Simple SummaryPymetrozine is one of the most common insecticides used to control the rice pestNilaparvata lugensin China. Because of its unique mechanism of action and the fact that there is no obvious lethal effect after pymetrozine treatment, it is unreasonable to determine the sensitivity of pymetrozine by only calculating the mortality rate. In the context of traditional bioassay methods, which do not reflect pymetrozine’s control effect on the pest populations in the field, we established fecundity assay bioassay methods to monitorN. lugens’resistance level to pymetrozine, and systematically evaluated pymetrozine’s effect on the fecundity ofN. lugens. Treatment with pymetrozine significantly reduced the number of offspring in bothN. lugensnymphs and adults.AbstractOn the basis of the inhibition effects of pymetrozine on the reproductive behavior ofN. lugens,we established a bioassay method to accurately evaluate the toxicity of pymetrozine inN. lugensand clarified the level of pymetrozine resistance ofN. lugensin the field. In this study, pymetrozine’s effects on the fecundity ofN. lugenswere evaluated using the topical application method and rice-seedling-dipping method. Moreover, the resistance ofN. lugensto 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 ofN. lugensthird-instar nymphs with LC15, LC50, and LC85doses of pymetrozine resulted in a significantly reduced fecundity ofN. lugens. In addition,N. lugensadults 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 LC50values 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 ofN. lugens. The fecundity assay results showed thatN. lugensonly developed low to moderate levels of resistance to pymetrozine, indicating that pymetrozine can still achieve effective control on the next generation ofN. lugenspopulations.