Abstract: The toxicity of insecticides to S.exigua different in generations and instars was tested by topical application. The results that the toxicity of alpha-cypermethrin, methomyl, profenofos, and chlorfluazuron to 4th instar was decreased among second, fourth and sixth generations, and that of alpha-cypermythrin, methomyl, profenofos, and chlorfluazuron to third, fourth and fifth instar of sixth generation was also decreased. These gave a viewpoint that the resistance develops very quickly with the generations and instars increasing.