A population model of cotton growth and development in response to weather, agronomic factors, and insect pests has been successfully used to examine cotton production in California. The impact of most arthropod pests on cotton yields is currently under investigation. Only the effects of defoliating insects,Spodoptera exigua(Hübner),Trichoplusia ni(Hübner), andHeliothis zea(Boddie), are reported here.The impact of moderate defoliation byS. exiguaandT. nipopulations causes only slight yield reductions. More important is the predation byS. exigualarvae on immature fruit (i.e., squares). Investigations on the interaction of plant age and developmental stage with defoliator damage (defoliation and predation on squares) indicates that the greatest yield reductions occur from attack early in the squaring period. The late season effects ofH. zeaare easy to measure and incorporate into the model.