The goals of this study were to determine the population dynamics of sweetpotato whitefly, Bemisia tabaci (Gennadius); green peach aphid, Myzus persicae (Sulzer); and the cotton/melon aphid, Aphis gossypii Glover, as well as to explore the incidence of the complex of aphid-borne viruses of watermelon, Citrullus lanatus L., on different planting dates. Four (1998–1999) and six (1999–2000) watermelon plots were monitored under grower management conditions in La Huerta and Casimiro Castillo, Mexico. Plant growth, sweetpotato whitefly, and aphid (adult and nymph) populations, and percentage of virus-infected plants were measured in fall–winter watermelon plots planted at different dates. The population of sweetpotato whitefly varied significantly by planting date and showed two different population patterns. Watermelon planted in early October had higher population density at young developmental stages, whereas plantings from mid-October had more adults at flowering or harvesting. Aphid species may be present at any time in fall–winter watermelon. However, higher levels of infected plants and more damage caused by viruses occurred when watermelon was planted on 16 November or later. Several factors acting separately or interactively may influence the population dynamics of vector populations and the complex of viruses in watermelon. Current levels of vector populations (nymphs) may not be high enough to produce direct damage to watermelon plants, but adults may be important factor in the viruses’ infection process. Planting before mid-November may aid in getting lower levels of infected plants and improving yield.