## 4 Discussion
The terpenoids \(\delta\)-elemene and L-linalool were only detected in young leaves. In our subsequent experiment using potted plants (A1-A5), 14 unique compounds were found, mostly aldehydes and alkanes (Table 1, Figure 2). In the PCA analysis from samples A1 to A5, the abundant aldehydes nonanal and decanal were found to have a strong discrimination effect among the LEM infestation levels, separating them into two clusters (Figure 3). The first cluster included the initial infestation (A1), heavy infestation (A3) and uninfested plant (A5) treatments, and the second cluster included the intermediate infestation (A2) and overexpression (A4) treatments. In addition, the terpenoids limone, \(\beta\)-caryophyllene, _ar_-curcumene and \(\alpha\)-zingiberene also contributed greatly to discriminate among LEM infestation levels. Hence, the terpenoids \(\beta\)-caryophyllene, _ar_-curcumene and \(\alpha\)-zingiberene were found to play an important role in the discrimination among both the infested and uninfested lychee plants (A), besides their participation in the discrimination among the field-collected tissues (B). Importantly, while the levels of the most abundant aldehydes (nonanal, decanal) decreased with the progression of the LEM infestation, the most abundant terpenoids (sabinene, limonene, _ar_-curcumene and \(\beta\)-caryophyllene) and alkanes (tetradacane, hexadecane, heptadecane) increased in the presence of LEM, suggesting that those major compounds may play an important role in lychee plant defense and LEM attraction/repellence for these compounds might differ. Recently, [PERSON] and [PERSON] [54] showed that lychee leaflets infested by LEM were rich in sugars, terpenoids, linoleic acid esters, vitamins, antioxidants, glycerol, and steroids. Our results were different from those reported by [PERSON] and [PERSON] [54] and we attribute these differences to the sampling methods. They used dried, powdered leaves extracted by Soxhlet apparatus with methanol. Our analyses using the HS-SPME method possess unsurpassed sensitivity and specificity in volatile sampling. In addition, we used authentic standards to confirm the identity of the compounds, while [PERSON] and [PERSON] [54] based their identification solely on the mass spectra of peaks. Second, our study offers further knowledge, demonstrating for the first time that the volatile profile of LEM infested lychee plants changes according to LEM infestation level. It has been shown by [PERSON] et al. [71] that the erineum-forming mite _Aceria erinea_ induced significant emissions of terpenoids in Persian walnut leaves (_Juglans regia_) and these emissions scaled positively with the percentage of the infested leaf area. In agreement with these findings, our study showed that terpenoids correlated positively with the LEM progression of infestation. These findings raised the hypothesis that LEM responses to the main terpenids emitted by lychee are dose dependent. Aiming to address this question, we designed a dual-choice arena (Figure 1) to investigate the LEM preferences when offering the terpenoids \(\beta\)-caryophyllene, _ar_-curcumene, limonene and sabinene on different concentrations. In fact, we confirmed our hypothesis by showing that key terpenoids played an important role in LEM attraction or repellence, depending on their concentration (Figures 8-11). Terpenoid VOCs play an important role in direct and indirect plant defenses [3]. These compounds can be toxic and repellent to the attacking insects [72] and mites [73], and can attract natural enemies of herbivores [72; 74]. For example, the terpenoids \(\alpha\)pinene, eugenol, limonene, and terpinolene repel several mosquito species [75; 76], and \(\alpha\)-pinene repels the tick _kNodes ricinus_[77]. Additionally, the sesquiterpene \(\beta\)-caryophyllene has been shown to repel mosquitoes [78] and the Asian citrus psyllid, _Diaphorina citri_ Kuwayama (Hemiptera: Lividae) [79]. Terpenoids can also act as elicitors that trigger plant defense mechanisms to target pests.