Although it is well established that biodiversity plays an important role in pest control, there is limited knowledge on how the origins of agriculture may shape arthropod biodiversity. Arthropod food webs likely have coevolved with wild crop relatives before domestication, but not all arthropod taxa may be able to inhabit cultivated habitats. We sampled wild and cultivated rice fields in the Mekong Delta of Vietnam to determine how season (dry versus wet) and rice cultivation influenced arthropod diversity and community composition. We found that cultivated rice fields supported ≈50% fewer taxa than wild rice fields, and that there were widespread losses in taxonomic richness within all of the major orders. Wild rice fields supported 173 unique taxa that were not found in any cultivated rice fields, whereas cultivated rice (Oryza sativa L.) supported only 23 unique taxa. Furthermore, wild and cultivated rice supported different arthropod communities. Predators and parasitoids were the most abundant in wild rice during the dry season. Herbivore densities were similar in wild and cultivated rice, but were reduced in wild rice (Oryza rugipogon L.) during the wet season. Neither season nor habitat influenced the densities of detritivores, but aquatic predators were more abundant during the dry season. This study provides empirical evidence that large-scale losses of species richness and changes in arthropod community structure are associated with the cultivation of domesticated rice.