The pyrokinin (PK)/pheromone biosynthesis-activating neuropeptide (PBAN) family, which is defined by a conserved C-terminal pentapeptide (FXPRLamide), is involved in many physiological processes in insects. In the oriental armyworm Mythimna separata, the larvae exhibit a variety of color patterns in response to changes in population density, which are caused by melanization and a reddish coloration hormone (MRCH), which is a member of the FXPRLamide neuropeptides. Interestingly, in some lepidopteran insects, MRCH is known as a PBAN, which activates the pheromone gland to produce sex pheromones. PBAN is encoded by a single gene, dh-pban, which encodes additional FXPRLamide neuropeptides, such as the diapause hormone (DH) and subesophageal ganglion neuropeptides (SGNPs). To determine the roles of the dh-pban gene, which produces multiple types of FXPRLamide neuropeptides after post-transcriptional cleavage of the precursor protein, we performed CRISPR/Cas9-mediated targeted mutagenesis in M. separata. We demonstrated that knockout armyworm larvae lost density-dependent cuticular melanization and retained yellow body color, even when reared under crowded conditions. Moreover, our rescue experiments using the synthetic peptides showed that not only PBAN but also β- and γ-SGNPs significantly induce the cuticular melanization in a dose dependent manner. Taken together, our results provide genetic evidence that neuropeptides encoded by the single dh-pban gene act redundantly to control density-dependent color pattern formation in M. separata.