## 1 Introduction  
Pine wilt disease (PWD) is a devastating disease of pine trees caused by _Bursaphelenchus xylophilus_ (pine wood nematode; PWN) Nickel (Aphelenchida: Parasitaphelenchidae). The PWN is native to North America and is now found in the USA, Canada, Mexico, China, Korea, Japan, Portugal and Spain [1, 2, 3, 4]. Since the introduction of PWN into China in 1982, despite the adoption of various measures to prevent its spread, the occurrence of PWD has exceeded expectations. PWD has occurred in 666 county-level administrative regions of 19 provinces (autonomous regions and municipalities directly under the Central Government), with a total area of 1,114,600 \(\mathrm{h}\mathrm{m}\mathrm{{}^{2}}\) and 19,467,400 dead pine trees in 2019, which has resulted in severe direct economic losses and losses of ecological service value equal to hundreds of billions of yuan [5]. The control of PWD has been studied for decades, including chemical, physical and biological, but prevention spread has not yet been achieved [6; 7; 8]. In field studies on the control of PWD where a large number of trees had died, a small number of trees survived [9]. It is apparent that trees have some resistance to PWNs, and the underlying mechanism of this resistance has attracted the attention. Studies suggest that pine trees resist infection through a combination of internal tissue structural changes, physiological and biochemical responses [10; 11; 12; 13], and gene regulation. After the PWN infects a pine, the tree produces a series of defense responses. In previous studies, it was found that the main signaling molecules for plant defense responses are reactive oxygen species (ROS), Ca\({}^{2+}\), phytohormones and volatile organic substances (VOS) [14; 15]. Previous studies on pine resistance have mainly focused on ROS metabolism related substances and the regulation of plant hormones. The role of VOS in pine resistance to PWN has yet to be elucidated. [PERSON] and [PERSON] show that confers produce terpenoids that negatively affect the physiology of invading species or offspring [16]. [PERSON] et al. confirmed the inhibitory effects of trans-2-hexenal on the growth, reproduction and efficacy against of PWNs [17]. In addition, coniferous trees emit more monoterreponids and sesquiterpenoids [18], and terpenoid concentrations may vary among different provenances of trees [19], therefore, it has been speculated this differentiation may partly explain resistance to PWD. Further research is needed to determine the differences in VOS among the families of the same pine tree species exhibiting varying resistance. It has also been found that _Pinus. sylvestris_ (L.) synthesizes and releases a large number of secondary metabolites, of which VOS account for a large proportion [20]. These VOS play an important role as chemical signals in the resistance of the host plant to disease. [PERSON] et al. [21] found that the main VOS of the _P. massoniana_ (Lamb) with different levels of resistance were terpenes such as \(\alpha\)-pinene, camphene, \(\beta\)-pinene and \(\beta\)-watercressene, but the pines species that are highly resistant to the PWN released more types of odor components than the susceptible _P. massoniana_, indicating that there may be correlation between the VOS of _P. massoniana_ and the ability to resist the PWN [21]. By measuring the major terpenes in two _P. thunbergii_ (Parlatore) stands infected with PWD, [PERSON] and Futai [22] found that the release of some terpen, such as \(\alpha\)-pinene, began to increase in summer, overlapping with the egg-laying season of the vector beetle, which may influence the development of symptoms in trees. It has also been suggested that large amounts of volatile terpen are produced in pine trees inoculated with PWN, which enter the canal cell of xylem and lead to the formation of canal cell cavities that impede the water transport process, thus causing the plant to wilt and die [23]. However, VOS and their role in PWD is an understudied area; the differences between resistant and susceptible _P.