There is increasing evidence that exposure to neonicotinoid insecticides at sublethal levels impairs colonies of honeybees and other pollinators. Recently, it was found that sublethal contamination with neonicotinoids also affect growth and behavior of ants. In this study, we exposed red imported fire ants, Solenopsis invicta Buren, to sublethal dosages of dietary imidacloprid and investigated its effect on ant feeding, digging, and foraging behavior. S. invicta consumed significantly more sugar water containing 0.01 µg/ml imidacloprid than untreated sugar water. Ants fed with 0.01 µg/ml imidacloprid also showed significantly increased digging activity than ants fed with untreated sugar water. However, imidacloprid at ≥ 0.25 µg/ml significantly suppressed sugar water consumption, digging, and foraging behavior. These results indicate that imidacloprid at sublethal concentrations may have a significant and complicated effect on S. invicta.