Pathogenicity and immune response of Spodoptera frugiperda to oral infection by Metarhizium rileyi CDTLJ1

• Oral infection by Metarhizium rileyi CDTLJ1 caused 78–100% mortality across different larval instars of Spodoptera frugiperda , with LC 50 values increasing with larval age. • While oral inoculation alone was effective, its combination with spray application further enhanced lethal efficacy. • The midgut and hindgut were identified as primary colonization sites, with hyphal bodies peaking in the hemocoel at 120 h post-inoculation. • Oral and spray infection significantly downregulated PGRP expression in intestinal and integumentary tissues, while Dorsal expression remained unchanged. To elucidate the mechanisms of oral infection by Metarhizium rileyi strain CDTLJ1 in Spodoptera frugiperda and evaluate its biocontrol potential, this study assessed its pathogenicity via oral inoculation, and investigated the infection process and host immune gene expression. The infection dynamics within the host were observed using fluorescence microscopy, and the expression patterns of immune-related genes ( PGRP and Dorsal ) were analyzed by RT-qPCR. The results showed that oral inoculation of 2nd to 6th instar larvae with CDTLJ1 at a concentration of 1 × 10 8 spores/mL resulted in corrected mortality rates of 100%, 93.33%, 91.67%, 81.67%, and 78.33%, respectively. The median lethal concentration (LC 50 ) values increased with larval instar, measuring 2.27 × 10 5 , 1.45 × 10 6 , 2.06 × 10 6 , 7.00 × 10 6 , and 1.28 × 10 7 spores/mL for the respective instars. While oral inoculation alone effectively caused mortality, the addition of a spray treatment further enhanced lethal efficacy. Fluorescence microscopy confirmed that the midgut and hindgut were the primary sites of fungal colonization following oral infection, with hyphae also observed in the Malpighian tubules. Monitoring of the hemocoel revealed that hyphal bodies were first observed in small quantities at 72 h post-inoculation (hpi), with their population peaking at 120 hpi, reaching approximately 1.10 × 10 7 per μL. Gene expression analysis indicated that both inoculation methods significantly downregulated PGRP expression in intestinal and integumentary tissues, whereas no significant change was detected in Dorsal expression.This study clarifies the pathogenic mechanisms of CDTLJ1 during oral infection of S. frugiperda and provides a theoretical foundation for its development and application as a biocontrol agent.