By Neal Lineback and Mandy Lineback Gritzner, Geography in the NewsTM
Fire Ants: Surviving and Thriving
Summer is in full swing and an invasive pest is making life miserable for residents, visitors and native and domestic animals across the southern United States. Fire ants have an incredible ability to survive and geographically expand their territories despite all efforts to eliminate them or even to contain their diffusion.
A Georgia Tech study cited by the National Geographic and Smithsonian sheds fascinating light on fire ant survival skills. When enduring a flood, the insects can form a raft by locking arms, legs and jaws. The raft is buoyant and completely airtight, allowing the ants to float away and survive as a floating ant raft even for months.
Fire ants are not native to the United States. They hail from oft-flooded regions of Brazil and Argentina, which likely explains their raft-building abilities. Just as other exotic plants and animals from distant lands have made themselves at home in the United States, fire ants have adapted to the landscape and climates here.
The fire ant is a menace across the southern United States and continues to spread outside the region. The fire ant first entered the United States at Mobile, Ala., in 1918 aboard a cargo ship likely from Argentina. From that single introduction, colonies began diffusing inland from that Gulf Coast port. A classic pest diffusion study by Howard Adkins (Annals of the Association of American Geographers, September 1970) followed the waves of the infestation that moved as far west as Central Texas, north as far as southern Tennessee, and east as far as North Carolina’s southern coastal plain.
The scientific consensus was that the colder climates to the north and west might limit further diffusion of the fire ant. To some degree that seemed true, but plant nurseries and sod farms delivering soil across the country provided new avenues of diffusion. The fire ant’s geographic range continues to expand.
The fire ant is only about one-quarter inch (6 mm) long. It lives in huge colonies that produce mounds of dirt, sometimes one to three feet (0.3-0.9 m) across and rising from a few inches to two feet high (0.6 m). In heavily infested areas, more than 50 mounds may occupy a single acre, about the size of a football field.
Each colony has a queen ant with phenomenal egg-laying ability. Her colony may number 250,000 ants, occasionally including new queens. These new queens can fly to new territories several hundred yards or meters away, where they may lay eggs and establish new colonies.
A fire ant colony feeds mostly on seeds, young plants, crickets and other insects. The ants, however, will attack anything that inadvertently disturbs their mounds. Most other ants bite invaders and spray acid on the wound. Fire ants, on the other hand, bite only to attach themselves to the victim and then sting (from the abdomen) injecting toxic venom.
Visitors unaware of fire ant dangers may be curious about the origin of the low dirt mounds on the landscape. When they purposefully or unintentionally disturb a colony’s mound, however, thousands of ants flood to the surface seeking the source of the disturbance. Within seconds, they may swarm the victim, inflicting venomous stings.
From painful personal experience, the authors of this GITN article describe the sting as having a sensation similar to being burned by fire, thus the name “fire ant.” Also from personal experience, occasionally, victims may have extreme allergic reactions to the fire ant venom and experience anaphylactic shock. Without immediate medical treatment, death can result.
Farmers and gardeners in the South carefully watch for fire ant colonies. Crop harvests often create contact between humans and fire ants when mechanical combines scoop up the tops of colony mounds. Fire ants have been known, in fact, to kill young calves and other animals dropped during birthing on fire ant colonies.
The fire ant research on rafts originally was published in the Proceedings of the National Academies of Science (Apr. 25, 2011). Ants create a raft when their nest is flooded by water. The raft is completely impenetrable—even the ants on the bottom layer of the raft do not get wet. Researchers say this phenomenon occurs because ants’ skin is hairy and uneven. Those rough surfaces repel water leaving a layer of air around each ant.
Researchers found that when submersed in water, fire ants press together so efficiently that each ant’s air bubble merges with the others’ to produce a big bubble that protects the entire raft. In the lab, the ants were able to lock onto one another within 90 seconds, making a raft about two and a half ants thick.
When a group of insects behaves as one, entomologists refer to them as a super-organism. Fire ants, while considered a terrible nuisance, must be admired for their communal behavior and super-organism status. They survivor and thrive.
And that is Geography in the News.
Sources: GITN 1096 “Fire Ants: No Wonder They Survive,”June 3, 2010;GITN #726, “Dangerous Fire Ants,” Maps.com, April 30, 2004:; and http://www.pnas.org/content/108/19/7669.abstract?sid=f46a2537-1319-49c8-a35f-521205d78e36
Co-authors are Neal Lineback, Appalachian State University Professor Emeritus of Geography, and Geographer Mandy Lineback Gritzner. University News Director Jane Nicholson serves as technical editor. Geography in the NewsTM is solely owned and operated by Neal Lineback for the purpose of providing geographic education to readers worldwide.