Abstract: Temporal and spatial insecticidal activities of three different transgenic cotton cultivars were evaluated: NuCOTN 33B cultivar expressing Cry1Ac insecticidal protein gene from Bacillus thuringiensis (Bt), developed by the Monsanto Company; GK12 expressing modified Cry1A Bt gene, developed in China; and SGK321 expressing both Bt and cowpea trypsin inhibitor (CpTI) genes, also developed in China. Two strains of cotton bollworm (CBW), Helicoverpa armigera (Hübner) with different susceptibility to Bt were used. Insecticidal activity in all three cultivars declined through time as plants grew,leaves had the highest insecticidal activity early in the growing season, but bolls and square showed higher insecticidal activity in the middle or late growing season. All three cultivars showed higher insecticidal effect against the susceptible strain of CBW than against the resistant strain of CBW. SGK321 had significantly higher insecticidal activity to CBW in the middle and late growing season (August and September), and also displayed more consistent resistance to the resistant strain of CBW compared to the other two cultivars.

Key words: Helicoverpa armigera, transgenic insect-resistant cotton, insecticidal activity, resistance