Pesticide Research Journal

Differential responses of larval instars of Helicoverpa armigera tt Bacillus thuringiensis (Bt) insecticidal crystal proteins (ICPs) and suitability of bioassay method to study the toxicity were evaluated. Neonates of test insects were highly susceptible to Bt ICPs than second and third instars. The seventh day median lethal concentrations (LC₅₀) of Bt ICPs viz, Cry 1 Aa, Cry1 Ab and Cry1 Ac to the neneates were 225.2, 346.8 and 101.7 ng cm⁻² and 0.34, 0.69 and 0.24 μl ml⁻¹, respectively, for surface contamination method and diet incorporation method. The LC₅₀ of Cry1Aa, Cry1Ab and Cry1Ac were 203.9, 296.2 and 133.5 ng cm⁻² and 267.4, 456.6 and 160.5 ng cm⁻², respectively, to second instar and third instar larvae by surface contamination method. The order of susceptibility of different instars were neonates > second instar > third instar.