To study the changes of parasitic adaptability and pathogenicity of Bursaphelenchus xylophilus with different virulence after cyclic infection on pine trees with different resistances. We selected high virulent strain ZJ and low virulent strain YW4 of B. xylophilus and inoculate them into different resistant pine seedlings(resistant Pinus massoniana, P. taeda, and P. thunbergii). By measuring the pathogenicity, relative host suitability(RHS) coefficient, population sex ratio, and cellulase activity of B. xylophilus, the changes in parasite adaptability and pathogenicity of B. xylophilus were explored. The results showed that the susceptibility of different resistant pine seedlings, whether inoculated with high virulent strain ZJ or low virulent strain YW4, was in order of P. thunbergii > P. taeda > resistant P. massoniana. After 3 cycles of inoculation with nematodes, the incidence of the 3 pine

seedlings increased; the RHS coefficients of both the high virulent and low virulent B. xylophilus populations were significantly increased. The female-to-male ratio of most populations of B. xylophilus increased after cyclic infection of resistant hosts. The detection of changes in cellulase activity of the B. xylophilus populations showed that the cycle inoculation of different resistant hosts increased the cellulase activity of B. xylophilus populations to varying degrees, and the enhancement effect of circulating infection with high resistance P. massoniana seedlings was the most significant. The B. xylophilus strains after circulating inoculation were re-inoculated into P. thunbergii seedlings for pathogenicity determination. It was found that the incidence time of P. thunbergii seedlings inoculated with the high virulent and low virulent B. xylophilus populations was shorter than that of the controls. The pine seedlings inoculated with the ZJ-Ma and ZJ-Huo populations had the fastest incidence, and the pine seedlings inoculated with the YW4-Huo and YW4-Hei populations had the slowest incidence, but they still occurred 12 days earlier than the control. Also, the incidence of pine seedlings inoculated with YW4-Ma, YW4-Huo, and YW4-Hei populations was as high as 87.5%, 87.5%, and 75%, respectively. These results indicated that B. xylophilus can increase the population adaptability, proportion of female and pathogenicity by infecting pine seedlings multiple times, and the enhancement effect when circulating infesting resistant pine seedlings is more significant.