The Japanese pine sawyer (JPS), Monochamus alternatus Hope, is an important vector of the pine wood nematode, Bursaphelenchus xylophilus (Steiner and Buhrer), which causes pine wilt disease. JPS selects a characteristic life cycle of a four- or five-instar pathway, which indicates the number of instars before pupation. In this study, we investigated the potential presence of a three-instar pathway and sought to determine the minimum larval age required for JPS to construct a pupal chamber. The results showed that no third instars made pupal chambers, suggesting that there is no three-instar pathway. The initiation time of digging the tunnel to make the pupal chamber ranged widely from Julian date 274 (30 September) to 332 (27 November). This timing became an average age stage of 4.8 instar, which equals an 80% completion of the fourth instar. Thus, the minimum larval age at which JPS constructs the tunnel to pupate the next year is estimated to be late in the fourth larval instar. Further, we discuss the diapause ecology in relation to the larval development of JPS in different thermal environments. Tentatively, we suggest that the regulation of diapause induction in JPS involves a dual process of reaching a critical developmental stage and stimulus (tentatively a cold temperature). This hypothesis will be helpful for future studies of diapause mechanisms and the selection of the instar-pathway in JPS and related species.