The Red Imported Fire Ant, Solenopsis invicta, is a stinging pest notorious for constructing large, distinctive mounds. These ants are responsible for significant ecological and economic damage across invaded territories, prompting large-scale control efforts. To understand this widespread problem, one must look to the fire ant’s ancestral home: the continent of South America.
Defining the Native Range in South America
The Red Imported Fire Ant (S. invicta) and its close relative, the Black Imported Fire Ant (S. richteri), originated in the tropical and subtropical regions of South America. The native range of S. invicta is centered around the seasonally flooded savannas and wetlands of the Pantanal region, encompassing parts of Brazil, Paraguay, and Argentina. This distribution extends north into the Brazilian state of Mato Grosso and eastward toward São Paulo State, and into the northern provinces of Argentina.
The two primary invading species, S. invicta and S. richteri, come from slightly different, though overlapping, native zones. S. richteri is native to a more temperate area, primarily ranging from northern Argentina, throughout Uruguay, and into southern Brazil. This difference suggests S. richteri is better adapted to cooler climates than S. invicta.
The native ecosystem is home to numerous natural enemies, including specialized parasitic phorid flies (Pseudacteon spp.), which help keep fire ant populations in check. These biological controls are absent in the regions the fire ants have invaded, allowing them to proliferate unchecked. The high density and dominance of fire ants in their invasive range stand in stark contrast to their more constrained presence within their native South American habitat.
The Initial Journey: How Fire Ants Invaded North America
The arrival of fire ants in North America was an accidental consequence of international commerce, a common pathway for invasive species. The most widely accepted timeline places the first major introduction of the Red Imported Fire Ant (S. invicta) in the 1930s. This followed an earlier arrival of the Black Imported Fire Ant (S. richteri) around 1918.
The primary entry point for both species is believed to be the port of Mobile, Alabama, or other similar Gulf Coast ports. The ants likely traveled as stowaways within cargo shipments originating from South America, most probably from Argentina. The mechanism for transport involved queen ants and parts of colonies hidden within soil used as ship ballast or in materials like lumber, hay, or raw plant goods.
Once a mated queen or a small colony fragment established itself, the species found the American Southeast highly suitable. The region’s warm, humid climate and open grasslands closely resembled their native subtropical environment. This initial beachhead in Alabama became the epicenter from which the red imported fire ant began its rapid spread across the southern United States.
Current Global Distribution and Invasive Status
After establishing a foothold in the southeastern United States, the Red Imported Fire Ant became a global invasive species, using the U.S. population as a secondary source for subsequent invasions. The ant’s current range in North America covers approximately 128 million hectares, extending across nine southeastern states from Florida to Texas, and reaching as far north as North Carolina and Virginia. Isolated infestations also occur in the drier climates of Arizona, New Mexico, and California, often facilitated by human transport.
The expansion of S. invicta beyond the Americas highlights its ability to colonize new territories via global trade and travel. Since the late 20th century, the ant has been introduced to multiple other continents, including Australia and across Asia. Significant populations are now established in southern China, Taiwan, and parts of Japan and Korea, with detections also occurring in New Zealand, though eradication efforts there have been successful.
This widespread success is due to several biological advantages, including a high reproductive capacity and the ability of colonies to rapidly relocate or form “living rafts” to survive floods. The lack of natural predators and diseases present in their South American home allows the ants to achieve exceptionally high densities, outcompeting native insect species and causing environmental and economic harm in nearly every new region they invade.