HELICOVERPA-ZEA (LEPIDOPTERA, NOCTUIDAE) - LARVAL LOCATION, MORTALITY, AND LEAF-AREA CONSUMPTION ON BACILLUS THURINGIENSIS-TREATED COTTON

A commercial formulation of Bacillus thuringiensis variety kurstaki Be rliner (Javelin) was evaluated 2 d after application at 0.025, 0.1, an d 0.5 kg/ha to cotton leaves and terminals for its effect on location, mortality, and leaf area consumption of exposed bollworm, Helicoverpa zea (Boddie), larvae. On leaves held in paper cups the percentage of larvae in the control was typically higher on the lower than upper sur face, and results were similar at all B. thuringiensis rates and times . Increasing the B. thuringiensis rate also significantly decreased le af area consumption, which was correlated inversely with larval mortal ity. Application of B. thuringiensis to cotton terminals resulted in a decrease in the percentage of live larvae located on meristematic tis sue and squares at all sampling times (3, 6, 12, 24, and 48 h) and an increase in the percentage on the holding cup surface and the terminal leaves, except at 48 h. The percentage of live larvae on the meristem atic tissues and squares at the 2 highest B. thuringiensis rates after 6 h had been reduced 36 and 50% and 73 and 64%, respectively, from th e control. These results show that within hours of B. thuringiensis ap plication to cotton terminals there is a significant shift in the loca tion of H. zea larvae away from meristematic tissue and squares to exp anded leaves or off the terminal or both. This suggests that adequate B. thuringiensis consumption causes larval movement to locations where larvae are less damaging.