The Red Imported Fire Ant (RIFA), Solenopsis invicta, is an invasive species introduced to the United States from its native range in South America. These ants have become a major public health and agricultural pest due to their venomous sting and high population densities. This article identifies the specific states where these ants have successfully established persistent populations across the country.
Current Geographic Distribution
The primary range of the Red Imported Fire Ant is concentrated in the contiguous Southeastern United States. This core infestation includes the Gulf Coast states of Florida, Georgia, Alabama, Mississippi, Louisiana, and Texas, along with South Carolina. These areas offer the warm, humid climate necessary for the ants to thrive year-round.
The infestation extends north and west into states with partial or isolated populations, such as North Carolina, Tennessee, Arkansas, Oklahoma, and Virginia. Isolated infestations, often linked to urban or irrigated areas, have also been found farther west in California and New Mexico. The ants’ northern boundary is generally limited by cold temperatures, as prolonged soil freezing or minimum temperatures below 10°F can prevent colony survival.
Their westward spread is restricted by arid conditions, with an annual precipitation limit of around 510 mm defining a barrier in non-irrigated regions. This combination of temperature and moisture requirements establishes the biological boundary for the contiguous infestation, which currently spans over 367 million acres in the US and Puerto Rico.
How the Invasion Began
The Red Imported Fire Ant entered the United States between the 1930s and 1940s, likely through the Port of Mobile, Alabama. Originating from South America, the ants traveled hidden within soil used as ship ballast to stabilize cargo vessels. When the ships docked, this infested soil was offloaded, introducing the species to North America.
The ants initially spread naturally through mating flights and colonies floating during flood events. However, human activity soon accelerated their movement, particularly through the transport of agricultural products and other goods.
Understanding Quarantine Zones
To prevent the artificial spread of fire ants beyond their established range, the USDA Animal and Plant Health Inspection Service (APHIS) established official quarantine zones. These regulations apply to counties where fire ants are confirmed to be established and are intended to control the movement of high-risk materials.
The quarantine restricts the interstate and intrastate movement of items that could harbor a hidden colony. Regulated articles include:
- Soil, potted plants, and nursery stock.
- Grass sod.
- Baled hay or straw that has been stored in contact with the ground.
- Used soil-moving equipment.
For these regulated items to be moved from a quarantined area to a non-quarantined area, they must be inspected and certified as fire-ant-free or treated with an approved pesticide.
Health and Ecological Consequences
The presence of fire ants presents problems that extend beyond simple nuisance, impacting human health, local ecosystems, and infrastructure. For humans and animals, the ants are known for their aggressive stinging behavior, with each ant capable of stinging repeatedly. The venom produces a burning sensation followed by a characteristic sterile pustule at the sting site, and in about 0.5% to 2% of cases, can trigger a life-threatening systemic allergic reaction, including anaphylaxis.
Ecologically, Solenopsis invicta acts as a dominant predator, displacing native ant species and reducing the diversity of insects and small animals. They disrupt the local food web by preying on ground-nesting birds, reptiles, and the young of livestock. Furthermore, their mound-building activities cause economic damage by interfering with agricultural machinery and altering soil structure. The ants also frequently infest electrical equipment, such as utility boxes and traffic signal controls, where nesting can cause short circuits and power outages.