A proposed mechanism for the regulation of the annual life cycle based on pupal chamber construction behavior in the Japanese pine sawyer

Abstract 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 (PWD). Although more than 50 years have passed since JPS was proven to be a PWD vector, surprisingly, the exact life cycle has not yet been fully understood. We could interpret the selection of JPS’s life cycle based on the hypothesis established through this study: a dual regulation of the critical larval age (a minimum 4.2-instar and average 4.8-instar) and cue stimulus (tentatively a cold temperature) in diapause induction expressed by the behavior of pupal chamber construction. The fourth larval instars above the critical age can initiate the tunnel for their pupal chamber when they meet the cue, followed by the selection of the four-instar pathway (4-IP), which indicates the number of instars before pupation. If there is no cue during the fourth instar, the larvae will develop into the fifth instar, and 5-IP can be selected when meeting the cue. However, no cue leads to repeated molting up to the 10th instar. The third or fewer larval instars overwinter without the selection of IP and develop to the final instar the following year to emerge into adults the next year (entering a two-year life cycle). Also, we could explain most of the phenomena that are not well understood in the JPS’s overwintering behavior, such as the pupation on the wood’s surface and unplugged entrance holes. Consequently, our hypothesis will be helpful for future studies of diapause mechanisms and the selection of IP in JPS and related species.