Combined Effect of Sublethal Exposure to Cotton Expressing the Endotoxin Protein of Bacillus thuringiensis and Natural Enemies on Survival of Bollworm (Lepidoptera: Noctuidae) Larvae
The interactions of natural enemies and transgenic cotton expressing the endotoxin of Bacillus thuringiensis Berliner variety kurstaki (BTK cotton) on bollworm, Helicoverpa zea (Boddie), survival were studied. Survival of bollworm larvae was measured on cotton not expressing endotoxin (non-BTK cotton) in field plots with 4 different densities of natural enemies. Larvae were conditioned on BTK or non-BTK cotton for 1, 2, or 4 d before infestation of the non-BTK cotton in the field plots. Environments with varying densities of natural enemies was a significant factor affecting bollworm survival at all postinfestations periods. Larval mortality increased with increased density of natural enemies. Tissue genotype on which bollworm larvae were predisposed significantly affected larval survival in environments with low, medium, and high density of natural enemies at 24, 96, and 48 h postinfestation, respectively. Larvae that were conditioned on BTK cotton tended to survive at lower rates than larvae conditioned on non-BTK cotton. Length of exposure was a significant factor affecting larval survival at all postinfestation time intervals in a cage environment with no natural enemies. In the absence of natural enemies, larval survival was correlated positively with length of exposure. Larvae that were conditioned for 4-d before field infestation survived at significantly higher rates than larvae that were conditioned for 1 and 2 d. Significant interactions between the effects of tissue genotype and length of exposure on larval survival were detected by analysis of variance in the environment with low densities of natural enemies at 48 and 96 h postinfestation. Significant linear regression equations that described larval survival as a function of densities of natural enemies were detected for larvae conditioned on BTK cotton for 1 d at 96 h and 4 d at 48 h postinfestation. Similar equations were found to be significant for larvae conditioned on non-BTK cotton for 2 d at 48 and 96 h postinfestation and for larvae conditioned for 4 d at all postinfestation periods.