Insects are one of the biggest factors in reducing cotton (Gossypium hirsutum L.) yields. To study the effects of insects in the cotton yield declines that have occurred during the past 25 years, simulations were made with the crop simulation model GOSSYM. The 1970-1976 (especially 1976) period at Stoneville, MS, and the 1970, 1975 periods at Phoenix, AZ, were times of intense Insect pressures, as indicated by the breeders' notes. The simulated yields were much higher than the harvested yields. At Florence, SC, however, only a slight yield decline occurred. According to the breeder, the introduction of pyrethroids in the mid-1970's may have been one of the factors that prevented a yield decline. At other locations In this study, insects were not believed to be a problem or no notes were taken on the amount of damage in the breeder's trials.