Baseline susceptibility of the American bollworm, Helicoverpa armigera (Hübner) to Bacillus thuringiensis Berl. var. kurstaki and its endotoxins in India.

Abstract

Baseline susceptibility of larvae of the American bollworm, Helicoverpa armigera (Hiibner) to Bacillus thuringiensis Berl var. kurstaki was studied by a diet incorporation method. Ninety-six hour median lethal concentrations (LC 5 0 ) of Bt var. kurstaki strains and parasporal crystal toxins varied widely for neonate larvae of different populations. Insect populations from nine locations in India showed differences in their susceptibility to Bt var. kurstaki strains and individual Cry toxins, viz. CrylAa 10.5, CrylAb 12.8, CrylAc 16.2, HD-114.1 and HD-73 5.7-fold. Insect populations obtained from pigeon pea crops at Navsari from December 2000 to January 2001, and at Delhi from October 1998 to November 2000 showed temporal variation in their susceptibility to Bt var. kurstaki HD-1 and HD-73. Temporal variation in insect susceptibility was correlated with temperature at these two locations. Insect acclimation to pre-treatment temperature influenced the susceptibility of the F 1 generation to Bt var. kurstaki. An increase in ambient temperature (about 10°C) increased the susceptibility to Bt var. kurstaki HD-73 by 7.5-fold. The role of selection pressure, host-plant, xenobiotic and other agroecological conditions on the susceptibility of H. armigera is discussed in relation to development of tolerance/resistance and integrated pest management.