Helicoverpa zea (Lepidoptera: Noctuidae): Larval Location, Mortality, and Leaf Area Consumption on Bacillus thuringiensis-Treated Cotton

Abstract

A commercial formulation of Bacillus thuringiensis variety kurstaki Berliner (Javelin) was evaluated 2 d after application at 0.02.5,0.1, and 0..5 kg/ha to cotton leaves and terminals for its effect on location, mortality, and leaf area consumption of exposed bollworm, Helicocerpa zea (Boddie), larvae. On leaves held in paper cups the percentage of larvae in the control was typically higher on the lower than upper surface, and results were similar at all B. thuringiensis rates and times. Increasing the B. thuringiensis rate also significantly decreased leaf area consumption, which was correlated inversely with larval mortality. Application of B. thuringiensis to cotton terminals resulted in a decrease in the percentage of live larvae located on meristematic tissue 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 meristematic 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 the control. These results show that within hours of B. thuringiensis application to cotton terminals there is a significant shift in the location of H. zea larvae away from meristematic tissue and squares to expended leaves or off the terminal, or both. This suggests that adequate B. thuringiensis consumption causes larval movement to locations where larvae are less damaging.