Resistance Monitoring of Nilaparvata lugens to Pymetrozine Based on Reproductive Behavior

SIMPLE SUMMARY: Pymetrozine is one of the most common insecticides used to control the rice pest Nilaparvata lugens in 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 monitor N. lugens’ resistance level to pymetrozine, and systematically evaluated pymetrozine’s effect on the fecundity of N. lugens. Treatment with pymetrozine significantly reduced the number of offspring in both N. lugens nymphs and adults. ABSTRACT: On the basis of the inhibition effects of pymetrozine on the reproductive behavior of N. lugens, we established a bioassay method to accurately evaluate the toxicity of pymetrozine in N. lugens and clarified the level of pymetrozine resistance of N. lugens in the field. In this study, pymetrozine’s effects on the fecundity of N. lugens were evaluated using the topical application method and rice-seedling-dipping method. Moreover, the resistance of N. lugens 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 N. lugens third-instar nymphs with LC(15), LC(50), and LC(85) doses of pymetrozine resulted in a significantly reduced fecundity of N. lugens. In addition, N. lugens 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 LC(50) 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 (EC(50): 14.370 mg/L, RR = 12.4-fold; ED(50): 0.560 ng/adult, RR = 10.8-fold), YZ21 (EC(50): 12.890 mg/L, RR = 11.2-fold; ED(50): 0.280 ng/adult; RR = 5.4-fold), and QS21 (EC(50): 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 N. lugens. The fecundity assay results showed that N. lugens only developed low to moderate levels of resistance to pymetrozine, indicating that pymetrozine can still achieve effective control on the next generation of N. lugens populations.