Abstract
The evolutional relationship between insect phoretic nematodes and their vector insects are discussed from the viewpoint of the occurrence of plant parasitism of nematodes. The xylophilus group of the genus Bursaphelenchus is a group of phoretic nematodes that use the Lamiini longhorn beetles as their carrier insects, and several species in the group, B. xylophilus, B. mucronatus, and B. firmae are known to have strong or weak virulence in pine trees. However, regardless of the phylogenetic closeness to those virulent species, B. doui does not have virulence to their host (habitat) pine trees. Then, the presence/absence of plant pathogenicity of nematodes and the life history traits of their vector insects were compared, suspecting that the beetles carrying virulent species have more opportunities to be associated with the healthy wood tissue of the plants during their feeding and oviposition. This tendency seemed also applicable to other two plant parasitic Bursaphelenchus species, B. cocophilus, and B. sexdentati. The plant parasitism (pathogenicity) of Bursaphelenchus nematodes was hypothesized to have occurred as an ability to enter and survive in the healthy plant tissue, i.e., the occurrence of plant pathogenicity seems to depend on the life history traits of carrier (vector) insects.
