## 4 Discussion
For instance, tomato plants exposed to the volatiles of transgenic tobacco enriched in \(\beta\)-ocimene decreased aphid, _Macrosiphum euphoriae_, development and reproduction (direct defense) and increased attraction of its natural enemy, the parasitoid _Aphidius ervi_ (indirect defense) [72]. Similarly, by inhibiting the emission of terpenoid HPIVs, [PERSON] et al. [74] demonstrated that the inhibition of those terpenoids in mite-infested plants decreased the attractiveness of the predatory mite _Phytoiseulus persimilis_. Like terpenoids, aldehydes can also act as specific elicitor molecules emitted after herbivory, pathogen infection, or oxidative stresses [80; 81]. For example, [PERSON] et al. [81] showed that Lima bean plants growing adjacent to plants chemically induced with benzothiadiazole up-regulated the production of nonanal and were more resistant to infection by the pathogen _Pseudomonas syringae_. In insect-plant systems, infestation by the peach aphid (_Myzus persicae_) and the Colorado potato beetle (CPB), (_Leptinotars decminineata_) drastically increased the amounts of nonanal in attacked plants [82]. The increased amounts of nonanal in potato plants infested with CPB played an important role in the attraction of the two predators, _Podisum maculiventris_ and _Perillus bioculatus_[80]. However, nonanal has also been shown to increase the attraction and oviposition of _Helicoverpa assulta_ (Lepidoptera: Nocutiidae) moths in tobacco plants [83]. In the current study, the emissions of nonanal and decanal decreased with increasing LEM infestation (Table 1). Although the results from the behavioral assays were inconsistent with increasing doses of nonanal and decanal (Figures 6 and 7), in the trials with lower concentrations of decanal (3, 5, and 7%) the treated-side was more attractive. Taken together, these data may suggest that decanal may initially elicit attraction to LEM, and as infestation progresses and decanal emissions increase, infested tissues may become less attractive to LEM. The attractiveness of volatile odors to most insects and mites is dose dependent [84; 85; 86; 87]. Similarly, in the present study, we found that the compound concentration was the most important factor determining attraction or repellence of LEM. A trend towards attraction of LEM at low and intermediate concentration and repellence at high concentration was found in three out of the six volatiles tested. Decanal (Figure 7), \(\beta\)-caryophyllene (Figure 8) and limonene (Figure 10) were mostly attractive to LEM at low and intermediate concentrations. Sabinene (Figure 11) promoted attraction of LEM in almost all concentrations tested, and nonanal (Figure 6) promoted repellence or had no effect on LEM preferences. Importantly, since we tested individual compounds, the outcome might be different when testing them in a mixture. Further research should investigate if volatile emissions of these compounds are associated with natural enemies of LEM. Several studies have shown that the ratio of odor blends are important in insect attraction [88; 89; 90]. Blends of volatiles in the proper ratios rather than single compounds can mimic natural conditions. Thus, the next step in understanding the attractive role of the lychee volatiles identified in this study to LEM is testing different odor blends at different ratios. Nevertheless, the findings that the LEM in this study significantly preferred some volatile compounds at lower concentrations is an important first step in understanding the attractive behavior of this pest. Further, to our knowledge, this is the first description of a bioassay for testing LEM odor preferences. It is important to note that because LEM hides in the erineum it was not possible to assess the number of mites released in the arena. In an erineum disc of 0.5 mm diameter the LEM population may vary from 0 to more than 8000 mites. This information, however, can only be confirmed after the mites exit the erineum.