Synergistic effects of <i>Bacillus thuringiensis</i> (Bacillales: Bacillaceae) and nucleopolyhedrovirus against <i>Agrotis ipsilon</i> (Lepidoptera: Noctuidae) larvae

The black cutworm, Agrotis ipsilon (Hüfnagel) (Lepidoptera: Noctuidae), is a globally significant underground pest known for its broad host range and destructive feeding habits. While nucleopolyhedrovirus (NPVs) and Bacillus thuringiensis (Bt) Berliner (Bacillales: Bacillaceae) have been widely used as microbial insecticides, their combined effects against A. ipsilon remain poorly understood. This study evaluated the pathogenicity of different NPVs and Bt strains against A. ipsilon larvae, both individually and in combination. Pathogenicity varied significantly among treatments, with Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) exhibiting the highest virulence, causing 84.62% insect mortality at the lowest tested concentration (1 × 105 OBs/ml). Insect mortality generally increased with higher NPVs concentrations, reaching over 90% at 1 × 108 OBs/ml. In contrast, Spodoptera litura (Fabricius) multiple nucleopolyhedrovirus (SlMNPV) showed moderate lethality compared to MbMNPV, while Spodoptera exigua (Hübner) multiple nucleopolyhedrovirus (SeMNPV) and 2 Bt strains exhibited even lower pathogenicity. The interactions between NPVs and Bt varied depending on pathogen combinations and concentrations. Synergistic effects were observed in SlMNPV-Bt mixtures, leading to increased larval mortality and distinct survival curves compared to treatments with SlMNPV or Bt alone. MbMNPV-Bt combinations produced stable additive effects in most cases, whereas SeMNPV-Bt combinations exhibited varied effects, including synergistic, antagonistic, and additive effects. These findings provide insights into optimizing NPV-Bt combinations for A. ipsilon control, offering a potential alternative to chemical insecticides.