Field experiments were conducted from 1999 to 2000 to assess the effect of nitrogen fertilization levels (0, 225 and 450 kg·hm-2 ) on the population dynamics of cotton bollworm (Helicoverpa armigera) and cotton aphid ( Aphid gossypii), and on the numbers of cotton buds and bolls in North China. The population densities of cotton bollworm and cotton aphid increased with increasing nitrogen fertilization during 1999 to 2000, but the differences between treatments were not significan( P>0. 05). There were sign...