Laboratory assays showed that dual toxin B. thuringiensis (Bt) cotton cultivars expressing both Cry1Ac and Cry2Ab endotoxins were more toxic to S. litura as compared to single toxin cultivars expressing Cry1Ac and non GM cotton.  First instar larvae that survived beyond 72h, when reared continuously on the leaves of dual toxin expressing Bt cotton cultivars resulted in death of the larvae subsequently and none of the larvae survived beyond five days.  When reared on squares the surviving first instar larvae continued to suffer from metamorphic aberrations even after four days, while those on single toxin Bt cultivars and non Bt cottons moulted to higher instars on two to three days of experimentation.  In case of second, third fourth and fifth instars feeding on leaves and squares of dual toxin Bt cotton cultivars, though survival was noticed, the final weight of each instar was reduced and developed into smaller pupae when compared to the single toxin Bt and normal cotton cultivars.  The growth and survival indices of III instar were significantly low when the larvae were fed with dual toxin Bt cotton cultivars as compared to single toxin Bt cultivars and non Bt cotton.