American bollworm, Helicoverpa armigera (Hubner) was selected for eleven subsequent generations with purified toxin Cry1Ac Bacillus thuringiensis under laboratory condition. The resistance ratio (RR) reached 56.73 fold-resistances. The larval development of resistant, susceptible and their F₁ generation were significantly varied while reared on normal artificial diet. The growth rate of resistant strain was slower compared with susceptible and F₁ strains. The present study shows asynchronism effect for Cry1Ac resistant strain in relation with susceptible and F1 strains. The overlapping distribution between the adult peak of resistant and the adult peak of susceptible were investigated and found to be apart from each other. Subsequently, the probability of random mating among resistant individuals would be more likely to be happened that helps insect resistance development within the population. Thus, the current study would recommend reviewing the implementation of refuge strategy (non Bt cultivated area) while adopting Bt modified crops.