Abstract:
Comparative response of five Bt-cotton cultivars (FH-142, FH Lalazar, VH-259, CIM-602, BH-178) and one non-Bt (CIM608) cultivar on mortality of the 1st, 2nd and 3rd instars Helicoverpa armigera (Hubner) was evaluated on different plant parts of Bt cultivars and synthetic diet incorporation bioassays. The 1st instar H. armigera showed complete (100%) mortality after feeding on leaves, squares, flowers and bolls of Bt cotton cultivars; the 2nd instar showed 100% mortality after feeding on only leaves of Bt cultivars (FH-142 and CIM-602), whereas the 3rd instar indicated comparatively less mortality (80-95%) on Bt cultivars during the two years. Overall, the highest mortality was observed on leaves compared to squares, flowers and bolls, and relatively less mortality was observed in 3rd instar compared to the 1st and 2nd instars after feeding on plant parts. The toxicity of Cry1Ac was evaluated using diet incorporation method. The lethal concentrations of Cry1Ac against 1st, 2nd and 3rd instars of H. armigera were 0.34, 0.77 and 1.37 µg mL-1 in diet incorporation method.