Objective  Population density of the typical borer pest, Monochamus alternatus, in the Bursaphelenchus xylophilus forest was investigated to seek effective means for detecting and controlling the difficult-to-manage infestation.

  Method  The vertical distributions of oviposition grooves, invasion (entry) and eclosion (exit) holes of M. alternatus inside the infected tree trunks were obtained by systematic random sampling and tree-by-tree, section-by-section close observation on evidence of infestation on trees at Fengshan National Forest Park in Ganzhou, Jiangxi from 2015 to 2017.The data were analyzed for correlations among them.

  Result  The oviposition tracks left on the trunks by M. monochamus constituted 46.07% of the total in the section half way above ground level on a tree less than 8 m high. For the trees equal or taller than 8 m, 53.687% of the marks were found in the lower part of a trunk. The quantitative distribution of larval invasion and adult eclosion holes on the diseased trees ranked in the order of middle section > lower section > upper section. Of which, approximately 50% were in the middle section of a tree trunk. The total number of oviposition grooves and invasion and eclosion holes correlated with tree height. No oviposition groove was observed on dead tree stumps, and few invasion and eclosion holes were found which amounted to merely 0.54% and 0.29%, respectively, of the total. The number of boreholes (i.e., the combined total of oviposition grooves and invasion and eclosion holes) in the lower part of the trunk significantly correlated with the number of eclosion holes in the entire tree. However, it was more accurate and reliable to estimate the total number of eclosion holes on a tree by using the count of invasion holes in the lower section.

  Conclusion  Using the invasion hole count in lower trunk section to estimate the total number of eclosion holes on a infested tree was considered a convenient, quick and reliable way for studying the M. monochamus infestation in a B. xylophilus forest.