Dispersal of Bursaphelenchus xylophilus Invading Dead Pinus thunbergii Trees through Oviposition Wounds Made by Monochamus alternatus and Transfer of the Nematodes to Vector Insects.

Abstract

The pinewood nematode, Bursaphelenchus xylophilus, is the causative agent of pine wilt disease (PWD). The nematode invades healthy pine trees via feeding wounds and dead pine trees via oviposition wounds made by the Japanese pine sawyer, Monochamus alternatus. We investigated the dispersal pattern of the nematodes invading Pinus thunbergii logs via oviposition wounds and the number of nematodes carried by the vector insects emerging from the logs. Trees killed by felling (felled trees) and those killed by natural infection of PWD in the field (PWD trees) were prepared, and then they were oviposited by M. alternatus. Nematode positive reaction in wood without oviposition wounds, larval frass and emergence holes of the adult insects was significantly higher in the PWD than the felled trees. Although there was no difference in the number of nematodes carried by the insects between the felled and PWD trees, the frequency of the insects carrying more than 10,000 nematodes was significantly higher in the felled than the PWD trees. Our results experimentally suggest that the nematodes invading dead trees from oviposition wounds did not tend to spread throughout the trees and that numerous nematodes were carried by the vector insect even if the trees were killed by some factor other than PWD.