To prevent pine wilt disease (PWD) from spreading over pine forests, elimination of pine trees killed by the pinewood nematode (PWN), Bursaphelenchus xylophilus is desirable, though this method is very laborious and time-consuming. If such dead trees are left in the field, pathogenic nematodes and their vector, Monochamus beetles, could spread from tree to tree without any difficulty. In our university arboretum, where many precious foreign pine species are planted in the field, all pine trees killed by PWD have been eradicated thoroughly before the next pine wilt season. Despite intensive efforts in removing dead trees from the stands, new dead trees tend to appear in the vicinity of the stumps of trees killed in the previous year, and wilting recurs in the same pine stand every year. Why does PWD recur at the same stand even after thorough eradication of dead pine trees?