[Objectives]  To evaluate the effects of suppressing natural enemies on cotton aphids on cotton seedlings and during summer. [Methods]  Field investigations and enclosures. [Results]  Propylaea japonica and Harmonia axyridis were the main natural enemies of aphids during the cotton seedling stage and could effectively suppress the aphid population achieving a biocontrol services index (BSI) > 91%. Araneae, Chrysopidae and Propylaea japonica were the main natural enemies of aphids in summer. These predators could not effectively suppress the population of summer aphids; because of low population densities and low natural enemy to aphid ratio the BSI was always < 20%. [Conclusion]  The results indicate that the effectiveness of natural enemies to control aphids in cotton fields varies between the seedling stage and summer.