A new direction to understand the life cycle of the Japanese pine sawyer considering the selection strategy of instar pathways

Su Bin Kim,Dong-Soon Kim

doi:10.1038/s41598-020-73344-1

Abstract

The Japanese pine sawyer, Monochamus alternatus Hope (Coleoptera: Cerambycidae), transfers the pine wood nematode, Bursaphelenchus xylophilus (Steiner and Buhrer) that causes pine wilt disease (PWD), especially in Asian countries. The key for the control of PWD is primarily focused on vector management. Thus, understanding the exact life history of M. alternatus is required. Since the late 1980s, the life cycle of M. alternatus has been accepted under the assumption that the final larvae pass four instars in the field. This study is revising the previous error for the life cycle hypothesis of M. alternatus by finding various instar pathways, which pathway is defined as the number of instars that larvae pass through prior to pupation. We confirm experimentally that the overwintered fourth or fifth instar larvae directly pupate to emerge as adults, indicating the presence of four and five instar pathways, respectively. The selection of instar pathway might be determined primarily by habitat temperature. This information will be useful to explain the variation of life history in M. alternatus populations worldwide based on the thermal environments, and also can be served to predict the northern distribution limit by applying the threshold degree-days for the completion of four instar pathway.

Highlights

The Japanese pine sawyer, Monochamus alternatus Hope (Coleoptera: Cerambycidae), transfers the pine wood nematode, Bursaphelenchus xylophilus (Steiner and Buhrer) that causes pine wilt disease (PWD), especially in Asian countries
The hypothesis of this life cycle was subjected to serious challenge, because the fact that most M. alternatus larvae pass five instars was proved in the field experimentally in Jeju, K orea[10]
The approach for determining the number of instars in insect species has frequently applied the frequency distribution of head capsule width[16,17,18]. This method can mislead actual instar numbers, since the head capsule width (HCW) of successive instars largely overlap in the field[18]

Summary

Introduction

The Japanese pine sawyer, Monochamus alternatus Hope (Coleoptera: Cerambycidae), transfers the pine wood nematode, Bursaphelenchus xylophilus (Steiner and Buhrer) that causes pine wilt disease (PWD), especially in Asian countries. This study is revising the previous error for the life cycle hypothesis of M. alternatus by finding various instar pathways, which pathway is defined as the number of instars that larvae pass through prior to pupation. In M. alternatus, the instar pathway has not yet been exactly established, it was partially known that the overwintered third and fourth instars[6], or fourth and fifth instars[12], could emerge as adults. This may be because it is difficult to examine the development of instars that are being processed secretly under bark in the field.

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Conclusion