Spodoptera litura is an important polyphagous pest that causes great damage to agriculture. We performed RNA-seq from 15 individuals of S. litura, including different larval (fifth and sixth instar larvae), chrysalis, and adult developmental stages. We also sequenced the Spodoptera frugiperda to compared with the S. litura. A total of 101,885 differentially expressed transcripts (DETs) were identified in the S. litura. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses indicated that the S. litura may undergo active development and basic metabolism, such as xenobiotic and detoxifying metabolism, during its larvae and adult stages, which explains the difficulty to control it. We also found that DETs of single-copy orthologous genes between S. litura and S. frugiperda were involved in basic metabolism and developmental, but S. litura were highly enriched in energy and metabolic processes, whereas S. frugiperda had stronger nervous and olfactory functions. Metagenomics analysis in larval S. litura and S. frugiperda revealed that microbiota can both participate in the detoxification and metabolism processes in them, but the relative abundance of detoxification-related microbiota was more abundant in S. frugiperda. Transcriptome results also confirmed the detoxification-related pathway of S. frugiperda was more abundant than that of S. litura. Therefore, we can conclude that the detoxification ability of S. frugiperda is stronger than that of in S. litura, which is related to the wide adaptation and strong resistance of S. frugiperda. Our study might provide clues to the development of novel pest control strategies.