What Are Fire Ants?
When people say "fire ants" in the United States, they're almost always talking about the red imported fire ant, or RIFA (Solenopsis invicta). These ants are not native to North America. They arrived from South America — most likely from Brazil or Argentina — through the port of Mobile, Alabama sometime in the 1930s or 1940s, probably as stowaways in soil used as ship ballast.
Since then, they've spread across most of the southern United States, from the Carolinas to California. They now infest more than 300 million acres across at least 14 states and Puerto Rico, according to the USDA.
Why Fire Ants Are a Problem
Fire ants are aggressive. Unlike most native ant species that will try to avoid confrontation, fire ants attack in large numbers when their mound is disturbed. They grip the skin with their mandibles and sting repeatedly, injecting a venom that produces a burning sensation — which is how they got their common name. Within a day, stings typically develop into raised white pustules. For most people this is painful but not dangerous; for those with allergies, fire ant stings can trigger serious allergic reactions including anaphylaxis.
Beyond the stings, fire ants cause significant agricultural and infrastructure damage. They damage crops, kill ground-nesting wildlife, and have a well-documented attraction to electrical equipment where they cause short circuits and equipment failures. The USDA has estimated that fire ants cause billions of dollars in damage annually across the United States.
Red Imported Fire Ants vs. Native Fire Ants
North America does have several native fire ant species, including Solenopsis xyloni (the southern fire ant) and Solenopsis geminata (the tropical fire ant). These native species are smaller, less aggressive, and build smaller colonies compared to the imported variety. In many areas, the red imported fire ant has actually displaced native fire ant populations because it out-competes them for food and territory.
The key differences between imported and native fire ants:
- Colony size: RIFA colonies can exceed 200,000 workers. Native species typically top out at a few thousand.
- Aggression: RIFA swarm and sting much more aggressively when disturbed.
- Mound size: RIFA mounds can reach 18 inches tall and extend several feet underground. Native fire ant mounds are generally smaller and less defined.
- Worker size variation: RIFA workers come in a wide range of sizes (polymorphic), from about 1/16 inch to 1/4 inch. This size variation within a single colony is a good identification clue.
For more on telling fire ants apart from lookalikes, see the fire ant identification guide.
How Fire Ant Colonies Work
A fire ant colony is a highly organized social structure centered around one critical individual: the queen. She is the only member of the colony that reproduces, and everything the workers do — foraging, tunneling, defending the mound — serves the goal of keeping the queen alive and producing more ants.
This is why bait-based treatments are so effective: they exploit the colony's own food-sharing behavior to deliver poison directly to the queen. Understanding the biology is the key to effective treatment. You can learn more about the colony's internal structure in the mound anatomy page and the full fire ant life cycle breakdown.