Antagonism between Formulations of Different Bacillus thuringiensis Subspecies in Heliothis virescens and Helicoverpa zea (Lepidoptera: Noctuidae)

Interactions between formulations of the aizawai and kurstaki subspecies of Bacillus thuringiensis Berliner were evaluated by bioassay in Heliothis virescens (F.) and Helicoverpa zea (Boddie). In preliminary experiments, a formulation of subspecies aizawai, Xentari AS^®, had significantly (P < 0.05) higher median lethal concentrations (LC₅₀s) in both insect species than formulations based on subspecies kurstaki. Helicoverpa zea was significantly (P < 0.05) more susceptible than H. virescens to one formulation of subspecies kurstaki (Dipel ES^®), but the two insects did not differ in susceptibility to Xentari AS^® or to a second formulation of subspecies kurstaki (Dipel 6AF^®). In H. virescens, Xentari AS^® was additive with Dipel 6AF^® and significantly (P< 0.05) antagonistic with Dipel ES^® and with a third formulation of subspecies kurstaki, Dipel 48A^®. In H. zea, Xentari AS^® was significantly antagonistic with all three formulations of subspecies kurstaki. This suggests that certain toxin combinations from B. thuringiensis subspecies might not be effective for managing H. virescens and H. zea populations.