Abstract: The efficient and stable expression of exogenous insecticidal genes in transgenic cotton is the key of transgenic cotton application. In order to compare the insect-resistant effect of transgenic cotton under different seeds purity, two transgenic varieties GK12 and SGK321 were mixed with non-genetically modified cotton seeds in eight different proportion for evaluating insecticidal protein expression level, the molecular detection and indoor hygiene tests showed that with the decline in the purity of insect resistance, cotton bollworm damage level gradually increased, corrected mortality of H. armigera larvae decreased, varieties of insect-resistant capacity declined. The field experiments indicated that with the decline of the insect resistant purity, boll number reduced, the number of empty fruit branches increased, which ultimately led to a sharp decline in production, and thus gave rise to the so-called “Bt cotton is not insect resistant”. In conclusion, the decline of seed purity had significant effection on insect resistance and cotton yield. In order to ensure high yield, the purity of transgenic insect-resistant cotton should be higher than 97%.

Key words: insect resistance, yield, seed purity, transgenic cotton