AN OUTBREAK OF THE CRYPTOMERIA BARK BORER, SEMANOTUS-JAPONICUS LACORDAIRE (COLEOPTERA, CERAMBYCIDAE) IN A YOUNG JAPANESE CEDAR (CRYPTOMERIA-JAPONICA D DON) PLANTATION .1. ANNUAL FLUCTUATIONS IN ADULT-POPULATION SIZE AND IMPACT ON HOST TREES

(1) The number of adults of the cryptomeria bark borer, Semanotus japonicus LACORDAIRE, was examined for each generation in a young plantation of Japanese cedar, Cryptomeria japonica D. DON, in Kyoto, Japan, throughout a period of population increase and decline from 1976 to 1985. Annual occurrences of damaged and killed host trees caused by the borer were also recorded. (2) The borer established its population in the study plantation 5 years after planting. It increased continuously in number, reached a peak abundance (about 34,000 adults/ha) 6 years later, and subsequently declined rapidly. Such a gradual outbreak occurring only once in early plantation ages was suggested to be a characteristic common to the infestation of the borer in susceptible host plantations. (3) The annual fluctuation in the number of trees from which adult borers emerged coincided with the yearly trend of the number of emerged adults in the whole plantation. The number of damaged trees in the study plantation amounted to 50.5% of the total population. Nearly 37% of these damaged trees were killed by the borer. The larger, dominant trees were more vulnerable to borer infestation and mortality. Tree diameter or growth rate is proposed as an indicator of the favourability of each host tree as a food resource for the borer. The abundance of the borer in host plantations was considered to be limited by the availability of favourable host trees.