Relationship between the initial number of carried Bursaphelenchus xylophilus and its transmission by Monochamus carolinensis with reference to virulence

The pine wood nematode (PWN), Bursaphelenchus xylophilus, is the causative agent of pine wilt disease and is transmitted by cerambycid adults of the genus Monochamus. It is believed to have been introduced into Japan from North America in the 1900s and since then it has been devastating the native pine forests. By contrast, the epidemics of the disease have never occurred in pine forests native to North America in the original habitats, indicating no expression of virulence. To determine the evolutionary change in the transmission traits of PWN, the temporal pattern of PWN transmission to fresh branch sections of Pinus sylvestris was studied using 38 Monochamus carolinensis adults in Illinois, USA, and compared with previous studies on the PWN-M. alternatus and B. mucronatus-M. saltuarius systems in Japan. A great difference was observed in the initial nematode load among vectors irrespective of the three systems. The maximal numbers of PWN transmitted (5 days)-1 by individual vectors were greater and the efficiencies of nematode transmission and invasion were higher for the Japanese PWN-M. alternatus system than for the North American PWN-M. carolinensis system in two heavy classes of initial load of over 1000 nematodes. The proportion of PWN transmitted to healthy pine trees by reproductively immature female vectors was small (2.5-11.5%) in the three nematode-vector systems in most cases. The evolutionary changes in the transmission traits and the control of the range expansion of the PWN were discussed in relation to the expression of virulence.