Abstract: In this paper,the life tables and the interference index of population control (IIPC) were used as the methods to evaluate the effectiveness of insecticides on the population dynamics of brown rice planthopper,Nilaparoata lugens The brown plantehopper,is an insect with high fecundity but with high vulnerability to natural enemies.The results showed that spraying an area with wide-spectrum insecticides,such as quinophos,interfered with the efficacy of the natural enemies markedly,giving an IIPC as high as 2.69.That is to say,the population size of the next generation will be increased to 2.69 times as against the original.At the same time,spraying with the highly selective insecticide buproferin,killed mainly the pest and favoured the effcacy of the natural enemies,giveing an IIPC as low as 0.37.That is the population size of the next generation will be decreased to 0.37 times as against the original.The interference index of population control showed a general effect on the pest insect,including side effects on its natural enemies.The highly selective insecticides are important in integrated pest management (IPM).