Process of Mass Oak Mortality and the Relation to Platypus quercivorus and Its Specific Fungus

Abstract

The emergence of the ambrosia beetle (Platypus quercivorus Murayama) and its relation to dead oak trees were studied for six years in the mass oak mortality area in Yamagata Prefecture. Investigations of the insect and its specific unknown fungus, considered a possible cause of the mass mortality, have focused on the mutual relationship between the two. Soon after the insects emerge in large numbers from dead oak trees at the end of June, they intensively assault healthy oak trees, and those trees infected are killed in early August. A strong relationship is therefore apparent between the timing of the insect invasion and the tree mortality process. This became clear in an experiment in which the insect disseminated this fungus in the trees. In inoculation tests on oak trees conducted at different times of the year, only those conducted in June resulted in tree mortality. These things strongly suggested that the fungus moved into the trees and was disseminated with the insect invasion and this was therefore related to the mortality process of oak trees.