A 4-yr study (1997–2000) was conducted in the northern Texas Rolling Plains to determine whether the pyrethroid insecticide λ-cyhalothrin was associated with changes in nutritional quality (nonstructural carbohydrates, percentage leaf nitrogen and moisture, and total amino acids) of cotton leaves. Another objective was to determine the relationship between nonstructural carbohydrates (glucose, fructose, sucrose, and starch) in cotton leaves and change in cotton aphid, Aphis gossypii Glover, numbers during late summer and on formation of dark-colored morphs. Carbohydrate concentrations, percentage leaf moisture and nitrogen, and total amino acids were not significantly altered in cotton leaves by λ-cyhalothrin. Glucose, fructose, sucrose, sugar ratio [(glucose fructose)/sucrose concentrations], leaf nitrogen, and moisture were significantly influenced by year and irrigation treatment. Regression analysis indicated that change in aphid numbers was influenced by numbers of aphids per leaf, temperature, leaf moisture and nitrogen, and sugar ratio. A negative linear relationship was observed between change in aphid numbers and sugar ratio; population growth was limited by high levels of glucose and fructose in cotton leaves, especially when temperatures were high and leaf moisture low. Percentage of dark-colored aphids was negatively correlated with temperature and daylength and positively correlated with leaf moisture and nitrogen and the sucrose/glucose ratio. Some of the nutritional and abiotic environmental variables that interact to regulate the occurrence of dark morphs are also interacting with other variables to influence the extent to which the reproductive potential will be expressed.