The field efficacy of two transgenic rice lines, containing the Cry1Ac gene from Bacillus thuringiensis (Bt) and the CpTI (cowpea trypsin inhibitor) gene, on the striped stem borer, Chilo suppressalis (Walker), and population variation in susceptibility of C. suppressalis to the Bt insecticidal protein Cry1Ac were evaluated in Fuzhou, Fujian Province of China. The results of an open field test showed that the two transgenic lines exhibited high efficacy against natural infestations of C. suppressalis. In contrast, the untransformed parental control line showed serious damage symptoms not only in untreated plots, but also in plots treated with insecticides. The cumulative number of C. suppressalis adults derived from transgenic rice lines was significantly lower than that from control lines with and without chemical control treatments. Our results show that transgenic rice lines expressing Cry1Ac and CpTI have great potential for protecting rice from C. suppressalis damage with reduced use of insecticides. Comparison of pupal weight and fecundity of C. suppressalis between transgenic and control treatments indicated that pupal weight significantly decreased after continuously feeding on transgenic rice lines, but there was no significant effect on fecundity. Adult females of C. suppressalis were collected from light traps to ascertain population variation in susceptibility to Bt toxins. Each female producing enough fertile eggs for bioassays was used as the parent of an isofemale line. Of 1,100 females collected, 395 isofemale lines produced enough offspring to be screened on a diet assay treated with Cry1Ac. The results showed that 51.7% of isofemale lines were differentiated as highly susceptible, 23.0% as moderately susceptible, and 23.8% with low susceptibility; 1.5% were nonsusceptible, with a survival rate >70% and similar development as on the non-Bt control diet. The results indicated significant variation in susceptibility to Cry1Ac in the C. suppressalis population, which should be considered for insect resistance management of Bt rice in the future.
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