The Japanese pine sawyer (Monochamus alternatus Hope; Coleoptera: Cerambycidae) is a primary vector of the pine wood nematode, Bursaphelenchus xylophilus (Steiner and Buhrer), which causes pine wilt disease (PWD). We investigated the progression of larval diapause in M. alternatus, focusing on changes in diapause intensity under controlled chilling at 9 °C and validation with field-aged larvae. Larvae collected in late October did not pupate after 0–10 days of chilling, representing an initial refractory phase in which diapause intensity was maximal. With increasing chilling duration, development time to pupation progressively decreased, reaching a stable baseline after 110 days, which marked diapause termination. Diapause intensity was expressed as the additional time to pupation relative to post-diapause larvae and visualized as a declining curve fitted with an extreme value cumulative function. Following transfer to different incubation temperatures, additional chilling produced disproportionately large reductions at 18 °C compared with warmer conditions, indicating that the residual inhibitory state at transfer strongly influences subsequent development. Field-aged larvae yielded a precise estimate of diapause termination on 15 March, and cumulative chilling within 2–15 °C by this date matched the 110-day benchmark from laboratory experiments. These findings demonstrate that the 2–15 °C range defines the effective chilling window for this species. By integrating laboratory and field evidence, we provide the first complete diapause intensity curve for M. alternatus and establish an empirical basis for predicting spring adult emergence. This framework advances our understanding of diapause ecology and contributes to phenology-based management of pine wilt disease.