Heliothis virescens and H. zea (Lepidoptera: Noctuidae): Dosage Effects of Feeding Mixtures of Bacillus thuringiensis and a Nuclear Polyhedrosis Virus on Mortality and Growth1

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Abstract
Tobacco budworm, Heliothis virescens (F.), and corn earworm, H. zea (Boddie), were reared on diets containing various concentrations of the multiple-embedded, nuclear polyhedrosis virus (NPV) from H. armigera (Hübner) and Bacillus thuringiensis (Berliner) (B.t.) (HD-1 isolate), both individually and as mixtures. Addition of NPV did not significantly affect the 7-day mortality due to normally lethal concentrations of B.t., but the addition of B.t. to normally lethal concentrations of NPV reduced mortality. An apparent synergistic relationship occurred between the pathogens at a normally sublethal concentration. H. virescens larvae fed on NPV were smaller at all concentrations tested, but their weight was not related to concentration of the virus. Weight of surviving H. zea larvae was affected only by high concentrations of NPV. Normally sublethal concentrations of B.t. affected larval weight of both species, and the weights were inversely proportional to the concentration of B.t.