## 3 Results
### Volatile Compounds Analysis by HS-SPME-GC/MS  
Volatile monoterepenes in pine plants are primarily emitted in response to external stimulation, such as a disease and wound [19; 31]. A total of nine volatile compounds were identified in _P. densiflora_ by HS-SPME-GC/MS analysis (Figure 3). The order of abundant monoterepene in _P. densiflora_ without PWNs infection was \(\alpha\)-pinene \(>\beta\)-pinene \(>\beta\)-phellandrene \(>\) camphene \(>\beta\)-myrcrene \(>\) 3-carene (Table 2). Among them, five monotepenes (\(\alpha\)-pinene, \(\beta\)-pinene, \(\beta\)-myrcrene, 3-carene, and \(\beta\)-phellandrene) were increased by at least two times in the PWN-infected _P. densiflora_ compared with the control trees (Supplementary Table S1). Interestingly, the content of 3-carene was faintly detected in _P. densiflora_ without PWN infection (0.79 \(\upmu\)g/g), but highly enhanced (7.65 \(\upmu\)g/g) after PWN infection. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Peak**} & \multirow{2}{*}{**Compound**} & \multirow{2}{*}{**Classification**} & \multirow{2}{*}{**Retention Time (min)**} & \multicolumn{2}{c}{**Peak Area \(\times\) 10\({}^{-7}\)**} \\ \cline{5-6}  & & & & **Control Tree (\%)** & **Infected Tree (\%)** \\ \hline
11 & \(\alpha\)-cubeene & Sesquiterpene & 15.77 & 0.30 \(\pm\) 0.01 (100) & 0.58 \(\pm\) 0.08 (193.33) \\
12 & copaene & Sesquiterpene & 16.16 & 0.26 \(\pm\) 0.04 (100) & 0.70 \(\pm\) 0.02 (269.23) \\
13 & \(\beta\)-caryophyllene & Sesquiterpene & 17.63 & 0.71 \(\pm\) 0.14 (100) & 1.17 \(\pm\) 0.08 (164.79) \\
14 & \(\beta\)-cubeene & Sesquiterpene & 19.17 & 0.60 \(\pm\) 0.05 (100) & 2.17 \(\pm\) 0.23 (361.67) \\
15 & \(\delta\)-cadinene & Sesquiterpene & 20.19 & 0.15 \(\pm\) 0.03 (100) & 0.55 \(\pm\) 0.02 (366.67) \\
16 & isocombrol & Diterpene & 31.27 & 1.29 \(\pm\) 0.06 (100) & 4.87 \(\pm\) 1.03 (377.52) \\
17 & resin acids & Diterpene & 42.15 & 54.30 \(\pm\) 1.12 (100) & 64.27 \(\pm\) 1.66 (118.36) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 1: Time-lapsed changes of _Pinus densiflora_ and _P. koraiensis_ after infection of pine wood nematode (PWN): (**a**) _P. densiflora_ 0, 20, 40, and 70 days after PWN infection, and (**b**) _P. koraiensis_ 0, 20, 40, and 70 days after PWN infection. Figure 3: HS-SPME/GC-MS analysis of volatile organic compounds in pine wood nematode (PWN)-infected and non-infected _Pinus densiflora_ plants: (**a**) Monoterpenes in total ion chromatograms (TIC) of non-infected _P. densiflora_, (**b**) monoterpenes in TIC chromatograms of _P. densiflora_ after PWN infection, and (**c**) chromatograms of monoterpenes authentic standards. 1. oxime-methoxy-phenyl, 2. tricyclene 3. \(\alpha\)-pinene, 4. camphene, 5 \(\beta\)-pinene, 6. \(\beta\)-myrpene, 7. 3-carene, 8. \(p\)-cymol, 9. \(\beta\)-phellandrene. Figure 2: GC-MS analysis of terpenoids in resin obtained from pine wood nematode (PWN)-infected and non-infected _Pinus densiflora_ plants: (**a**) Resin compounds of control _P. densiflora_, and (**b**) resin compounds of PWN-infected _P. densiflora_. 1. \(\alpha\)-pinene, 2. camphene, 3. \(\beta\)-pinene, 4. \(\beta\)-myrcene, 5. 3-carene, 6. \(\beta\)-phellandrene, 7. terpinolene, 8. L-pinocarveol, 9. methylthymylether, 10. acetic acid, 11. \(\alpha\)-cubebene, 12. copaene, 13. \(\beta\)-caryophyllene, 14. \(\beta\)-cubebene, 15. \(\delta\)-cadinene, 16. isocembrol, 17. resin acids. Another susceptible pine species, _P. koriensis_, had represented the different monoterpenes proportion composed of a total of nine volatile compounds (Figure 4). The order of major monoterpenes in _P. koriensis_ were in the order \(\alpha\)-pinene \(>\beta\)-pinene \(>\) 3-carene. The amount of volatile monoterpene of _P. koriensis_was lower than _P. densiflora_ (Table 2). The compositions of volatile monoterpene released from _P. densiflora_ and _P. koriensis_ were similar, except \(\beta\)-phellandrene and D-limonene. The \(\beta\)-phellandrene was only identified in _P. densiflora_, but D-limonene was identified only in _P.