─ 87 ─ INTRODUCTION Pine wilt disease, caused by the pinewood nematode (PWN), Bursaphelenchus xylophilus Steiner & Buhrer, is a major threat to pine forests in Japan. In Japan and elsewhere its spread is so rapid and the resulting damage is so serious that to date extensive areas of pine forests have been destroyed (Mamiya, 1988). The PWN occurs over a wide area in Japan and in other Asian countries (Mamiya, 1988, 2004; Yang, 2004). This disease is supposed to have originated in North America (de Guiran and Bruguir, 1989) and recently it has been found in Portugal (Mota et al., 1999). A cerambycid beetle, Monochamus alterna- tus Hope, vectors the PWN (Mamiya and Enda, 1972; Morimoto and Iwasaki, 1972). Numerous PWNs aggregate around M. alternatus pupal chambers (PC) in wilt-killed pine trees and the emerging beetles carry many nematodes and they inoculate them into healthy trees during maturation feeding. The profuse growth of blue stain fungi on the walls of M. alternatus PC serve as a food source for PWNs which prolifer- ate and aggregate in the chambers. Thus, the beetles that emerged from the chambers acquire many PWNs both externally and internally (Maehara et al., 2005). Japanese Journal of Nematology Vol. 36 No. 2 December, 2006 Seasonal changes in the nematode fauna in pine trees killed by the pinewood nematode, Bursaphelenchus xylophilus Rina Sriwati, Shuhei Takemoto and Kazuyoshi Futai 1 The seasonal changes in the number of free-living nematodes and the pinewood nem- atode (PWN), Bursaphelenchus xylophilus, in 15-year-old Japanese black pine (Pinus thun- bergii) trees killed by the PWN were determined. Fifteen species of nematodes were isolat- ed, i.e. one species, each of Mononchida and Plectidae, two species, each of Monhisterida, Rhabditida and Tylenchida, three species of Bursaphelenchus including the PWN, and four species of Diplogasterida. The PWN and one species of Diplogasterida were the most prevalent nematodes isolated. The numbers of PWNs decreased from August to December, 2004, but then increased in February, 2005, then decreased again until June, 2005. During the experimental period the population changes of the Diplogasterida nema- tode mimicked those of the PWN, however, correlations between the numbers of the two nematodes varied considerably both among trees and seasons. Both the PWN and the Diplogasterida nematode were more abundant around the pupal chambers of the vector beetle, Monochamus alternatus, than elsewhere in the tree wood. Jpn. J. Nematol. 36 (2), 87-100 (2006). Key words: pine wilt disease, nematode community, population dynamics. Laboratory of Environmental Mycoscience, Graduate School of Agriculture, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan. Tel. +81-75-753-6060; Fax +81-75-753-6429. 1 Corresponding author, e-mail: futai@kais.kyoto-u.ac.jp