Watermelon Citrullus lunatus (Thunberg, Matsumura & Nakai) is an ecosystem having a variety of arthropods, each one playing a specific role. Although some of them are considered pest to crops, some others are responsible for soil aeration, nutrient release and predation of pest species and are, therefore, considered beneficial to crops. The intensive farming practiced for watermelon cultivation in Brazil is based on the use of tiamethoxam and deltamethrin, which may not only kill target but also nontarget organisms such as beneficial arthropods. Research data regarding the influence of insecticides on arthropods in watermelon cropping is scarce. This study aimed to evaluate the effect of the insecticides deltamethrin and thiamethoxam on soil surface and watermelon canopy arthropod community. The study was carried out in the State of Tocantins, Brazil. Although the application of thiamethoxam and deltamethrin was efficient in controlling populations of Aphis gossypii (Glover), as we expected, they negatively affected non-target arthropods such as detritivores insects in the canopy and soil surface. Ecological implications of the impact of such pesticides on beneficial arthropod species are discussed.