Efficacy and Ultrastructural Impact of Metarhizium anisopliae and Metarhizium robertsii on Myllocerus subfasciatus

Abstract

The ash weevil, Myllocerus subfasciatus, is a significant insect pest that infests brinjal. Both the adults and grubs feed on the leaves and roots, respectively, leading to considerable yield loss. The subterranean habits of the larvae limit the effectiveness of insecticide applications, necessitating the implementation of integrated pest management programs that utilize entomopathogenic fungi. This investigation aimed to identify potential Metarhizium species against ash weevil larvae through scanning electron microscopy (SEM) and histopathology. The ash weevils were mass-cultured on brinjal plants under insect-proof conditions. Eleven Metarhizium sourced from the departmental repository were subjected to pathogenicity tests on second-instar ash weevil larvae, revealing that a concentration of 1 × 10⁶ conidia/mL was optimal for SEM and histological studies. Among the 11 Metarhizium strains examined, the TNAU ENTMA TDM 8 strain produced spores measuring 5.8 µm in length and 2.4 µm in width in both potato dextrose agar (PDA) medium and larvae. SEM analysis indicated that the mycelial adherence and penetration of Metarhizium were most pronounced in the larvae 5 days post inoculation (DPI). Histopathological investigations demonstrated that the TNAU ENTMA TDM 8 strain caused degradation of fat bodies and hemocytes at 3 DPI, and complete body distortion at 7 DPI, while the untreated control exhibited no such effects. The M. robertsii strain TNAU ENTMR GYU 1 displayed slower infectivity compared to the M. anisopliae strains. The TNAU ENTMA TDM 8 strain was determined to be the most effective against M. subfasciatus larvae and can be utilized for managing ash weevil populations.