Red imported fire ant, Solenopsis invicta Buren, is a serious pest in the southeastern United States. It has caused economic losses in agricultural products, stings can cause anaphylactic shock, and it has reduced biodiversity. S. invicta has displaced native ant species throughout its range, presumably by competitive exclusion. In 1998, S. invicta populations were confirmed in southern New Mexico, resulting in a quarantine of Doña Ana County. Although large populations of the native southern fire ant, S. xyloni McCook, are common in the Mesilla Valley, only two small populations of S. invicta have been found since the initial quarantine. It is unclear why these populations have not become more dominant. This research studied the influence of the fire ant parasitoid Pseudacteon tricuspis Borgmeier (Phoridae), low humidity, and competition with S. xyloni on the survival and fitness of S. invicta. Competitive and noncompetitive foraging experiments with and without parasitoids were conducted in the laboratory to evaluate food retrieval and worker activity of each species. Overall, S. invicta was able to retrieve more food than S. xyloni. Pseudacteon flies reduced foraging and worker activity of S. invicta under low humidity. S. invicta dominated at high and low humidity in the absence of P. tricuspis. However, S. xyloni dominated at high humidity and coexisted with S. invicta at low humidity when P. tricuspis was present. This may help explain the lack of establishment and spread of S. invicta in the southwestern United States and encourage the use of Pseudacteon flies as a control option in arid environments.