The impact of nitrogen fertility to cotton plant (Gossypium hirsutum L., c.v. Acala) on silverleaf whitefly (Bemisia argentifolii Bellows & Perring) population dynamics and the honeydew production and the related biochemical and physiological mechanisms were investigated in California. Five nitrogen levels were evaluated using urea in a randomized complete block design with five replicates. Treatments consisted of soil applications of 0, 100, 150 and 200 lbs nitrogen per acre, and a soil application of 100 lbs nitrogen together with a foliar application of 10 gal of low-biuret urea per acre. Applied nitrogen linearly increased densities of both adult and immature whiteflies during their peak population growth on cotton. Higher nitrogen treatments also resulted in higher densities of honeydew drops produced by the whiteflies. Also, the nitrogen treatments linearly enhanced cotton foliar photosynthetic rates and altered concentrations of soluble proteins, soluble amino acids and several soluble carbohydrates such as glucose, fructose and sucrose in cotton petiole. However, the applied nitrogen had no effect on seedcotton yield. Glucose levels were significantly correlated with densities of whitefly adults during the peak population size. Significant correlations between densities of adult or immature whiteflies and other cotton physiological parameters occurred on only a few sampling dates.