**3. The hypothesis**

Given that there had been no report on the occurrence of *B. xylophilus* in Burma (CABI & EPPO, 1997), it was not logical to speculate the pine wood nematode in Wanding came from any Burmese source. Moreover, Wanding, the only site of infestation, lies in the west portion of the LRGR, which is regarded as natural barriers obstructing organisms from expanding by natural means (He et al., 2005). Therefore, it was also not logical to speculate that the nematodes came from adjacent areas in Yunnan. However, after a careful patrol of the initial infested pine stand, investigators discovered that the infested trees was centered an under-

Fig. 1. The distribution pattern of the pine wilt disease (red dots) in mainland China based

With the expansion of pine wilt disease, an interesting distribution pattern was revealed. In eastern and southern China, the distribution concentrated near harbors, like the deltas of the Yangtze River and the Pearl River, while in the central portion of China, the infested areas are mostly along the Yangtze River. Such a distribution pattern suggested that the dispersal

The occurrence of the pine wilt disease in Yunnan Province, southwestern China was distinct. Not only because it was one of the latest infested areas after the massive expansion, but also due to its unique geographical position. Unlike eastern China where many harbors are distributed, the first, single, and isolated pine stand (*Pinus kesiya* var. *langbianensis*) in Yunnan infested by *B. xylophilus* was located in the westernmost corner, a remote township

Given that there had been no report on the occurrence of *B. xylophilus* in Burma (CABI & EPPO, 1997), it was not logical to speculate the pine wood nematode in Wanding came from any Burmese source. Moreover, Wanding, the only site of infestation, lies in the west portion of the LRGR, which is regarded as natural barriers obstructing organisms from expanding by natural means (He et al., 2005). Therefore, it was also not logical to speculate that the nematodes came from adjacent areas in Yunnan. However, after a careful patrol of the initial infested pine stand, investigators discovered that the infested trees was centered an under-

on the historical data from 2004 ~ 2011, data source: SFA (2004 ~ 2011).

of *B. xylophilus* was closely related to transportation.

**3. The hypothesis** 

named Wanding near the Sino-Burmese border (SFA, 2007).

constructing telecommunication facility, which consumed a considerable amount of electronics manufactured in eastern China (Z.Q. Li, pers. comm.). Hence, a reasonable hypothesis was conceived that the nematodes in Wanding were casually introduced with non-local *M. alternatus* populations hidden in the wood-packaging materials. Providing that *M. alternatus* from different localities possess different genetic profiles, the non-local individuals can then be distinguished by analyzing the genetic differences on population level, and the source can be traced by comparing the genetic profiles with the samples taken from the suspected region.
