Abstract: 【Aim】 The objective of this study is to evaluate the toxicities of nC22 mineral oil against the Asian citrus psyllid, Diaphorina citri, when used alone or mixed with imidacloprid in the laboratory, and to screen the mixing ratios that show synergistic effect on imidacloprid, so as to provide scientific support for the application of mineral oil against D. citri. 【Methods】 In the laboratory, the lethal effect of nC22 mineral oil and the positive controls nC23 and nC28 mineral oils against eggs, and the early instar nymphs, late instar nymphs, and adults of D. citri were determined by immersion method and spraying method, respectively, and the toxicities were calculated based on the LC₅₀ values (7 d after treatment for eggs, and 1 d after treatment for nymphs and adults). The nC22 mineral oil was mixed with imidacloprid at various ratios, and their lethal effects on the early instar nymphs of D. citri were determined. The synergistic effect of nC22 mineral oil on imidacloprid was evaluated by interactive determination, co-toxicity factor (CTF), and co-toxicity coefficient (CTC) methods. 【Results】 When used alone, the LC₅₀ value of nC22 mineral oil against D. citri eggs was significantly lower than those of nC23 and nC28 mineral oils. The LC₅₀values of nC22 mineral oil against both the early and late instar nymphs were similar to those of nC23 mineral oil, and lower than those of nC28 mineral oil. The LC₅₀ value of nC22 mineral oil against adults was similar to that of nC28 mineral oil, and lower than that of nC23 mineral oil. The mixtures of nC22 mineral oil and imidacloprid at the ratios of 3∶7, 4∶6 and 7∶3 showed remarkable synergistic effect on imidacloprid: the actual mortalities of the early instar nymphs of D. citri were above the equivalent line in the interactive determination method, the CTF value were 34.807, 22.655, and 40.798, and the CTC values were 187.430, 183.876 and 222.936, respectively. The mixture of nC22 mineral oil and imidacloprid at the ratio of 7∶3 showed the strongest synergistic effect on imidacloprid. 【Conclusion】 nC22 mineral oil shows higher toxicity against D. citri than the imported nC23 and traditional nC28 mineral oils. When mixed with imidacloprid at proper ratios, nC22 mineral oil shows strong synergistic effect, and theoretically it can significantly reduce the use of chemical insecticide, which could be further tested in fields.

Key words: Diaphorina citri; imidacloprid, mineral oil, synergistic effect, mixing ratio, toxicity