RESISTANCE - A THREAT TO THE INSECTICIDAL CRYSTAL PROTEINS OF BACILLUS-THURINGIENSIS

Insecticidal crystal proteins (also known as delta-endotoxins) synthes ized by the bacterium Bacillus thuringiensis Berliner (Bt) are the act ive ingredient of various environmentally friendly insecticides that a re 1) highly compatible with natural enemies and other nontarget organ isms due to narrow host specificity, 2) harmless to vertebrates, 3) bi odegradable in the environment, and 3) highly amenable to genetic engi neering. The use of transgenic plants expressing Bt delta-endotoxins h as the potential to greatly reduce the environmental and health costs associated with the use of conventional insecticides. The complex mode of action of Bt is the subject of intensive research. When eaten by a susceptible insect delta-endotoxin crystals are solubilized in the mi dgut; proteases then cleave protoxin molecules into activated toxin wh ich binds to receptors on the midgut brush border membrane. Part of th e toxin molecule inserts into the membrane causing the midgut cells to leak, swell, and lyse; death results from bacterial septicemia. Insec ticides formulated with Bt account for less than 1% of the total insec ticides used each year worldwide because of high cost, narrow host ran ge, and comparatively low efficacy. Environmental contamination, food safety concerns, and pest resistance to conventional insecticides have caused a steady increase in demand for Bt-based insecticides. The rec ent escalation of commercial interest in Bt has resulted in more persi stent and efficacious formulations. For example, improved Bt-based ins ecticides have allowed management of the diamondback moth, Plutella xy lostella (L.). Unfortunately this has resulted in the evolution of res istance to delta-endotoxins in P. xylostella populations worldwide. Th e recent appearance of Bt resistance in the field, corroborated by the results of laboratory selection experiments, demonstrates genetically -based resistance in several species of Lepidoptera, Diptera, and Cole optera. The genetic capacity to evolve resistance to these toxins is p robably present in all insects, and the heritability, fitness costs, a nd stability of the resistance trait are documented in several insect populations. In two strains of Bt-resistant lepidopteran species, mech anisms of resistance involve reductions in the binding of toxin to mid gut receptors. Research on other resistant strains suggests that other mechanisms are also involved. Unfortunately, the high stability of th e resistance trait, as well as broad spectrum cross-resistance to othe r delta-endotoxins, undermines many potential options for resistance m anagement. Genetically engineered plants, expressing delta-endotoxin c ontinuously and at ultrahigh doses, ensure intense and rapid selection of the target insect population. The efficacy of transgenic plants ca n be preserved only by developing an integrated pest management progra m that is designed specifically to reduce selection pressure by minimi zing exposure to Bt and increasing other mortality factors, thereby sl owing the rate of pest adaptation to Bt.