Commensal relation between Bursaphelenchus xylophilus (Nematoda: Aphelenchoididae) and Monochamus alternatus (Coleoptera: Cerambycidae) within pine trees

Abstract

Bursaphelenchus xylophilus (Steiner et Buhrer) Nickle causes pine wilt disease and is transmitted by Monochamus alternatus Hope adults in Japan. To determine the effects of the beetle larvae on the nematode population and the effects of the nematode population on beetle larval growth, we inoculated fresh pine bolts with various combinations of B. xylophilus, M. alternatus, and the blue-stain fungus Ophiostoma minus (Hedgc.) Syd. & P. Syd., the food of the nematodes. We also compared the content of ergosterol among the phloem, xylem, and larval frass. The nematode density was 22 or 25 times higher in frass than in phloem and the lowest in xylem, indicating positive effects of beetle larvae on the nematode population growth. Frass had twice higher ergosterol content than the phloem, indicating the fungal mycelial content was greatest in the frass. The difference in the content of fungal mycelia could explain in part the difference in B. xylophilus density between the frass and phloem. By contrast, B. xylophilus did not affect the growth and developmental rate of M. alternatus. This study showed that the relation between the beetle and nematode populations changes from mutualism to commensalism corresponding to the dispersal to reproduction stages of the nematode population.