Galls are abnormal proliferation and differentiation of plant tissue cells caused by insect feeding or oviposition stimuli, forming protrusions or tumor-like structures. The gall wasps (Dryocosmus kuriphilus) provide food and protection for their larvae within the galls. According to the nutrition hypothesis, galls provide rich nutrients to the gall-inducing insects, and as such, the nutrient content in galled tissues should be higher than that in non-galled tissues, while the content of secondary metabolites should be inversely related. Previous studies have found that the content of nutrients like sugars in the galls induced by D. kuriphilus are higher than in non-galled tissues, while the content of metabolites like tannins is conversely lower, which is consistent with the nutrition hypothesis. Metabolomic and transcriptomic studies have separately revealed that the content of nutrients such as soluble sugars is elevated in galls compared with non-galled tissues, whereas the content of secondary metabolites like flavonoids shows an opposite trend, accompanied by downregulation of photosynthesis-related gene expression. However, there are currently no reports on combined transcriptomic and metabolomic analysis of the differences between the galled and non-galled tissues. This is the first time to integrate metabolomics and transcriptomics to comprehensively compare differential metabolites and differentially expressed genes between galled and non-galled tissues of the host plant Castanea mollissima by Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. The results indicated that the levels of amino acids and peptides in the galls were upregulated, possibly related to the growth and development of D. kuriphilus, while the flavonoid content was downregulated, potentially reflecting a defensive mechanism initiated by the insect for survival. The results align with the nutrition hypothesis. Furthermore, KEGG pathway enrichment analysis showed that the expressions of most photosynthesis-related genes were down-regulated in galls, inhibiting photosynthesis in host plants.