Watermelon, Citrullus lanatus (Thunb.) Matsum. & Nakai (Cucurbitales: Cucurbitaceae), is one the 5 most-consumed fresh fruits in the world. The thrips Frankliniella schultzei (Trybom) (Thysanoptera: Thripidae) is an important pest of watermelon crops in tropical regions. Among the principal factors that regulate pest populations in crops are phenological stage of the host plant, weather and climate, and natural enemies. Thus, knowledge of such factors may allow the prediction of the risk of pest damage to such crops. The objective of this study was to identify factors that drive F. schultzei population densities in watermelon crops. During 2014 and 2015, we evaluated the effect of abiotic (weather) and biotic (phenological stage of leaves, and occurrence of natural enemies) factors on F. schultzei population densities on watermelon commercial crops. Frankliniella schultzei densities were higher in dry periods with more intense winds. Insect pest density was higher on younger leaves of plants in the vegetative stage. Frankliniella schultzei preferred to attack younger leaves of the plant located at the apex of the branches. The results obtained in this work suggest that the population growth of F. schultzei in watermelon crops is higher in periods of low rainfall. The population densities of F. schultzei depend on the phenological stage of plants, weather, and populations of natural enemies. Farmers should seek to preserve the populations of Chrysoperla sp. (Neuroptera: Chrysopidae), which are an important natural enemy of F. schultzei.