Pine wilt disease has been a serious threat to pine trees. Extensive efforts have been carried out to elucidate the disease mechanisms and to control the disease from a variety of research disciplines such as plant pathology, entomology, forest ecology, and mathematical modeling approaches. It has been known that some pine trees infected by the pine wood nematode do not exhibit the disease symptom and these asymptomatic carrier trees could be a source of new beetles that contribute to spread the disease to new susceptible trees. In this study, I define asymptomatic carrier tree as those that exhibit the disease symptom in the next year after the nematode infection. Mathematical analysis shows that 1) The delay in time to exhibit the disease itself does not contribute much to the disease spread, but 2) The effect of asymptomatic carrier trees to attract beetles in early summer is crucial for the disease spread by increasing contacts of beetles with susceptible tress. Empirical measurement and test of the effect of asymptomatic carrier trees to attract beetles is worth to challenge.