Toxicity profiling of indigenous Bacillus strains with insecticidal activity for the management of Spodoptera litura
摘要
Bacillus thuringiensis (Bt) is widely recognized as an eco-friendly alternative to chemical insecticides, and continuous efforts are underway to discover strains with improved efficacy against major agricultural pests. In this study, bacterial isolates with Bt-like activity were isolated from agricultural soils of Aligarh and industrial soils of Ghaziabad, Uttar Pradesh, India. Colony counts indicated a higher abundance of Bacillus-like colonies in agricultural soils (1.4 × 109 CFU g⁻¹) than in industrial soils (1.18 × 109 CFU g⁻¹). A total of 30 purified Bacillus isolates were characterized, of which 10 (A1–A6 from Aligarh; G1–G4 from Ghaziabad) formed spores and parasporal crystals. Scanning electron microscopy confirmed bipyramidal and cuboidal crystal morphologies, while SDS-PAGE revealed protein bands ranging from ~ 65 to 130 kDa, consistent with Cry-like protoxins. Third- and fourth-instar larvae of Spodoptera litura were exposed to spore–crystal suspensions at concentrations of 50–200 µg mL⁻¹ using a leaf-dip assay. No mortality occurred at 50 µg mL⁻¹, whereas higher concentrations produced dose- and time-dependent effects. Probit analysis of 96 h mortality revealed LC₅₀ values between ~ 100 and 145 µg mL⁻¹ among the most potent isolates, with strain A6 exhibited the highest potency (LC₅₀ ≈ 100 µg mL⁻¹; LC₉₀ ≈ 151 µg mL⁻¹). Less effective isolates (A5, G1, G3) showed higher LC₅₀ values, indicating lower potency. Other isolates (A1, A2, A3, A4, G2, G4) also achieved complete larval mortality at the highest tested dose (200 µg mL⁻¹) by 96 h, whereas A5, G1, and G3 did not reach full mortality within the tested dose range. These findings highlight the presence of native Bt isolates with promising insecticidal activity against S. litura, particularly strain A6, which may serve as a candidate for developing biopesticides in integrated pest management programs.