Abstract: 【Aim】 To clarify the effects of the medium lethal concentration (LC₅₀) of avermectin on the development of short-term resistance of Psylla chinensis to avermectin. 【Methods】 The LC₅₀ value of avermectin to the 3rd instar nymphs of the field population (F) of P. chinensis was determined at 24 h after treatment by leaf-dipping method. Based on the F population, the relative avermectin-sensitive strain (SS), the field resistant strain from the F population after exposure to LC₅₀ of avermectin for six consecutive generations (FR) and the field control strain from the F population without exposure to avermectin for six consecutive generations (FS) of P. chinensis were established under indoor conditions. The resistance ratios of the 3rd instar nymphs of the F population and various strains of P. chinensis to avermectin were measured, and the developmental duration and survival rate of each developmental stage, and adult longevity and numbers of eggs laid per female of various strains were measured to construct a two-sex life table. 【Results】 The LC₅₀ values of avermectin to the 3rd instar nymphs of the F population and the SS strain of P. chinensis at 24 h after treatment were 2.458 and 0.706 mg/L, respectively. The resistance ratios of the 3rd instar nymphs of the F population, FR strain and FS strain to avermectin were 3.458, 11.965 and 1.782-fold, respectively. The developmental duration and survival rate of each developmental stage, and adult longevity and number of eggs laid per female of the FR strain changed as compared with those of the FS strain and SS strain. The egg duration and nymphal duration of the FR strain were significantly longer than those of the FS strain and SS strain. The adult longevity of the FR strain was shorter than that of the FS strain and SS strain. The number of eggs laid per female of the FR strain was decreased by 57.000 and 74.300 grains, respectively, as compared with those of the FS strain and SS strain. The intrinsic rate of increase (r), finite rate of increase (λ) and net reproductive rate (R₀) of the FR strain were also decreased as compared with those of the FS strain and SS strain. The relative fitness of the FR strain was 0.743, which was 0.807-fold as high as that of the FS strain. 【Conclusion】 The resistance level of P. chinensis to avermectin increased and the fitness decreased by continuous treatment with LC₅₀ of avermectin for six generations. Therefore, avermectin should not be continuously used in pear orchards in order to delay the increase of the resistance of P. chinensis to avermectin.

Key words: Psylla chinensis, avermectin, age-age, two-sex life table, growth and development, population parameters