Modeling the spread of pine wilt disease caused by nematode (Bursaphelenchus xylophilus) associated with its vector, the Japanese pine sawyer (Monochamus alternatus)

Akiko Yoshimura, Kohkichi Kawasaki*, Fugo Takasu, Katsumi Togashi**, Kazuyoshi Futai***, and Nanako Shigesada

Department of Information and Computer Sciences, Nara Women's University, Kita-Uoya Nishimachi, Nara 630-8506, Japan
*Department of Knowledge Engineering and Computer Sciences, Doshisha University, Kyo-Tanabe 610-0321, Japan
**Faculty of Integrated arts and Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 793-8521, Japan
***Laboratory of Applied Botany, Faculty of Agriculture, Kyoto University, Kyoto 606-8224, Japan

Ecology, 80(5), 1999, pp. 1691-1702.

Abstract: An epidemic of pine wilt disease has been spreading in wide areas of Japan for nearly a century. The disease is caused by the pinewood nematode, Bursaphelenchus xylophilus, with the pine sawyer, Monochamus alternatus, as vector. The spread of disease is facilitated by an obligatory mutualism between the nematode and the pine sawyer, such that the pine sawyer helps the nematode transmit to a new host tree, while the nematode supplies the pine sawyer with newly-killed trees on which to lay eggs. We present a mathematical model to describe the host-vector interaction between pines and pine sawyers carrying nematodes on the basis of detailed data on the population dynamics of pine sawyers and the incidence of pine wilt disease at a study site located on the northwest coast of Japan. We used the model to simulate the dynamics of the disease and predict how the epidemic could be controlled by eradication of the pine sawyer. The main results are: 1) There is a minimum pine density below which the disease always fails in invasion. However, even if the pine density exceeds this minimum, the disease fails in invasion due to the Allee effect when the density of pine sawyers is very low. 2) The minimum pine density increases disproportionately with increase in the eradication rate. 3) The probability for a healthy tree to escape from infection until the epidemic dies out decreases sharply with increase in the initial pine density or the initial density of pine sawyers.