## 4 Discussion
In our study, the LCT\({}_{99}\) value of EF was 65.43, 45.85, and 46.90 mg\(\cdot\)h/L for eggs, nymphs, and adults, respectively. The differences between these two studies might be considered because of the different temperature conditions of 8 \({}^{\circ}\)C [7] and 20 \({}^{\circ}\)C in this present study. EF treatment might be dependent on temperature. On the other hand, LCT\({}_{99}\) values of PH\({}_{3}\) treatment show similar results between [PERSON] et al. (2016) and our study (Table 1). The EF + PH\({}_{3}\) combined treatment against _P. citri_ has been reported in a previous study [7], in which eggs, nymphs, and adults were treated at 8 \({}^{\circ}\)C with different treatment times: 2 h and 4 h. However, in our study, the temperature for the combination treatment was set to 20 \({}^{\circ}\)C, and the treatment time was set to 4 h. The fumigation toxicity of PH\({}_{3}\) to eggs was not found at 4 h at 20 \({}^{\circ}\)C, so it was treated at the level of 1 mg\(\cdot\)h/L as other developmental stages, which was the level of LCT\({}_{25}\) (Table 1). The EF treatment concentration for the combination treatment was simultaneously treated with LCT\({}_{10}\), LCT\({}_{25}\), and LCT\({}_{50}\) concentrations, and the treatment time was uniformly applied at 4 h. It was intended to shorten the treatment time because the PH\({}_{3}\) treatment time of 20 h was too long in the quarantine field. With this combined treatment, a synergistic effect was confirmed in eggs, nymphs, and adults (Table 2), and thus there was no need to use EF or PH\({}_{3}\) alone. This combination treatment caused more potent fumigation toxicity than that of the single treatment. As we determined four different enzyme activities after individual treatment of EF and PH\({}_{3}\), there was no significant change in AChE and GST activities, but it was confirmed that there were changes in COX and CE activities (Figure 2). Among them, a previous report found that the activity of COX in _Myzus persicae_ Sulzer (Hemiptera: Aphididae) nymphs increased in response to EF treatment [36]. This result is similar to our study as EF treatment on _P. citri_ nymphs enhanced COX activities (Figure 2a). Interesting molecular-level changes in this study are changes in genes related to chitin biosynthesis, especially in _P. citri_ nymphs (Figures 2 and 3). It indicates that treatment with EF and PH\({}_{3}\) affects _P. citri_ chitin production, which can be considered as a major route of the mode of fumigation action. On the other hand, the synergistic effect might be related to the biochemical changes. Through proteomic analysis, protein expression changes after individual treatment of EF and PH\({}_{3}\) were found in _P. citri_ adults, and several proteins after both treatments were commonly down-regulated which are involved in mitochondrial proteins related to energy production and protein biosynthesis. Among them, putative dihydroplipoyl lysine-residueacetyltransferase mediates the conversion of pyruvate to acetyl-CoA and CO2 as one of the pyruvate dehydrogenase components [37]. Therefore, it is a linker for the glycolytic pathways to the tricarboxylic cycle in living organisms. DLD is another important protein in relation to PH3 fumigation, because the unique biochemical mechanism for insect pest resistance to PH3 is a genetic change in DLD. DLD is known to be primarily activated in mitochondria and involved in energy generation. PH3-resistant insects develop genetically modified DLDs in their amino acid sequences [2; 19]. Similarly, the significant reduction in DLD level may produce lower energy in _P. citri_ after the combined treatment, indicating that it could be considered as a vital target site. Another down-regulated protein is phosphoribosyl formimino-5-aminoimidazole carboxamideribotide isomerase which is involved in histidine biosynthesis. The decreased expression of this enzyme may reduce histidine production during the fumigation and inhibit the regulation and metabolism of trace elements, and the repair and growth of tissue [38; 39].