Spore-Toxin Interactions and Sublethal Effects of Bacillus thuringiensis in Spodoptera frugiperda and Pseudoplusia includens (Lepidoptera: Noctuidae)

Interactions between the spores and δ-endotoxins of Bacillus thuringiensis Berliner were tested in a low-susceptibility insect, Spodoptera frugiperda (J. E. Smith). The spores (formulation MYD^™) and one δ-endotoxin (formulation MVP^™) had additive effects on mortality at a MVP^™ dose of 117,500 μg/g of diet (P < 0.01) and synergistic effects at a MVP^™ dose of 235,000 μg/g diet (P < 0.01). The spores and another δ-endotoxin (formulation MYX^™) were antagonistic at a MYX^™ dose of 117,500 μg/g diet (P < 0.01) and additive at a MYX^™ dose of 235,000 μg/g diet (P < 0.01). The two δ-endotoxin formulations were additive with one another (P < 0.01). Sublethal concentrations of MVP^™ fed to larvae retarded the development of larvae and pupae for 5 d (P < 0.01) and 1.2 d (P < 0.01), respectively, and decreased pupal weight by 48 mg (P < 0.01). The spore formulation did not affect (P > 0.05) pupal weight or the life span of larvae or pupae. Median lethal concentrations of MVP^™ and MYX^™ were 6,904 and 7,561 × greater, respectively, in S. frugiperda than in Pseudoplusia includens (Walker). In P. includens, sublethal concentrations of MVP^™, MYX^™, and Dipel^™ fed to larvae significantly (P < 0.05) reduced pupal weight and increased pupal life span compared to control insects. MVP^™ and Dipel^™ increased larval life span significantly (P < 0.05), but MYD^™ did not.