Frequency distribution of the initial number of Bursaphelenchus xylophilus carried by adult Monochamus carolinensis in Illinois, with reference to that by M. alternatus in Japan

The pine wood nematode (PWN), Bursaphelenchus xylophilus, is the causative agent of pine wilt disease and is transmitted primarily by cerambycid adults of the genus Monochamus. It is believed to have been introduced into Japan from North America in the 1900s and confirmed to have invaded western Europe in 1999. The nematode has been devastating pine forests in invaded areas because the native pine species are susceptible. This study examined the number of PWN carried by individual M. carolinensis adults (initial PWN load) immediately after emerging from dead Pinus sylvestris and P. strobus trees in Illinois, USA. The data were compared with the initial PWN load on M. alternatus adults emerging from dead P. thunbergii trees in Japan ca 80 years after the PWN invasion. When the zero-inflated negative binomial model was fitted to the initial PWN loads it provided three results. First, the PWN exhibited clumped distributions among vectors in the PWN-M. carolinensis-P. sylvestris or P. strobus system in Illinois and the PWN-M. alternatus-P. thunbergii system in Japan. Second, there was no difference in the frequency distribution of the initial PWN load between the three disease systems. Third, the initial PWN load increased as the overwintering PWN density in wood or the adult body mass increased, whereas it decreased as the emergence date of vectors increased. Consequently, the frequency distribution of initial PWN load exhibited no evolutionary change in Japan and the initial PWN load was considered to be determined by environmental factors and conditions.