Population densities of the pine wood nematode, <i>Bursaphelenchus xylophilus</i>, in dead pine trees caused by pine wilt disease in cool areas of Japan

Pine wilt disease caused by the pine wood nematode, Bursaphelenchus xylophilus, (PWN), is the most devastating disease of pine trees (Pinus densiflora and P. thunbergii) in Japan. The disease has spread to the northern part of Japan, where the cool climate (annual mean temperature < 12℃) delays disease development. The current study, which was conducted in northern Japan, demonstrated PWN population dynamics in dead pine trees infected with PWN in the previous year and died in early spring through early summer. PWN numbers in dead trees were lower in the cold climate than previously reported in warm climates. PWNs were widely distributed throughout each dead tree and had a contagious distribution. Oviposition scars of the PWN vector, Monochamus alternatus, were not detected on the dead trees killed in spring to early summer because the time of disease development and tree death did not coincide with the time of insect oviposition.