A new <i>Metarhizium</i> strain suppresses <i>Spodoptera frugiperda</i> (Lepidoptera: Noctuidae) by disrupting key physiological systems

The increasing global demand for agricultural production is threatened by fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith, 1797), a highly polyphagous pest. The biocontrol of this pest is beneficial over chemical control. The new Metarhizium lepidiotae (Driver & Milner) (Hypocreales: Clavicipitaceae), strain ML was isolated from Mount Heng, Hunan Province in China. The biocontrol potential of ML on FAW was evaluated through laboratory bioassays, while physiological effects were examined via histopathological assays. Enzyme activity assays were conducted to assess fungal enzyme activity levels, and the relative expression levels of fungal virulence-related genes were analyzed using RT-qPCR. This study explored that the strain ML caused a high larval mortality of 97.92% at 1 × 109 spores/ml, with an LC50 of 1.26 × 104 spores/ml. The pupal mortality was 87.5% at 1 × 108 spores/ml, with an ST50 of 8.33 d. The results showed that the high virulence was driven by elevated expression of fungal virulence-related genes and enzymes. The histopathological assays proved that the extensive hyphal invasion by ML disrupted larval tissues such as cuticle, epidermis, hemolymph, fat body, and muscles, damaging the respiratory, digestive, and excretory systems. The elevated levels of chitinase (35.82 µg/h/g), protease (3,844.02 nmol/min/g), and lipase (2,288.50 nmol/min/g) activities were synergized with the expression of M. lepidiotae elastase-like serine protease gene (MLpr1), chitinase gene (Mlchit1), secreted lipase gene (MlSecL), and lipase A gene (MlLIPA) throughout the infection. These findings provided valuable insights into the pathological mechanisms driving the high virulence of M. lepidiotae against FAW, offering potential for sustainable biocontrol.