Spore-toxin interactions and sublethal effects of Bacillus thuringiensis in Spodoptera frugiperda and Pseudoplusia includens (Lepidoptera: Noctuidae)

Interactions between the spores and delta-endotoxins of Bacillus thuringiensis Berliner were tested in a low-susceptibility insect, Spodoptera frugiperda (J. E. Smith). The spores (formulation MYD(TM)) and one delta-endotoxin (formulation MVP(TM)) had additive effects on mortality at a MVP(TM) dose of 117,500 mu g/g of diet (P < 0.01) and synergistic effects at a MVP(TM) dose of 235,000 mu g/g diet (P < 0.01). The spores and another delta-endotoxin (formulation MYX(TM)) were antagonistic at a MYX(TM) dose of 117,500 mu g/g diet (P < 0.01) and additive at a MYX(TM) dose of 235,000 mu g/g diet (P < 0.01). The two delta-endotoxin formulations were additive with one another (P < 0.01). Sublethal concentrations of MVP(TM) 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(TM) and MYX(TM) were 6,904 and 7,561 X greater, respectively, in S. frugiperda than in Pseudoplusia includens (Walker). In P. includens, sublethal concentrations of MVP(TM), MYX(TM), and Dipel(TM) fed to larvae significantly (P < 0.05) reduced pupal weight and increased pupal life span compared to control insects. MVP(TM) and Dipel(TM) increased larval life span significantly (P < 0.05), but MYD(TM) did not.