Relative Toxicity of Subspecies of bacillus Thuringiensis against Lepidopterous Insect Pests of Agricultural Importance

The relative toxicity of various subspecies of Bacillus thuringiensis (Bt) against important lepidopterous insect pests was determined. Bt kurstaki HD-1 was toxic against all the five insect pests tested; however, with low toxicity against Spodoptera litura . Larvae of Helicoverpa armigera and Pieris brassicae were five fold more susceptible to Bt kurstaki HD-73 that produces only Cry1Ac toxin. However, Bt kurstaki HD-73 was non-toxic against the larvae of S. litura and Spilarctia obliqua . Bt aizawai HD-137 was equally active against S. litura and Sesamia inferens but non toxic against P. brassicae and S. obliqua . On the other hand Bt tolworthi strain was highly toxic against P. brassicae and S. obliqua . Bt japonensis T23 001was non-toxic to all the insect pests tested. Larval growth of H. armigera was severely inhibitied at extremely low concentration of Bt kurstaki HD-73 (EC50 0.07 ng/ml) and Bt kurstaki HD-1 (EC50 0.16 ng/ml). The toxicity of indigenous Bt strains belonged to subspecies tolworthi and galleriae were also promising against H. armigera .