Insect Gut Microbiota Diversity Reveals Differences in Pathogenicity Between <i>Metarhizium rileyi</i> and <i>Beauveria bassiana</i>

Beauveria bassiana and Metarhizium rileyi are entomopathogenic fungi that are widely present in nature. Due to their excellent insecticidal activity, they have been widely developed as microbial preparations and utilized in the biological control of pests. To clarify differences in pathogenicity between B. bassiana and M. rileyi against Spodoptera frugiperda, third-instar larvae were subjected to immersion treatments, whereas fifth-instar larvae were inoculated via injection with various doses of fungal strains B. bassiana AJS91881 and M. rileyi SXBN200920. Our results demonstrated differential virulence between fungal species depending on the infection method. Specifically, in immersion assays, M. rileyi displayed significantly higher pathogenicity compared with B. bassiana. Survival assessments of mixed-infection groups (1:9, 1:1, and 9:1) revealed reduced larval LT₅₀ values corresponding to elevated proportions of M. rileyi. Conversely, B. bassiana exhibited significantly enhanced pathogenicity relative to M. rileyi in injection assays, with higher proportions of B. bassiana (9:1, 1:1, 1:9 groups) resulting in reduced LT₅₀ values. GM diversity analysis further illustrated distinct microbiota alterations correlated with varying fungal ratios. Divergent virulence observed between immersion and injection protocols likely stems from fundamentally different infection mechanisms employed by B. bassiana and M. rileyi. In vitro cultivation experiments demonstrated superior growth of B. bassiana compared to M. rileyi. Thus, B. bassiana was more pathogenic in intra-hemoceol injection bioassay, whereas M. rileyi demonstrated greater efficacy in immersion bioassay. Overall, this study provides preliminary evidence for differential pathogenicity mechanisms between B. bassiana and M. rileyi toward S. frugiperda larvae, contributing important knowledge for agricultural biocontrol programs.