|On the southern slopes of Purple Mountain in the Zhongshan Park area to the west of Nanjing there are several culturally significant buildings including Dr. Sun Yat-sen's Mausoleum. More than 300 steps must be climbed to reach this sacred shrine.|
|In 1982, five black pines, Pinus thunbergii, in the area turned red. Close examination showed that they were infested with the pinewood nematode, Bursaphelenchus xylophilus. This nematode feeds within the tissues of the tree blocking the sapwood which causes the death of the tree. The first signs of attack are the red needles and thinning of the crown.|
|The pinewood nematode is not able to move from tree to tree on its own. The main vector in this part of China is the sawyer beetle Monochamus alternatus. The beetle is infested by the pine wood nematode when it transforms from the pupa to the adult. Young LIII pine wood nematodes find their way into the spiracles of the adult. The beetle emerges and flies to the crown of a healthy tree for maturation feeding on the young branches. While the beetles are feeding, the LIII nematodes enter the wounds in the tree and infest the sapwood and bark tissues.|
|The nematodes rapidly colonize the new host tree blocking the sapwood. These symptoms of dying branches are known as pine wilt disease. The dead tree is now a suitable host for rearing the young of the sawyer beetles.|
|The female lays her eggs under the bark and the young round-headed borer larvae feed in the softer inner bark tissues. When they have developed strong mandibles, they bore into the trunk itself. They continue to feed here until they are mature. The mature larvae bore back to the sapwood area in the outer region of the trunk where they excavate a larger chamber in which they pupate. It is during the transformation to the adult stage when the pinewood nematode, which has been feeding and multiplying in the tree tissues, infests the tracheal system of the adult. The adult emerges by chewing a neat circular exit hole. The larval galleries in the trunk show up as grub holes when the trunk is milled.|
Chinese scientists have suggested the following control strategies.
Application of these tactics in an integrated pest management strategy has reduced the rate of spread from 20,000 ha/yr to less than 10.000 ha/yr. More than 2 million pine trees have been killed since 1982.
The major method of spread of this disease throughout the world has been in the low grade packing timbers that have been used for packaging of heavy freight. Grub holes with blue staining fungi in such crating materials indicate that sawyer beetles have been in the log. It takes a year for the sawyer beetle to complete its growth in a log or piece of lumber. Young beetle grubs can survive very well in cut lumber. During the time the lumber is used for crating up heavy freight, the grubs are able to continue growing. Importing countries generally discard these packing timbers. At this time the beetles complete their development and emerge into a new habitat (complete with their pine wood nematodes! Extra quarantine measures need to be taken to prevent accidental introductions. All packing cases and ship dunnage used to secure cargo in holds during sea voyages must be collected and destroyed portside as soon as possible.
Research is being carried out in Jiangsu Province to determine the resistance of Pinus massonia to pinewood nematode. Inoculation trials are currently underway in a large provenance study at the Jiangsu Forest Research Institute to determine those provenances that are most resistant to the pinewood nematode.
Acknowledgment: I thank Ge Minghong, Wang Qiming and Xu Fuyan of the Jiangsu Forest Research Institute for their kind hospitality on June 12, 1996 and for showing me the details of their work. For further information on the pine wood nematode, see the Abstracts from the International Symposium on pine wilt disease caused by the pine wood nematode that was held at the 1995 International Congress of Entomology held in Beijing, China.
John A. McLean
Aug 12, 1996
For those who are interested in the view from the top of the 300 steps climbed to Dr. Sun Yat-sen's Mausoleum, click here.