Evaluation of Cost-effective Substrates for Mass Production of Metarhizium rileyi and Its Bioefficacy against Spodoptera litura (F.) (Lepidoptera: Noctuidae)

Abstract

Spodoptera litura (F.), a polyphagous pest affecting over 100 plant species, poses a serious threat to agricultural productivity, particularly in crops such as groundnut, soybean, cotton and pulses. Increasing concerns over pesticide resistance and environmental hazards have driven the search for effective biological alternatives. Metarhizium rileyi, an entomopathogenic fungus, has emerged as a promising biocontrol agent against lepidopteran pests. The present study aimed to evaluate various solid substrates for the mass production of M. rileyi and to assess its pathogenic potential against third instar larvae of S. litura under laboratory conditions. Seven substrates viz., rice, sorghum, bajra, maize, red gram, green gram and finger millet were tested for conidial yield. Substrates were inoculated with fungal spores and incubated at 25°C for 15 days. Conidial yield was assessed using a hemocytometer. Among them, rice supported the highest spore production (3.7 × 10⁸ spores/mL), followed by sorghum (3.2 × 10⁸ spores/mL) and bajra (2.8 × 10⁸ spores/mL), suggesting their suitability for large-scale fungal cultivation. Pathogenicity was evaluated through larval dip bioassays using third instar S. litura larvae exposed to graded conidial concentrations (1 × 10² to 1 × 10⁹ spores/mL). Bioassays revealed a dose-dependent increase in larval mortality, with the highest mortality (93.3%) recorded at 1 × 10⁹ spores/mL. The LC₅₀ value was estimated to be 2.87 × 10⁴ spores/mL, with a well-fitting probit regression model (χ² = 0.442; P = 0.976). These findings establish the efficiency of M. rileyi both in terms of mass multiplication and virulence against S. litura. The fungus demonstrates strong potential as a biopesticide and a viable component of integrated pest management (IPM) strategies. Future research should focus on field-level validation and formulation optimization to enhance its practical utility in sustainable crop protection.