Spodoptera frugiperda (Lepidoptera: Noctuidae) is a migratory agricultural pest that is devastating on a global scale. Beauveria bassiana is a filamentous entomopathogenic fungus that has a strong pathogenic effect on Lepidoptera pests but little is known about the microbial community in the host gut and the dominant populations in fungus-infected insects. B. bassiana AJS91881 was isolated and identified from the infected larvae of Spodoptera litura. The virulence of AJS91881 to the eggs, larvae, pupae and adults of S. frugiperda was measured. Moreover, the gut microbial community diversity of healthy and fungus-infected insects was analyzed. Our results showed that after treatment with B. bassiana AJS91881, the egg hatching rate, larval survival rate and adult lifespan of the insects were significantly reduced, and the pupae rigor rate was significantly increased compared to that of the control group. Additionally, the gut microbial community was reconstructed after B. bassiana infection. At the phylum and genus level, the relative abundance of the Proteobacteria and Serratia increased significantly in the B. bassiana treatment group. The KEGG function prediction results showed that fungal infection affected insect gut metabolism, environmental information processing, genetic information processing, organism systems and cellular processes. Fungal infection was closely related to the metabolism of various substances in the insect gut. Serratia marcescens was the bacterium with the highest relative abundance after infection by B. bassiana; intestinal bacteria S. marcescens inhibited the infection of insect fungi B. bassiana against the S. frugiperda. The presence of gut bacteria also significantly reduced the virulence of the fungi against the insects when compared to the group with the larvae fed antibiotics that were infected with fungal suspension (Germfree, GF) and healthy larvae that were infected with fungal suspension prepared with an antibiotic solution (+antibiotic). In conclusion, the reconstruction of the insect intestinal bacterial community is an indispensable link for understanding the pathogenicity of B. bassiana against S. frugiperda. Most importantly, in the later stage of fungal infection, the increased abundance of S. marcescens in the insect intestine inhibited the virulence of B. bassiana to some extent. The findings aid in understanding changes in the gut microbiota during the early stages of entomopathogenic fungal infection of insects and the involvement of insect gut microbes in host defense mediated by pathogenic fungal infection. This study is also conducive to understanding the interaction between entomopathogenic fungi, hosts and gut microbes, and provides a new idea for the joint use of entomopathogenic fungi and gut bacteria to control pests.

Western flower thrips (WFT), Frankliniella occidentalis (Pergande), is a highly invasive pest which is harmful to many cash crops globally and resistant to various insecticides. Entomopathogenic fungi (EPF), as biological control agents, have demonstrated a good control effect on WFT. The aim of this study was to evaluate the synergistic and pathogenicity efficacy of the fungal strain Metarhizium flavoviride WSWL51721 when distributed with diatomaceous earth (DE) and the active ingredient imidacloprid using four bioassay methods against adult and second instar larvae of WFT. The data of the four bioassays have been fitted to the time–concentration–mortality (TCM) model. The corrected mortality ranges of WFT adults were 75–100%, 82.69–100%, 78.85–100%, and 92.31–100%, and the corrected mortality ranges of WFT second instar larvae were 72.22–100%, 85.19–100%, 77.77–100%, and 100% in the four bioassays at concentrations of 1.2 × 10⁶ to 1.2 × 10⁸ conidia/mL, respectively. At 1.2 × 10⁸ conidia/mL, assays 2 (M. flavoviride with DE), 3 (M. flavoviride with imidacloprid), and 4 (M. flavoviride with DE and imidacloprid) had the shortest median lethal time (LT₅₀), compared with that of assay 1 (M. flavoviride alone) for adults at 2.26 d, 2.06 d, and 1.53 d, and second instar larvae at 2.45 d, 1.70 d, and 1.41 d, respectively. The median lethal concentration (LC₅₀) in the four bioassays decreased within 3–10 days of inoculation. On the third day, it was found that the lowest median lethal concentrations in assays 2, 3, and 4 were 1.58 × 10⁷, 1.13 × 10⁷, and 3.39 × 10⁶ conidia/mL, respectively, which were significantly different from that in assay 1 for the adults. For the second instar larvae, assays 2, 3, and 4 also had the lowest lethal concentrations and were significantly different from those of assay 1. There were significant differences in sporulation between adults and second instar larvae under the four bioassays. Our results indicate that assays 2 (M. flavoviride with DE), 3 (M. flavoviride with imidacloprid), and 4 (M. flavoviride with DE and imidacloprid) demonstrate synergistic effects on the control of both adult and second instar larvae of WFT under laboratory conditions.