Herbivorous insects in the order Lepidoptera cause massive losses to agricultural yields. One such species, the fall armyworm (Spodoptera frugiperda) causes the greatest damage to maize and exhibits host plant preference for Poaceae species. The beet armyworm (Spodoptera exigua) also feeds on monocots but is more commonly a pest on dicot vegetable crops. In response to herbivory, plants produce toxic and deterrent metabolites. For example, maize synthesizes indole-derived benzoxazinoids that limit the growth of non-adapted insects. Green foxtail (Setaria viridis), the wild progenitor of domesticated foxtail millet (Setaria italica), synthesizes the neurotransmitter serotonin as a defensive compound. Although insect feeding induces the accumulation of hundreds of metabolites in both maize and green foxtail, the biosynthetic pathways and defensive functions of most of these metabolites remain unknown. This work aims to compare the transcriptional and metabolic impacts of herbivory from the monocot-adapted fall armyworm and the generalist beet armyworm on maize and green foxtail. The results of these experiments show transcript-level changes in plant defensive pathways, including jasmonic acid biosynthesis and signaling, and secondary metabolic pathways such as indole-derived metabolism and terpenoids. Global analyses of differentially expressed genes between herbivory and controls show enriched gene ontology functional categories, including terpene synthase activity, indole metabolism, and lipid metabolism. Differences in plant defense responses against the two caterpillars also identify genes that are specifically affected by the monocot-selective fall armyworm. Additionally, these results show both shared defensive pathways, as well as divergent strategies between the domesticated and undomesticated plant hosts. Defense responses unique to green foxtail may contribute to herbivore resistance that is lacking in domesticated crops such as maize. The experimental results may alsobe utilized to further define defense-related metabolic pathways in both maize and green foxtail.