## 1 Introduction
_Monochamus alternatus_ is a wood-boring insect, widely distributed from East to South Asia, including China, Japan, Korea, Laos, Taiwan, Vietnam, and some other parts of the world, including Canada, Denmark, and the United States [35]. The larvae of this beetle feed and pupate in the phloem and xylem of host plants, the newly emerged adults mate after pre-mating feeding, and the female adults spawn in the ovipositing incisions [36] (Figure 1). _M. alternatus_ can spread pine wood nematodes (_Bursaphelenchus xylophilus_), which are the causal agent of pine wilt disease, during adult feeding [37; 38]. _P. massoniana_ is an important tree species widely distributed in southern China because of its high tolerance of acidic soils and high oleoresin yield, with a distribution area of about 20 million square kilometers in China. However, _P. massoniana_ is the preferred host of _M. alternatus_ and is seriously damaged by pine wilt disease. Since the disease was first discovered in China in 1982, it has killed billions of pine trees, causing huge economic losses, and is the main threat to the healthy pine forests widely distributed in China [39]. In our previous study, the terpene-based defense of _P. massoniana_ was induced by _M. alternatus_ adult feeding. Transcriptome sequencing and paraffin sections showed that most TPSs of _P. massoniana_ showed increased expression, which was followed by the formation of traumatic resin ducts and release of terpenoids [18; 40]. Among the released volatile terpenoids, monoterpenoids and sesquiterpenoids had the highest contents, including \(\alpha\)-pinene, \(\beta\)-phellandrene, \(\beta\)-pinene, \(\beta\)-myrcene, camphene, caryophyllene, and longifolene. In addition, many diterpenes are released. We also proved that methyl jasmonate induction significantly promoted the release of volatile terpenoids from _P. massoniana_[41]. At the same time, the pre-induction of methyl jasmonate significantly reduced the feeding area of the _M. alternatus_ adult [41]. These findings indicate that the terpenoids are important secondary metabolites of _P. massoniana_ in response to _M. alternatus_ adult feeding, and TPSs are involved in the biosynthesis of terpenoids. Terpenoids play a major role in _M. alternatus_ host and mate localization, as well as in chemical communication among the pine wood nematode, _M. alternatus_, and _P. massoniana_[42; 43; 44]. Some TPSs in _P. massoniana_ have been identified, such as Pm GGPPS2, which affects the synthesis of diterpenoids [45], as well as Pm TPS4 and Pm TPS21, which regulate the synthesis of \(\alpha\)-pinene and longfolene and participate in the defense against pine wood nematodes [46]. Nevertheless, the links between the structures of TPSs and their products in _P. massoniana_ remain unclear. In this study, we first modelled the structures of six TPSs in _P. massoniana_ that had been found to be significantly induced by _M. alternatus_ adult feeding, including four mono-TPS genes, _TPS (+)-\(\alpha\)-pinene, TPS (\(-\))-\(\alpha\)-pinene, TPS (\(-\))-\(\beta\)-pinene_, and _TPS limonene_, and two sesqui-TPS genes, _TPS selinene_ and _TPS longfolene_, after cloning and analyzing their sequences. Prokaryotic expression was then performed for monoterpene synthesis to determine their final products. The results from this study will help reveal the potential correlations between the structures of TPSs and their corresponding terpene products, which will provide a theoretical basis for the transgenic breeding of _M. alternatus_ resistant Masson pines to control pine wilt disease.