop novel control strategies based on transposons and microorganism-based incompatibility, genetic engineering could be used to improve the efficiency of classical genetic control strategies, such as introduction of conditional lethal genes or sex-ratio distortion genes (Gould and Schliekleman, in prep.). Researchers using classical genetic techniques have been severely limited in their ability to find genes that confer conditional lethality or sex-ratio distortion. Each time a project is developed for a specific pest, a new search must be undertaken to find useful genes or translocations. With the tools of genetic engineering, promoters and useful genes could be found that would be effective in many species. Once transformation of diverse insect species becomes more routine, it will be possible to transform a variety of species with appropriate genes.
Whether genetic engineering can be used for the direct control of pest species is an open question. In many cases, it seems easier to engineer crop plants for insect and disease resistance than it is to engineer the pest or vector. Most genetic control strategies to be used on pest species (such as the insertion of dominant lethal genes) work only for obligately sexual, outcrossing species (Atkinson and O'Brachta 1999, Thomas et al. 2000). Because many weeds are self-fertilized, they might not be amenable to this approach, and other weeds that currently outcross could become resistant to genetic control by increasing their frequency of inbreeding. Many pathogens and insect pests are parthenogenetic; these species would not be candidates for the new genetic control strategies.
Genetic control strategies are exceptionally specific in their action, so they are likely to be environmentally friendly. Once a genetic control strategy is put into place, it can maintain itself and not require continued expenditure of funds. Many obligately outcrossing pests (such as weeds and household and veterinary pests) could be difficult to control through plant engineering. There is a need to assess carefully what the niche might be for direct genetic control techniques. The niche and the environmental benefits might be larger if substantial funding is used immediately to support research. If research support is insufficient or too late, the niche for this environmentally benign control technology could become occupied by other more expensive and less benign techniques. Public concern about the use of genetically engineered insects could, however, overshadow any of the potential environmental benefits of this technology and could block its implementation.