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On the basis of the inhibition effects of pymetrozine on the reproductive behavior of <i>N. lugens,</i> we established a bioassay method to accurately evaluate the toxicity of pymetrozine in <i>N. lugens</i> and clarified the level of pymetrozine resistance of <i>N. lugens</i> in the field. In this study, pymetrozine’s effects on the fecundity of <i>N. lugens</i> were evaluated using the topical application method and rice-seedling-dipping method. Moreover, the resistance of <i>N. lugens</i> to pymetrozine in a pymetrozine-resistant strain (Pym-R) and two field populations (YZ21 and QS21) was determined using the rice-seedling-dipping method and fecundity assay methods. The results showed that treatment of <i>N. lugens</i> third-instar nymphs with LC₁₅, LC₅₀, and LC₈₅ doses of pymetrozine resulted in a significantly reduced fecundity of <i>N. lugens</i>. In addition, <i>N. lugens</i> adults treated with pymetrozine, using the rice-seedling-dipping and topical application method, also exhibited a significantly inhibited fecundity. Using the rice-stem-dipping method, pymetrozine resistance levels were shown to be high in Pym-R (194.6-fold), YZ21 (205.9-fold), and QS21 (212.8-fold), with LC₅₀ values of 522.520 mg/L (Pym-R), 552.962 mg/L (YZ21), and 571.315 (QS21) mg/L. However, when using the rice-seedling-dipping or topical application fecundity assay method, Pym-R (EC₅₀: 14.370 mg/L, RR = 12.4-fold; ED₅₀: 0.560 ng/adult, RR = 10.8-fold), YZ21 (EC₅₀: 12.890 mg/L, RR = 11.2-fold; ED₅₀: 0.280 ng/adult; RR = 5.4-fold), and QS21 (EC₅₀: 13.700 mg/L, RR = 11.9-fold) exhibited moderate or low levels of resistance to pymetrozine. Our studies show that pymetrozine can significantly inhibit the fecundity of <i>N. lugens</i>. The fecundity assay results showed that <i>N. lugens</i> only developed low to moderate levels of resistance to pymetrozine, indicating that pymetrozine can still achieve effective control on the next generation of <i>N. lugens</i> populations.