DEVELOPMENT OF BACILLUS-THURINGIENSIS CRYIC RESISTANCE BY SPODOPTERA-EXIGUA (HUBNER) (LEPIDOPTERA, NOCTUIDAE)

Selection of resistance in Spodoptera exigua (Hubner) to an HD-1 spore -crystal mixture, CryIC (HD-133) inclusion bodies, and trypsinized tox in from Bacillus thuringiensis' subsp, aizawai and B. thuringiensis su bsp. entomocidus was attempted by using laboratory bioassays. No resis tance to the HD-1 spore-crystal mixture could be achieved after 20 gen erations of selection. Significant levels of resistance (11-fold) to C ryIC inclusion bodies expressed in Escherichia coil were observed afte r seven generations, Subsequent selection of the CryIC-resistant popul ation with trypsinized CryIC toxin resulted, after 21 generations of C ryIC selection, in a population of S. exigua that exhibited only 8% mo rtality at the highest toxin concentration tested (320 mu g/g), wherea s the 50% lethal concentration was 4.30 mu g/g for the susceptible col ony. Insects resistant to CryIC toxin from HD-133 also were resistant to trypsinized CryIA(b), CryIC from B. thuringiensis subsp. entomocidu s, CryIE-CryIC fusion protein (G27), CryIH, and CryIIA. In vitro bindi ng experiments with brush border membrane vesicles showed a twofold de crease in maximum CryIC binding, a fivefold difference in K-d, and no difference in the concentration of binding sites for the CryIC-resista nt insects compared with those for the susceptible insects, Resistance to CryIC was significantly reduced by the addition of HD-1 spores, Re sistance to the CryIC toxin was still observed 12 generations after Cr yIC selection was removed. These results suggest that, in S. exigua, r esistance to a single protein is more likely to occur than resistance to spore crystal mixtures and that once resistance occurs, insects wil l be resistant to many other Cry proteins, These results have importan t implications for devising S. exigua resistance management strategies in the field.