## 4 Discussion  
The PWN symptoms after PWN infection in _P. densiflora_ and _P. koraiensis_ were investigated. The external symptoms (wilting leaves and discoloration) of pine tree had appeared after 20 days of PWN infection and showed clear wilting of needles after 40 days. Thus, there will be a lot of physiological and chemical changes at around 20 days after PWN infection. PWNs are used parenchyma cells in the resin canal as food sources [3]. PWN infection affected the chemical compositions of oleoresins [9]. We investigated the secreted resin in control and PWN-infected _P. densiflora_ by the GC. The resin acids were highly increased in the resin of PWN-infected plants. 3-Carene is one of the monoterpenoids, which was undetected in control resin but somewhat prominently increased in the resin of PWN-infected plants. The biosynthesis and emission of terpenids are rearranged by the abiotic and biotic stress factors such as drought, temperature fluctuations, air and soil pollution, or pathogen attack [32; 33]. The PWN migrates to the host tree when insect vectors (_Monochamus_ spp.) feed on the young leaves of pine tree. The insect injury was associated with profile of volatile terpenes of the beetles' host tree [33]. PWD after PWN infection occurs by interrupted water-conduction in sapwood by cavitation of tracheids [34]. The tracheid cavitation (embolism) is induced by a significant increase in volatile terpen (monoterpenes) [35]. [PERSON] et al. [36] reported that limonene and abietadiene were emitted by PWN-infected _P. pinaster_ branches, and abietadiene emission in PWN-infected branches was interpreted as a result of tree defensive response. The chemical variability of volatile essential oil profiles was found in some pine species, but obvious changes of the essential oil chemicals were showed in the PWN inoculation area [37]. SPME has a number of advantages, including simplicity, high sensitivity, and a relatively non-invasive nature [38]. In HS-SPME/GC-MS analysis of volatile monoterpenes emitted from _P. densiflora_ and _P. koraiensis_ after PWN infection, emission of some monoterpenes was clearly enhanced in PWN-infected pine trees. The amount and proportion of monoterpenes were different between _P. densiflora_ and _P. koraiensis_. \(\beta\)-Phellandrene and D-limonene were specifically detected in _P. densiflora_ and _P. koraiensis_, respectively. The other five monoterpenes were commonly detected in both pine species, but the proportion of monoterpenes was different between the two species. Other studies also confirmed that three monoterpenes, \(\alpha\)-pinene, \(\beta\)-myrrene, and \(\beta\)-phellandrene, from _P. densiflora_ are emitted dominantly, depending on location or season in the forest of Korea [39; 40; 41] and Japan [42]. The major monoterpenes in _P. koraiensis_ were \(\alpha\)-pinene, camphene, \(\beta\)-myrrene, and D-limonene [43]. [PERSON] et al. [44] reported that the terpene emission rates of _P. koraiensis_ were much lower than the other conifers. This result suggested that types and amounts of monoterpene emission are different among _Pinus_ species. Interestingly, 3-carene was highly correlated to PWN infection among the volatile compounds in _P. densiflora_ and _P. koraiensis_. After PWN infection, 3-carene was increased clearly, but it was rarely detected without PWN infection in _P. densiflora_. In _P. koraiensis_, emission of 3-carene was remarkably enhanced (54.7 times in 20 days) after PWN infection. This result indicates that biosynthesis and emission of 3-carene may be involved in the reaction of PWN infection. It was reported that the specific ratio of monoterpenes influenced PWN behavior [45]. The monoterpenes \(\alpha\)-pinene and \(\beta\)-pinene were reported as predominant constituents of Masson pine, whereas their relative amounts and proportions had changed with respect to the progression of PWD [45; 46; 47]. According to our continuous literature review, no report has been found on the change of 3-carene in pine plant after infection of PWNs.