Temporal Variation in Susceptibility of American Bollworm, Helicoverpa armigera to Bacillus thuringiensis (Bt) var. kurstaki HD-73, its Cry 1 Ac Toxin and Bt Cotton

The temporal susceptibility of neonates of the American bollworm, Hellcoverpa armigera to Bacillus thuringiensis (Bt) var. kurstaki HD-73 or its Cry1Ac toxin in the artificial diet assays was investigated for the populations of four different locations. The insect populations, collected post-Bt cotton introduction, showed less susceptibility to Cry1 Ac/HD-73 in the artificial diet assays. The larval susceptibiilty to Cry1Ac in the cotton leaf dip assays was higher than in the artificial diet assays, possibly due to the synergism of Cry1Ac with cotton leaf allelochemicals. The toxicity of Bt cotton leaves to neonates of H. armigera remained high during the course of studies from 2204–2000. The absence of resistance to Bt cotton in H. armigera is discussed in relation to Bt resistance traits and current management practices.