Effects of spring-killed pine trees on the epidemics of pine wilt disease

Pine wilt disease (PWD) is caused by the pinewood nematode (PWN), Bursaphelenchus xylophilus (Steiner et Buhrer) Nickle, and is transmitted by cerambycid beetles. In some pine trees infected with the PWNs in Japan, foliage changes from green to brown in summer to autumn of a nematode infection year (summer- autumn-killed trees) and the others in the following spring of a nematode infection year (spring-killed trees). The vector beetles require 1 or 2 years for development in cool summer areas and 1 year in warm summer areas. To evaluate the effects of the spring-killed trees and vectors with a long developmental time on the PWD epidemics, we presented simple mechanistic mathematical models. The models showed that it was possible for spring-killed trees to cause PWD epidemics when the transmission rate was high, and the efficacy of spring-killed trees as infection source was similar to that of summer- autumn-killed trees. Spring-killed trees and vector beetles with a developmental time of 2 years harbored in summer- autumn-killed trees delayed epidemic timing by 3-10 years or actually suppressed epidemics.