# Economic Loss of Pine Wood Nematode Disease in Mainland China from 1998 to 2017
## 4 Discussion  
The pine wood nematode disease in China has continuously expanded since 1998. Due to global warming, the suitable range of pine wood nematodes has continued to expand, and new vector insects and susceptible tree species have been developed in the process. In 2012, pine wood nematode disease was found in Tianzhushan Forest Park, with an annual average temperature of only 7.9 \({}^{\circ}\)C, in Shanyang County, Shaanxi Province, at an altitude of 1700-1800 m. For the first time, _Pinus armandii_ [PERSON] was identified as a natural susceptible tree species for pine wood nematodes [54]; for the first time in 2016, the _Monochamussaltuarius_ [PERSON] was confirmed to be a transmission vector of pine wood nematodes, and Korean pine is another new host of Chinese pine wood nematodes [55]. The number pine wood nematode outbreaks in China is soaring, and human activity is an important factor affecting the spread rate and scale of pine wood nematode disease [56]. Coastal cities such as Shantou City in Guangdong Province and Dalian City in Liaoning Province became the first portals in the province infected and spread inward, indicating that the long-term expansion of pine wood nematodes is due to the transportation of pine wood nematode logs [25], as sea transportation played a huge role in transportation. In addition, Henan and Guangxi Zhuang Autonomous Regions have relapsed after the epidemic area was successfully eliminated, which may be due to the fact that the two provinces have planted horsetail pines and black pines with different degrees of latent infection [57]. Pine wood nematode disease has an incubation period, which cannot be detected and treated in time, leading to re-spread. The original technology of Yunnan Province to use _Poria cocos_ for the harmless treatment of pine wood nematode disease has made outstanding contributions to the effective control of pine wood nematode disease. In 2004, after the first discovery of pine wood nematode disease in Ruili City, the area of the disease continued decreasing, and the affected area was  
Figure 4: Average economic losses of pine wood nematode disease in mainland China from 1998 to 2017.  
finally removed within 5 years. After that, the disease was discovered again in 2013. However, due to the small area of the disease, the treatment was quickly successful. From 1998 to 2017, the economic losses of the pine wood nematode disease in China showed regional disparities, and the southeast region was the worst-hit area for disease economic losses. On the one hand, due to the long outbreak and large disease area in the southeast region, the pine wood nematode disease occurred in Jiangsu, Anhui, Shandong, Zhejiang and Guangdong provinces for 20 consecutive years, and the scale of the disease is expanding. On the other hand, the quality of forest resources in southern China is high, and pine forests are widely distributed. Especially in Guangdong, where there is sufficient rainfall and suitable climate conditions, the forest ecosystem per unit area has high value, so the losses are huge. In addition, Liaoning Province is located in the northeast forest farm, and pine wood nematodes have developed new hosts and new vector insects here, so they spread very quickly. The area of disease increased 10 times within a year, and economic losses increased rapidly. In this study, a long-term series of dynamic evaluation was achieved using the forestry output value index and dynamic equivalent factor method. The forestry output value index is a continuity index that reflects the inter-annual changes in forestry production. Using this index, the calculation parameters and evaluation results can be converted and discounted to the currency level of the current year to achieve a dynamic evaluation of direct economic losses.