Fire ants can't survive for prolonged periods in conditions that freeze soil as deep as they are nesting. In the Deep South, it is unlikely that low winter temperatures will affect fire ant populations. However, at their extreme northern range, winters could be cold enough to significantly affect populations.
Ultimately, cold winter temperatures will limit how far north fire ants can spread. Generally, minimum temperatures below 10 degrees F are required to affect populations.
Sustained cold temperatures in late fall or early winter might be expected to have a greater impact than later in the winter.
Find more information about fire ants in eXtension's Imported Fire Ant Resource Area.