1997b). The new pyrrole insecticides originated in dioxypyrrolomycin, a natural product derived from an extract of Streptomyces spp. (Black et al. 1994).

Hypovirulence

Another potential use of microorganisms to protect plants is through hypovirulence—infection of a plant with benign strains that protect hosts from later infection by virulent strains. Hypovirulence is caused by a double-stranded RNA virus that infects the fungus, and reduces its virulence relative to the crop plant. The hypovirulent state can be induced through infection with viruses and related materials. Hypovirulence has achieved its widest success in combating chestnut blight, a disease that radically altered the eastern forest landscape after introduction of an invasive fungus (NRC 1996).

Transgenic Crops

Choice of crop variety has always been a cornerstone of crop protection, especially for disease and insect control. There are numerous review articles on plant resistance to insects (Harris 1980, Kogan 1982, Hedin 1983). The sources of resistance to pests in crops have been classified as nonpreferred, antibiosis, and tolerance (Pfadt 1971). Insect preference for a host plant is related to physical structure of the surface (hairs, wax, and so on), color, taste, odor, and light reflection. A resistant variety might be nonpreferred by virtue of lacking one or more of the preferred factors or characteristics of the host plant. Antibiosis is the adverse effect of the plant on the pest, which can be due to a deleterious chemical or the lack of specific nutrients in the plant. Tolerance is the ability of certain plants to withstand pest attack by virtue of general vigor or ability to repair tissue damage caused by a pest attack.

Plant resistance as a pest-management factor has achieved some outstanding results (for example, against grape phylloxera, woolly aphid, Hessian fly, and wheat stem sawfly) (Kogan 1982). Desirable features of pest-resistant plants are specificity, cumulative effectiveness (with effect on the pest compounded in successive generations), and persistence and harmony with the environment, ease of adoption, and compatibility with other IPM tactics (Kogan 1982). Disadvantages of using pest-resistant plants for pest management include the long development time (3-15 years), genetic limitations (due to lack of available resistance genes), and evolution of pests that have overcome and are no longer controlled by the bred-in resistance. To overcome some of the problems associated with traditional resistance-breeding approaches, transgenic technologies have



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