Development and mechanisms of resistance to <i>Bacillus thuringiensis </i>endotoxin Cry 1 Ac in the American bollworm, <i>Helicoverpa armigera </i>(Hübner)

Abstract

The American bollworm, <i>H. armigera, </i>evolved 31-fold resistance to selection pressure of <i>B. thuringiensis </i>endotoxin Cry 1 Ac within six generations. The Cry 1 Ac selected larvae of <i>H. </i>armigera showed cross-resistance to Cry1Aa and Cry1Ab both in terms of mortality and growth reduction. Studies on mechanisms of resistance to Cry1Ac showed that proteases of resistant insects degraded Cry1Ac faster than those of susceptible insects, which led to the relative unavailability of toxin of about 58 kDa for binding and perforation of midgut epithelial membrane of the target insect. Besides, resistant and susceptible populations of <i>H. armigera </i>differed in the binding of their receptors with Cry1Ac toxin. These results suggest the possibility of both mechanisms existing in imparting resistance. These findings mandate the necessity of <i>B.thuringiensis</i> resistance management for usage of <i>B. thuringiensis </i>either as a conventional insecticide or through transgenic crops.