Fenvalerate, omethoate and monocrotophos are the major pesticides for the chemical control of cotton aphid in Henan province. The experiment of application of these pesticides was done in cotton field in 1990, the result showed that plots by a successive three time spray with fenvalerate and four sprays with omethoate would induce the aphid population outbreak. The contrary was obtained in plots treated successively with monocrotophos. After the fourth time of using pesticide, population densities in two tr...