【Objective】 The pine wood nematode, Bursaphelenchus xylophilus, is an invasive species that damages pine trees, with significant ecological and financial consequences around the world. In China, the species has recently breached its optimal temperature line and has now spread to several districts in Liaoning Province. In this paper, we outline the reasons for the rapid spread of B. xylophilus to northern China by compiling evidence about its temperature adaptability. We compare two isolates, from southern and northern China, to understand differences in pathogenicity and reproductive ability. 【Method】 We compared the morphological index values of the southern B. xylophilus strain AMA3 with the northern B. xylophilus strain FCBX. We tested the virulence and reproductive ability of both strains of B. xylophilus in Pinus thunbergii material to assess its adaptability. We used a culture of Botrytis cinereal, a fungus which contributes to pine disease, to simulate field pathogenicity conditions, under different temperature conditions.【Result】There were no significant differences in the morphological index values of AMA3 and FCBX, all of which were typical of Umbellifera nematodes. Both strains were cultured with B. cinereal in the temperature range of 15-35 ℃, and we found that increases in temperature correlated with increases in reproduction. Reproduction of FCBX strains was generally higher than that of AMA3 strains, although this was reversed at -30 ℃. At low temperatures (15-20 ℃), FCBX produced significantly more replicates than AMA3 did. After vaccination against AMA3 and FCBX, P. thunbergii specimens had different susceptibility to disease, although FCBX maintained a higher relative pathogenicity than AMA3. Thirty-five days after inoculation, P. thunbergii specimens exhibited a mortality of 80% when infected with FCBX, and the disease index was 85; samples infected with AMA3 had a mortality of 60%, and the disease index was 73. Over this period, FCBX reproduced more than AMA3 strains in the P. thunbergii samples, but this difference was not significant. 【Conclusion】 Compared with southern strains (AMA3), northernB. xylophilus strains (FCBX) had slightly larger morphological indicators, higher reproductive capacity in B. cinereal, activity ability, higher reproduction in P. thunbergii, and greater pathogenicity. As B. xylophilus has moved northward, it has been subject to low temperature conditions, and we suggest it has acclimatized to these conditions. It has now broken through its optimal temperature line, and northern strains exhibit strong reproductive capacity, high pathogenicity and adaptability to low temperature conditions.