Gut microbiota contribute to host adaptability of Spodoptera frugiperda: evidence from host plant switching and antibiotic manipulation in a laboratory colony that lost the capacity to complete larval development on maize

The fall armyworm (FAW), Spodoptera frugiperda , is a major invasive polyphagous pest worldwide, and its robust host and environmental adaptability underpins its successful colonization. While gut microbes have been implicated in insect host adaptation, their functional roles in FAW remain incompletely understood. This study explored gut bacterial contributions to FAW host adaptability by adopting multigenerational host rearing assays, antibiotic ingestion and 16 S rDNA sequencing on a laboratory colony that had lost the capacity to complete larval development on maize. FAW larvae exhibited the strongest feeding preference for maize, followed by barnyard grass, mulberry, and pepper. Starvation resulted in reduced host selectivity of FAW, thereby increasing the probability of feeding on alternative hosts. Multigenerational rearing across four generations and measurements of fitness traits (body weight, developmental period, survival and fecundity) demonstrated that host plant type significantly shapes FAW fitness. Larvae raised on artificial diet maintained far superior fitness in every generation, while all fitness indices increased gradually across generations on the four tested plant hosts, reflecting gradual adaptive improvement to host plants. Antibiotic treatment significantly impaired the development, body weight, survival, and reproduction of FAW on all host diets, with only partial fitness recovery detected in their untreated progeny. Sequencing uncovered host-specific gut bacterial communities: control FAW were dominated by Enterococcus , Klebsiella and Tyzzerella , whereas antibiotic-treated FAW were dominated by Staphylococcus and Corynebacterium . Functional prediction via BugBase and FAPROTAX suggested marked differences in the top 10 microbial functional categories, where the control group harbored higher abundances of mobile genetic elements, chemoheterotrophy and fermentation pathways, whereas the antibiotic-treated group exhibited elevated facultative anaerobes and pathogenic potential, alongside reduced stress tolerance, chemoheterotrophy and fermentation capacity. Further reference-based functional prediction identified 63 putatively beneficial bacterial genera (such as lactic acid bacteria) and 43 potentially pathogenic bacterial genera (such as Clostridium , Klebsiella , Pseudomonas and Serratia ). Relative abundance analysis revealed significant enrichment of putatively beneficial bacterial genera in control larvae, whereas potentially pathogenic taxa prevailed in antibiotic-treated larvae. Host plant identity shapes the gut microbiota assembly of FAW, and gut symbiotic bacteria contribute to FAW’s adaptability to different host plants. Gut symbiotic microbes boost FAW fitness mainly via nutritional and metabolic pathways. Disruption of gut microbial homeostasis may compromise the functions of symbionts and even trigger pathogenic effects of opportunistic microbes, thereby constraining the growth, reproduction and host-adaptation capacity of FAW.