Compatibility and Laboratory Bioefficacy of Beauveria bassiana Isolates Combined with Selected Insecticides against Plutella xylostella L.

Abstract

The diamondback moth, Plutella xylostella (L.), is one of the most destructive pests of cruciferous crops and has developed resistance to several insecticides, highlighting the need for sustainable pest management strategies. The present study was undertaken to evaluate the compatibility of different isolates of Beauveria bassiana with selected insecticides and to assess their bioefficacy against P. xylostella under laboratory conditions. The experiment was conducted during 2024–2025 at the Biocontrol Research and Production Centre (BRPC), Department of Entomology, College of Agriculture, Jawaharlal Nehru Krishi Vishwa Vidyalaya (JNKVV), Jabalpur. Five isolates of B. bassiana (MTCC-892, NBAIM-92, NBAIM-93, Bb Local, and MTCC-1022) were evaluated in combination with emamectin benzoate, fipronil, indoxacarb, and spinosad. Compatibility was assessed by measuring radial mycelial growth and percentage inhibition at 3, 5, and 7 days after inoculation (DAI), whereas conidial production and spore viability were determined on the tenth day after inoculation. In addition, the bioefficacy of compatible B. bassiana–insecticide combinations was evaluated against third-instar larvae of P. xylostella under laboratory conditions. Significant differences were observed among fungal isolates, insecticides, and their interactions for all compatibility parameters. Bb Local recorded the highest radial growth at 3 DAI (44.03 mm) and 5 DAI (63.90 mm), whereas NBAIM-92 exhibited the highest growth at 7 DAI (77.29 mm). Among the insecticide treatments, emamectin benzoate caused the greatest reduction in fungal growth, recording mean radial growth values of 27.06, 40.95, and 58.13 mm at 3, 5, and 7 DAI, respectively. Fipronil recorded the highest mean spore count (1.69 × 10⁸ spores mL⁻¹) and spore viability (97.31%) among the insecticide treatments. Although all insecticides reduced fungal growth and sporulation, spore viability remained above 95% in all treatments, indicating good compatibility. Bioefficacy was evaluated by recording larval mortality at 24, 48, 72, 96, and 120 h after treatment. Spinosad recorded the highest mean mortality at 24 h (65.56%), 48 h (82.61%), and 72 h (96.10%) and achieved 100% mortality by 96 h, whereas all tested insecticide treatments resulted in 100% mortality by 120 h. Among the fungal isolates, NBAIM-92 recorded the highest mean mortality at 120 h (89.36%). Under the laboratory conditions of this study, the evaluated B. bassiana–insecticide combinations showed measurable compatibility and high bioefficacy against P. xylostella larvae.