The impact of planting date and nitrogen fertilization on cotton plants, silverleaf whitefly, Bermisia argentifolii Bellows and Perring, population dynamics and the related physiological mechanisms involved in the process were investigated in a field in Riverside, California. Treatments consisted of early and late planting dates (26 April and 8 June,) and 4 levels of fertilization (0, 100, 150 and 200 Ibs N per acre) for each planting date. Applied nitrogen linearly increased seed cotton yield of early planted cotton and early planted cotton treated with chemical defoliant but had no effect on yield of late planted cotton. The nitrogen also resulted in increased densities of whiteflies. Also, early planted cotton supported higher densities of whiteflies than late planted cotton. Petiole glucose and fructose levels were significantly correlated with densities of whitefly adults during the peak population size on late planted cotton. A significant correlation between densities of adult whiteflies and other cotton physiological parameters occurred on only a few sampling dates.