# Chinese Cabbage Changes Its Release of Volatiles to Defend against _Spodoptera litura_
## 4 Discussion  
The tritrophic interactions among plants, herbivores, and their parasitoids and influence each other in the long course of coevolution [12]. Plants use an array of biochemical and morphological properties to defend against attacks by herbivores. For direct defense, plants produce organic compounds that interfere with the behavior of herbivores [10; 32; 33]. For indirect defense, the HIPVs produced by plants can attract natural enemies of herbivores [6; 9; 23; 34]. This study shows that Chinese cabbage releases volatiles that can suppress _S. litura_ larvae from choosing leaves that are already exploited. However, when the larvae choose to feed on unattacked leaves, they face the risk of being found by _M. similis_. These patterns support the proposition that plants have evolved to manipulate the blend of volatiles that are emitted in order to maximize both direct and indirect defenses against herbivory. Olfactory receptors can be used by herbivovons insects to help locate hosts or to avoid danger [35; 36; 37; 38]. Studies on the oviposition behavior of _S. litura_ have shown that the adults of _S. litura_ can rely on plant volatiles to find the most suitable hosts but to avoid plants already infested by _S. litura_ larvae [39; 40]. Before the current study, however, little was known about how host plant volatiles affects the feeding of _S. litura_ larvae. We found that _S. litura_ larvae prefer undamaged to damaged cabbage tissue and showed significant responses to HIPVs (Figure 2b,d). The characteristics of this feeding behavior are consistent with those of other Nocutiade insects. _S. exigua_, for example, showed obvious antifeedant and evasive responses to corn HIPVs [17]. In the current study, we found that _M. similis_ females were attracted to both _S. litura_ regurgitate-treated plants and _S. litura_-infested plants, but when given a choice among volatiles produced by healthy, mechanically damaged, regurgitate-treated, and _S. litura_-infested plants, they strongly preferred volatiles produced by _S. litura_-infested plants (Figure 1). In response to damage caused by herbivovons insects, plants can regulate lipoxygenase, isoprenoid, and shikimate pathways in order to release compounds that attract natural enemies of herbivores for indirect defense [41]. This phenomenon has been previously reported. When infested by _S. exigua_, for example, _Zea mays_, _Nicotiana tabucum_, and _Gossypium hirsutum_ release volatiles that attract wasps that parasitize _S. exigua_[13; 23; 42]. To determine whether a single factor in _S. litura_ larval feeding affects the behavior of parasitic wasps, we simulated the effects of mechanical damage and herbivore reguritate on the behavior of _M. similis_. We found that, compared with responses to clean air, _M. similis_ females showed strong responses to the volatiles of regurgitate-treated plants but not to the volatiles of mechanically damaged plants (Figure 1a,b). Mechanical damage to plants also failed to attract _Microplitis palidipes_[43]. When comparing the effects of the treatments on the release of classes of volatiles, we found that there was no significant difference between the mechanical damage treatment and the healthy treatment, but that the _S. litura_ regurgitate treatment reduced the release of aldehydes and alkanes (Figure 3). We found that the _S. litura_ regurgitate treatment reduced the release of butane,1-isothiocyanato and 1-Butene,4-isothiocyanato, and increased the release of nonanal, pentadecane, and heptadecane (Figure 4). A reduction in the release of specific volatiles by plants has also been found for other Nocutiade. For example, glucose oxidase in _Helicoverpa zea_ regurgitate can reduce the release of nicotine in tobacco [44]. In another study, the regurgitate of _S. frugiperda_ reduced the release of corn HIPVs but did not affect the attraction of the parasitoid _Cotesia marginitvertis_ to _S. frugiperda_-infested corn [45].