Abstract: The temporal-spatial variation in efficacy of transgenic Bt cotton leaves from different nodes of the main stem and in different growth stages against neonate larvae of the cotton bollworm, Helicoverpa armigera (Hübner) were determined using the leaf-feeding method. The effects of leaves from both lateral branches of cotton in the field and seedlings in laboratory grown under different weather conditions on the neonates of the cotton bollworm were examined. Results show that efficacy of the leaves from the 2nd to 10th nodes on the main stem of Bt cotton R₁₉-137 was the highest, killing 97.0%-100% of larvae after 5 d feeding with 1.0-1.1 grades of leaf damage; the efficacy of the leaves from the 11th to 16th nodes decreased significantly with mortalities of 35.6%-67.6%, and survivors mainly in the 2nd instar. When the leaves from lateral branches of cotton in late July and late August were tested, mortalities of 30.9%-44.9% and 10.0%-30.0% occurred, respectively, indicating further decline of the control efficacy. The experiments also show significant influence of weather conditions, under which the Bt cotton grew, on the control efficacy of Its leaves. The possible reasons for temporal-spatial variations in efficacy of Bt cotton leaves against the cotton bollworm and the importance of weather conditions in Bt cotton application were discussed.

Key words: Transgenic Bt cotton, Helicoverpa armigera, leaf efficacy, temporalspatial variation, weather factors