The Japanese pine sawyer (JPS), Monochamus alternatus, is the primary vector of the pine wood nematode, the causal agent of pine wilt disease. Despite more than five decades of intensive research, its complete life cycle—particularly the developmental pathways defined by larval instar number prior to pupation—remains incompletely resolved. This study reexamines JPS’s life cycle from the perspective of instar-pathway (IP) selection. While four- and five-instar pathways (4-IP and 5-IP) have been documented, empirical support for a three-instar pathway (3-IP) has remained inconclusive. We tracked the full developmental trajectories of individually marked larvae from egg hatch to pupal chamber construction under near-natural field conditions. Special attention was given to tunnel-digging behavior, a characteristic sequence of boring and entrance-hole plugging that precedes overwintering. Our observations indicate that the initiation of tunnel-digging behavior coincides with the completion of IP selection. All individuals committed to an IP only after reaching at least the fourth instar, most frequently during the mid- to late fourth instar stage. No explicit evidence was found for pupation or adult emergence following a 3-IP. Reanalysis of previously published datasets likewise failed to provide convincing support for a 3-IP. These results suggest that the fourth instar represents a critical developmental threshold for progression toward pupation and adult emergence. Variation beyond this threshold allows some individuals to continue growth into a fifth instar, resulting in alternative 5-IP. The present study redefines JPS’s developmental framework and provides an improved foundation for future modeling of its phenology and population dynamics.
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