In the southeastern United States, Solenopsis invicta often occur at high density in association with an invasive mealybug, Antonina graminis. In a series of studies, we tested whether the abundance of these species are facilitated by a facultative mutualism. A field study showed that larger populations of S. invicta are associated with higher densities of A. graminis and their host plants. An experiment also showed that S. invicta colonies with access to honeydew-producing A. graminis and unlimited insect prey grow substantially larger than colonies with access to unlimited insect prey alone. Another experiment showed that S. invicta protects A. graminis from an introduced biological control parasitoid, Neodusmetia sangwani. Overall, the results suggest that mutually beneficial interactions between S. invicta and A. graminis are likely to be important in promoting larger populations of both species.