Abstract: 【Aim】To evaluate the effects of ebamectin benzoate and avermectin on the fecundity of Nilaparvata lugens and its parasitic natural enemy Anagrus nilaparvatae, to explore the reasons leading to the resurgence of N. lugens in the field caused by the two insecticides.【Methods】 The effects of application of 225 mL/hm² of 5% ebamectin benzoate microemulsion (ME) and 240 mL/hm² of 5% avermectin suspension concentrate (SC) on the population density of N. lugens and parasitism rate of A. nilaparvatae adults were investigated in the field conditions. The toxicity of the two insecticides to the 4th instar nymphs of N. lugens was tested with rice stem-dipping method in the laboratory. In addition, the effects of different concentrations (LC₁₅, LC₃₀, LC₅₀ and LC₇₅) of the two insecticides on the fecundity of N. lugens and the survival and parasitic ability of A. nilaparvatae adults were also tested with rice stemdipping method in the laboratory. The toxicity of the two insecticides to A. nilaparvatae adults was determined by the method of residual film in glass tube, and their risk levels were evaluated. 【Results】 The field investigations showed that the application of either 5% emamectin benzoate ME or 5% avermectin SC significantly increased the population density of N. lugens in the fields, and significantly reduced the parasitism rates of A. nilaparvatae adults. The laboratory bioassay results revealed that the LC₅₀ values of ebamectin benzoate and avermectin against the 4th instar nymphs of N. lugens were 1.20 and 2.03 mg/L, respectively. The number of eggs laid per female adult of N. lugens treated by either ebamectin benzoate (LC₁₅-LC₇₅) or avermectin (LC₁₅-LC₅₀) was significantly higher than that of the control. The LC₅₀ values of ebamectin benzoate and avermectin against A. nilaparvatae adults were 0.0145 and 0.0205 mg/L, respectively. The safety factors for the dosage of emamectin benzoate and avermectin used in fields were 4.52×10^-4-9.05×10^-4 and 8.52×10^-4-14.22×10^-4, respectively, suggesting that the two insecticides have extremely high risk to A. nilaparvatae adults.【Conclusion】 The field application of ebamectin benzoate or avermectin could cause the resurgence of N. lugens because of two reasons. The first was that both insecticides would stimulate the oviposition of female adults of N. lugens, and the second was that both insecticides had high risk to A. nilaparvatae and greatly weakened the control efficacy of this important parasitic wasp on N. lugens. Therefore, we should decrease the use of the two insecticides in paddy fields to avoid the resurgence of N. lugens.

Key words: Nilaparvata lugens, Anagrus nilaparvatae, emamectin benzoate, avermectin, resurgence, fecundity, parasitism rate