Abstract
Indigenous Abies firma trees were surveyed on Mount Unzen, Japan to clarify the dieback mechanisms of A. firma trees caused by herbivorous insects. A curculionid, Parendaeus abietinus, was revealed to be a univoltine species that caused the fall of needles as a result of adult and larval feeding on A. firma needles, and a scolytid, Polygraphus proximus, was regarded as a bivoltine species that produces subcortical galleries. Different degrees of needle infestation by P. abietinus among census years suggested that the population density of P. abietinus was remarkably high from 1995 to 1998, thereafter decreasing to 2001. Abies firma trees suffering from serious needle fall were frequently attacked and colonized by P. proximus in 1998 and 1999, and died soon after colonization; however, following the decline in the population density of P. abietinus, the number of A. firma trees colonized by P. proximus significantly reduced from 1999 to 2001. The possible dieback process of A. firma is considered as follows: (1) Around 1995, P. abietinus increased its population density and caused serious needle falling of A. firma; (2) in 1998 and 1999, P. proximus successfully colonized the bark of defoliated trees and played an important role in the dieback of A. firma; (3) after 1998, the population density of P. abietinus decreased; (4) thereafter, P. proximus could not colonize the bark of A. firma that survived serious defoliation and actively produced oleoresin.
