The Red Imported Fire Ant, Solenopsis invicta, is a species of ant that has gained worldwide recognition for its painful, venomous sting, which often results in a characteristic white, fluid-filled pustule on the skin of humans and animals. Workers are highly defensive of their nests and will swarm an intruder, biting and stinging repeatedly. Originating in South America, this ant has successfully established populations across numerous continents.
The Original Home (Native Range)
The native range of the Red Imported Fire Ant lies within the tropical and subtropical regions of central South America. The species is centered in the Pantanal and the floodplains of the Paraná River basin, spanning parts of Brazil, Argentina, and Paraguay. This environment is characterized by seasonal and intense flooding, which fostered unique survival strategies. The ants adapted by quickly evacuating their nests and forming waterproof, living rafts made of their own bodies. This behavior allows the entire colony, including the queen and brood, to float until they reach dry land to establish a new mound.
Global Invasion and Current Distribution
The fire ant’s global distribution began with its introduction into the United States, likely arriving in cargo ships at a Gulf Coast port in the 1930s or 1940s. The species rapidly spread across the Southeastern United States, currently infesting vast areas across nine states, including Texas, Florida, Georgia, and Louisiana. The northern and western boundaries of this expansion are generally defined by federal quarantine lines established to prevent the movement of infested materials. Although the main population is concentrated in the Southeast, isolated infestations have also been established in parts of California, New Mexico, and Puerto Rico. The US population later became the source for secondary invasions in other parts of the world, facilitated by global commerce, establishing populations in Australia, East and Southeast Asia (including mainland China, Taiwan, and Japan), and numerous Caribbean islands.
Ideal Environmental Conditions
The geographic spread of the fire ant is dictated by its environmental tolerances, favoring warm, moist conditions. Colonies thrive in tropical and subtropical climates where temperatures allow for year-round activity. The northern expansion in the US is limited by cold temperatures, as prolonged freezing conditions prevent colony survival. Foraging activity is optimal when surface temperatures range from 70°F to 95°F, often pushing activity into the night or early morning during summer. The ants prefer open, sunny habitats like lawns, pastures, and parklands, where they construct their characteristic dome-shaped soil mounds, often in heavy clay or loamy soils that provide stability, and they dig deeper tunnels in dry regions to access moisture.
Methods of Spread and Expansion
Fire ants expand their range through a combination of natural processes and human-assisted transport, the latter being the primary driver of long-distance spread. Human commerce inadvertently relocates colonies or individual queens by moving materials that harbor nests. The most common materials involved in this long-distance transport are potted nursery stock, sod, hay, and soil-moving equipment. Natural expansion occurs through nuptial flights, water transport, and colony budding. Nuptial flights spread new queens up to a few miles, while floating rafts allow for short-distance spread during floods, and colony budding allows multiple-queen colonies to split and migrate locally.