Cuticular Breakdown: Histopathological Manifestations of Beauveria Bassiana Infection in Spodoptera littoralis Larvae

Objective: The overreliance on chemical insecticides for controlling the devastating cotton leafworm, Spodoptera littoralis, has led to resistance and environmental concerns. This study investigated the biocontrol potential of the entomopathogenic fungus Beauveria bassiana by determining its median lethal concentration (LC₅₀) against fourth instar larvae and characterizing the ensuing histopathological damage to the larval cuticle, the primary barrier to infection. Methods: The pathogenicity of an indigenous B. bassiana isolate was assessed through a leaf-dipping bioassay using five conidial concentrations (1×10⁵ to 1×10⁹ conidia/mL). The LC₅₀ was calculated via probit analysis. For histopathological examination, larvae treated with the LC₅₀ (2.81 × 10⁶ conidia/mL) were dissected at 72 hours post-treatment. Cuticle samples were processed, embedded in paraffin, sectioned, and stained with Hematoxylin and Eosin (H&E) and Periodic Acid-Schiff (PAS) for detailed microscopic analysis. Results: Bioassay results revealed a significant dose-dependent mortality, with the highest concentration (1×10⁹ conidia/mL) causing 85.3% mortality. The calculated LC₅₀ was 2.81 × 10⁶ conidia/mL. Histopathological analysis demonstrated severe structural degradation in the cuticle of treated larvae. Key findings included the disintegration of the epicuticle and the lamellar structure of the procuticle, extensive invasion and proliferation of fungal hyphae and blastospores within the cuticular matrix, and the formation of lytic cavities. Fungal penetration reached the epidermal layer, which displayed significant disruption and vacuolization, confirming a complete breach of the host's primary physical defense. Conclusion: Beauveria bassiana proves to be a highly virulent pathogen against S. littoralis larvae. The histopathological evidence provides a mechanistic understanding of its efficacy, directly linking larval mortality to comprehensive cuticular disintegration and systemic fungal invasion. These findings strongly support the integration of B. bassiana as a sustainable and effective biocontrol agent within IPM programs to manage this economically important pest.