# Chinese Cabbage Changes Its Release of Volatiles to Defend against _Spodoptera litura_
## 4 Discussion
As part of their long-term coevolution with host plants, herbivores may reduce the release of HIPVs so as to reduce the indirect defense involving the attraction of natural enemies. To simulate the feeding of _S. litura_ in the current study, we simply soaked the plants in regurgitate. The total amount of volatiles released was smaller after regurgitate treatment than after larval infestation (Figure 3), which is reasonable because _S. litura_ larvae will both produce regurgitate and destroy a substantial quantity of host plant tissue. According to the results discussed above, we speculate that the backflow of _S. litura_ may have elicitors tochange the volatiles of Chinese cabbage, and the induced volatiles will affect the selection behavior of _M. similis_. Previous studies have shown that the length of time that plants are infested by herbivorous insects will affect the diversity and richness of HIPVs [1, 46, 47]. For example, tomato plants released more (32)-3-hexen-1-yl acetate, \(\beta\)-ocirene, and \(\beta\)-caryophyllene after 12 h than after 6 h of _Trichoplasma ui_ infestation [48]. In this study, we found that the effects of HIPVs released from _S. litura_-infested Chinese cabbage on the selection behavior of _S. litura_ and _M. similis_ differed depending on the duration of the infestation. When the plants were infested by _S. litura_ larvae for 12 h, the released volatiles significantly affected host location by _M. similis_ but not by _S. litura_ (Figures 1c and 2d). After the plants were infested by _S. litura_ for 12 h, the release of (_Z_)-3-hexen-1-ol, (_Z_)-3-hexenyl acetate, ally isothiamate, linalool, and octadecane increased relative to levels in healthy plants; limonene was detected only with infested plants, and butane,1-isothiocyanato only appeared after infestation for 12 h. However, _S. litura_ showed a reduced preference for plants that were infested by _S. litura_ larvae for 24 h (Figure 2d). The release of (_Z_)-3-hexen-1-ol, butane, 1-isothiocyanato, (_Z_)-3-hexenyl acetate, and linalool was highest after 12 h of _S. litura_ infestation and then gradually decreased. Release of 2-hexenal, (_Z_)-3-hexen-1-ol, (_Z_)-3-hexenyl acetate, and linalool was higher early during _S. litura_ infestation than later during _S. litura_ infestation. The release of nonanal, pentadecane, heptadecane and octadecane increased with the infesting time of _S. litura_. Chinese cabbages were able to reduce their attractiveness to _S. litura_ larvae after they had been infested for 24 h but not for 12 h. This is because plants require time for their resistance genes to recognize the effectors of herbivorous insects and to induce immune responses [49]. Plants differ in their immune response times, and Chinese cabbage in our experiment required 24 h to produce an immune response. Correlation analysis indicated that the selection rates of _M. similis_ and _S. litura_ were affected by volatile class and identity. The results showed that the increased release of limonene, linalool, and hexadecane increased the attractiveness of plant tissue to _M. similis_ but that the increased release of (_E_)-2-hexenal and 1-Butene, 4-isothiocyanato reduced the attractiveness of plant tissue to _M. similis_. The increased release of (_E_)-2-hexenal, 1-Butene, 4-isothiocyanato, and decanal increased the attractiveness of plant tissue to _S. litura_, but the increased release of limonene, nonanal, hexadecane, heptadecane, and octadecane reduced the attractiveness of plant tissue to _S. litura_. We can find the herbivore and the parasitoid are expressing opposite attraction to some volatiles; it has become clear that plants respond to arthropod herbivory with the induced production of volatiles, but because they have different degrees of evolution in three trophic level interactions (tritrophic interaction), the effects of HIPVs on _S. litura_ and _M. similis_ would also be different. Plants release specific volatile substances to affect the behavior of different insects to protect themselves.