Survival rates of tobacco budworm (Lepidoptera : Noctuidae) larvae exposedto transgenic cottons expressing insecticidal protein of Bacillus thuringiensis Berliner

The insecticidal activity of transgenic cottons expressing endotoxin protei n of Bacillus thuringiensis Berliner (Bt cotton) was quantified by measurin g survival of tobacco budworm, Heliothis virescens (F.), larvae caged on di fferent plant structures for varying lengths of exposure. Percentages of la rvae surviving were measured on St cottons expressing Cry1Ab and Cry1Ac pro tein. Plant structure (terminal, leaf, square or bell) did not affect larva l survival, and survival did not differ significantly between Cry1Ab and Cr y1Ac cottons. Larvae exposed to Bt cotton for only 24 h had higher initial survival than larvae exposed for 48, 72 and 96 h. Larvae first exposed to B t cotton at 4 d of age had higher survival than those first exposed as neon ate or 2-d-old larvae. Survivorship of neonate and 4-d-old larvae exposed t o Cry1Ac cotton was significantly reduced with only 48 h of exposure to the insecticidal plants. Seven-day-old larvae exhibited no significant reducti on in survivorship with exposure to Cry1Ac cotton for 48 h.