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.

Caenorhabditis japonica forms a species-specific and female host-biased phoretic association with the shield bug Parastrachia japonensis. To determine the distribution of C. japonica, nematodes from P. japonensis were surveyed in islands ranging from Kyushu to Okinawa of Japan. C. japonica was isolated from all 6 sampled populations of P. japonensis from Kyushu, Amami Oshima, and Tokunoshima Islands, but not from those from Okinawa Island. The limited distribution of C. japonica in these islands is suggestive of the origin of C. japonica and its limited phoretic association with P. japonensis in this region.