Fire ants generally survive the winter by employing specific strategies. The red imported fire ant (Solenopsis invicta), a pest primarily found across the Southern United States, is not cold-tolerant enough to withstand prolonged freezing temperatures at the soil surface. Their unique nest architecture allows the entire colony to persist by relocating the colony’s core to a stable, warmer environment deep within the ground.
Winter Survival Strategy: Vertical Migration
The primary mechanism fire ants use to survive winter is vertical migration, a collective movement deep into the soil. This action transforms the earth beneath the mound into an insulating thermal refuge that shields the colony from freezing air temperatures. The soil acts as an effective insulator against the rapid temperature fluctuations that occur above ground.
Colonies move the queen, the brood (eggs, larvae, and pupae), and the majority of workers into the deeper, warmer tunnels. The subterranean network of a mature colony can extend to depths of 10 feet or more, though most chambers are found within the top 3 feet of soil. By moving to this depth, the ants access stable soil temperatures that remain well above freezing, even when the air temperature is significantly colder.
Colony Activity During Cold Periods
Once settled in the deeper chambers, the ants enter a state of reduced activity known as quiescence, which is distinctly different from true hibernation. They remain non-dormant and non-freeze-tolerant, meaning they do not undergo the physiological changes necessary to survive ice formation. This state is marked by a significant slowdown in metabolic rate, conserving energy reserves when foraging is impossible.
Surface foraging activity ceases entirely when soil temperatures drop below approximately 50 to 59 degrees Fahrenheit. The queen minimizes or halts egg production, and the worker ants form tight clusters around the brood and the queen to maintain a stable, warmer temperature in the central nest area. This clustering behavior is a key part of their thermoregulation, maximizing heat retention. If a warm spell occurs, the ants can quickly become active and briefly resume foraging.
Lethal Temperature Thresholds
Mortality in fire ant colonies occurs when cold exposure is both severe and prolonged, typically limiting the northern spread of the species. Colony death is primarily caused by the freezing of the soil to the depth of the colony’s core. For the colony to be killed, the ground needs to freeze to a depth of at least 12 inches, which is where the ants typically aggregate in cold weather.
Research has established that fire ant workers have 100% mortality after approximately seven days of exposure to temperatures of 24.8 degrees Fahrenheit (-4 degrees Celsius). The geographic distribution of the red imported fire ant is largely limited by the frequency and severity of cold winters, specifically the minimum January air temperature. Areas where the mean minimum air temperature drops below approximately 10.4 degrees Fahrenheit (-12 degrees Celsius) are considered too cold for long-term colony survival.