Lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of glycosidic bonds in crystalline polysaccharides including chitin and cellulose. The recent discovery of LPMO family proteins in many insect species suggests that they presumably play a role in chitin degradation in the cuticle/exoskeleton, tracheae and peritrophic matrix during insect development. Insect LPMOs belong to auxiliary activity family 15 (AA15/LPMO15) and have been classified into at least four groups based on phylogenetic analysis. In this study, we identified and investigated the physiological functions of group I LPMO15 (MaLPMO15-1 and PhLPMO15-1) in two longhorn beetle species, Monochamus alternatus and Psacothea hilaris. In both species, depletion of LPMO15-1 transcripts by RNAi resulted in a lethal pupal-adult molting defect. The insects were unable to shed their old pupal cuticle and died entrapped in their exuviae. Furthermore, TEM analysis revealed a failure of degradation of the chitinous procuticle layer of their old cuticle, retaining intact horizontal laminae and vertical pore canals containing perpendicularly oriented chitin fibers (pore canal fiber, PCF) in their core. These results indicate that MaLPMO15-1 and PhLPMO15-1 are required for turnover of the chitinous old cuticle, which is critical for insect molting.