Abstract

BACKGROUND Cuticular microbial communities influence insect–pathogen interactions, yet their protective roles remain incompletely understood. These microbial defenses are critical in pest species like Bactrocera dorsalis , where microbial symbionts may shape resistance to biological control agents. RESULTS We show that the cuticular microbiota of Bactrocera dorsalis enhances resistance to the entomopathogenic fungi (EPF) Beauveria bassiana and Isaria fumosorosea through dual immune and chemical mechanisms. Gnotobiotic assays demonstrate that specific bacterial isolates ( Microbacterium , Psychrobacter , and Staphylococcus ) promote host survival and defense by stimulating Toll/IMD signaling pathway and up-regulating antimicrobial peptides (AMPs) such as defensin , attacin , and cecropin . In contrast, non-cuticle-associated bacteria ( Escherichia coli and Staphylococcus aureus ) failed to enhance survival and immune responses, indicating that protection is microbiota-specific rather than a result of general bacterial exposure. In parallel, bacterial volatiles, such as phenol, indole, and eicosane, along with extracellular enzymes including chitinase, cellulase, protease, and lipase, suppressed fungal germination and hyphal growth. Importantly, these protective effects were found to vary with host age, indicating age-dependent modulation of microbiota-mediated defense. CONCLUSIONS Our findings revealed that cuticle-associated bacteria protect their host through complementary immune and chemical pathways. These microbiota act as an active barrier, and microbial metabolism interacts with host immune signaling. By distinguishing specific cuticle-derived effects from non-specific bacterial stimulation, this study highlighted the role of microbiota in modulating fungal susceptibility and supports their application in developing next-generation biocontrol strategies in pest management. © 2026 Society of Chemical Industry.