A number of studies have been conducted to address this question. Fire ants can't survive for prolonged periods in conditions that freeze soil as deep as they are nesting. Estimated northern limit boundaries are areas with a 10 degrees F minimum temperature. However, fire ants finding warmer micro-habitats, such as curbs that act as heat sinks or indoor plant courts, can survive farther north than predicted. For literature, search the ant data base maintained by the USDA-ARS. Figures found here have been used to propose the potential range of this species as posted on Imported Fire Ant Range Studies.
For more information, see FAQ #1117: "Will a hard winter kill imported fire ants?"
