Isolation and Characterization of a Native Metarhizium rileyi Strain Mrpgbm2408 from Paralipsa gularis in Maize: First Data on Efficacy and Enzymatic Host Response Dynamics

<i>Paralipsa gularis</i> (Zeller) has become an increasingly destructive pest in both storage and field ecosystems, particularly affecting maize crops across China. As chemical control methods face limitations due to resistance development and environmental concerns, biological control presents a promising alternative. In this study, we isolated and identified a novel strain of <i>Metarhizium</i> sp. from naturally infected <i>P. gularis</i> larvae collected in Yunnan Province, China. Morphological characterization, along with ITS-rDNA and EF-1α-rDNA sequencing, confirmed the fungus as <i>Metarhizium rileyi</i>. The optimal growth medium for this strain was SMAY, and the optimal conditions were 25 °C under continuous light (L:D = 24:0). Laboratory bioassays showed that the strain exhibited high virulence against <i>P. gularis</i> larvae, with cumulative mortality reaching 82% following infestation with 5 × 10⁸ conidia/mL. Biochemical analyses revealed that fungal infection significantly inhibited the activity of the key antioxidant enzyme SOD in the host, while activities of POD, CAT, and detoxification enzymes (P450, CarE, AChE, and GSTs) were significantly increased. These results indicate that immune responses were triggered, and systemic colonization of the host was achieved. Overall, this native <i>M. rileyi</i> strain demonstrates strong potential as an effective biological control agent. Its ability to overcome insect defenses and induce high mortality supports its integration into pest management programs targeting <i>P. gularis</i>. This work advances the understanding of fungal-insect interactions and contributes to sustainable, environmentally safe strategies for managing a pest of economic importance in agricultural ecosystems.