The red imported fire ant arrived in the United States accidentally, carried in soil used as ballast in cargo ships from South America. The ants entered through the port of Mobile, Alabama, sometime between the early 1930s and 1945. From that single introduction point, they spread across the Southern states, first by natural expansion and then dramatically faster through the commercial nursery trade.
Origin in South America
Red imported fire ants are native to the lowland areas of Brazil and Argentina. Genetic studies have traced the specific source population of the U.S. invasion to northern Argentina, near the province of Formosa along the Paraguay River drainage. Researchers compared DNA from ant populations across six U.S. states with samples from multiple South American locations and determined, with high confidence, that the Formosa population was the origin of the original colonists. A neighboring population about 100 miles away in Corrientes, Argentina, may have contributed as well.
How They Crossed the Ocean
Cargo ships traveling from South America to the Gulf Coast routinely loaded soil as ballast to stabilize the vessel during transit. That soil, scooped from riverbanks and port areas in the ants’ native range, contained fire ant queens and workers. When ships unloaded at Mobile, the contaminated ballast soil was dumped on shore, releasing the ants into a new environment with no natural predators.
Fire ant queens are remarkably self-sufficient colonizers. A newly mated queen doesn’t need an existing colony to survive. She sheds her wings, burrows into the ground, and lays a small first batch of eggs on her own. Those first workers then help raise the next generation, and the colony grows from there. Fire ant colonies can also survive flooding by forming living rafts on the water’s surface, drifting until they reach dry ground to rebuild. These traits made the species well suited to surviving a chaotic journey in a ship’s hold and establishing itself quickly once on land.
Spread Through the Nursery Trade
For the first decade or so after arrival, the ants expanded slowly on their own around the Mobile area. The real acceleration came when they colonized commercial plant nurseries. Fire ants built nests in potted plants and around the base of larger shrubs and trees, hiding undetected in the root balls. Surveys from the late 1940s and early 1950s showed that shipments of infested nursery stock from Mobile to other states were responsible for much of the early geographic spread. A single delivery of ornamental plants could carry an entire colony, queen included, hundreds of miles from the original introduction site.
A federal quarantine on fire ant-infested nursery material, combined with insecticide treatments at nurseries, eventually slowed this pathway. But by then the ants had already established footholds across the Gulf states and beyond.
Early Attempts at Control
Authorities recognized the problem relatively quickly. The first organized control program began in 1937, just a few years after the ants likely arrived, in Baldwin County, Alabama. Federal, state, and county agencies cooperated on the effort. The method was crude by modern standards: workers opened each mound with a shovel, poured calcium cyanide dust inside, and covered the hole with soil. It was labor-intensive, mound-by-mound work that couldn’t keep pace with the ants’ reproduction and spread.
Later campaigns in the 1950s and 1960s attempted broader eradication using aerial spraying of chlorinated hydrocarbon insecticides over millions of acres. These programs killed fire ants but also devastated wildlife and beneficial insects, drawing sharp criticism from conservationists, including Rachel Carson. The ants recolonized treated areas within a few years. No eradication effort has succeeded.
Where They Are Now
Today, red imported fire ants occupy at least 14 states across the South and Southwest, from Virginia to Texas and into California. They’ve also been introduced to other countries, including Australia, China, Taiwan, and several Caribbean nations, typically through the same pathway of contaminated soil and cargo. The species causes an estimated $8.75 billion in damage per year in the U.S. alone, affecting agriculture, livestock, electrical equipment, and residential property.
On the health side, fire ant stings cause severe allergic reactions (anaphylaxis) in roughly 5 out of every 10,000 people in colonized states. A 1989 physician survey counted over 20,000 patients treated for sting reactions in a single year, with about 2% of those cases involving anaphylaxis. The numbers have likely grown as the ants’ range has expanded.
Why One Introduction Was Enough
It may seem unlikely that a single contaminated load of ship ballast could produce an invasion spanning a continent. But the biology of fire ants made it almost inevitable. A lone queen can found a self-sustaining colony. That colony can grow to hundreds of thousands of workers within a few years. Mature colonies produce thousands of new queens during mating flights, each capable of starting her own colony within a few miles. And the lack of natural enemies, such as the parasitic flies and competing ant species that keep populations in check in South America, allowed explosive growth that native ecosystems had no mechanism to resist.
The genetic evidence confirms just how small the founding event was. Analysis of genetic diversity in U.S. fire ant populations shows a sharp bottleneck compared to native South American populations, consistent with a very small number of original colonists, possibly even a single multiply-mated queen, giving rise to the entire North American invasion.