Solenopsis invicta

School of Ants collection
Literature record

Solenopsis invicta

From Dr. Eleanor's Book of Common Ants

A.K.A.: Fire ant

What’s the big deal?

Anyone growing up south of Virginia or east of New Mexico has likely experienced the supreme pleasure of jamming a stick into a Solenopsis invicta, or fire ant, mound and watching its angry inhabitants boil out from the earth. Fire ants are notorious for their brawly disposition and their pustule-producing stings, but beneath that surface lies an intricate, well-oiled machine, or perhaps the better word is superorganism, that’s worth a second look (hold your sticks, please).

Fire ants originally hail from Argentina, Brazil and Paraguay, but ever since they landed in Mobile, Ala., in the 1930s, they have made themselves at home in the United States. Their conspicuous, large mounds burst from the land across the Southeast like mini volcanoes, dotting open ground, roadsides, agricultural fields and residential areas (Callcott and Collins 1996). Fire ants fill their mounds with galleries and tunnels for storing food, raising young, and just hanging out being ants.

Fire ant mounds also have their own air-conditioning system. Because these mounds extend deep into the earth, the ants can move their colony down to cooler ground when it’s hot outside. When temperatures drop, they pull the colony up, closer to the surface, to take advantage of the sun’s warming rays (Taber 2000). Fire ant colonies can grow to be large. Some include as many as 200,000 workers (Tschinkel 1998), roughly the population of the inner core of my own city, Raleigh, North Carolina.

In order to make a nest large enough for so many respiring, moving, working, eating, bodies, they also have a lot of soil to haul. The dirt on the surface of some fire ant mounds (most of which corresponds to the holes they have dug below) can fill 10 gallon-sized milk jugs (Markin et al. 1973). Meanwhile, below the surface their underground foraging tunnels can radiate out 100 feet in all directions (Taber 2000), allowing them to reach a cool drink of water or even food even when the air above ground is too cold (or just as often in the South) too hot. All of that soil-turning aerates the ground and helps plants grow, but it gets them—and us—into trouble. Agricultural fields with plantings like soybeans and corn provide everything fire ants need in one place, a kind of agricultural Wal-Mart. The soft, tilled earth makes a perfect nesting ground. Pest insects cover the plants—easy prey for fire ants to pick up and take back home. Native ants are fewer, offering the fire ants less competition. Farms are where, to these ants, the livin is easy. Unfortunately, fire ants don’t realize farmers plant fields for people, not fire ants. Their mounds damage harvest machinery, and the ants nibble plant roots, resulting in lower crop production. In addition, fire ants encourage pest insects like aphids to grow so the ants can then snack on honeydew, the sweet substance aphids produce (Stimac and Alves 1994). Plus, they sting anyone unlucky enough to cross their threshold.

When a fire ant stings you, she first grabs your skin with her jaws. Once she has a tight grip, she jabs her stinger, located at the tip of her abdomen, into your skin as many times as she can before you knock her off. Her stinger is a tiny poisoned spear. Each time her stinger makes contact with you, she injects a small amount of toxic poison. This poison causes itching, oozing pustules on most people, but for some unlucky victims, the stings trigger an extreme allergic reaction, anaphylaxis.

For their nestmates, fire ants’ stingers work for more than just fighting. A fire ant’s stinger most often works as a trail marking tool. Using the pin-like tip of the stinger for precision, a fire ant returning from a particularly large food resource will release an invisible chemical called a pheromone as she moves back towards the nest. Her sisters follow that pheromone scent like bloodhounds, a fast track to a sure resource.

Everything I’ve told you so far is about how fire ants live in the places they have invaded (or rather, we have brought them). Back home in their native Argentina, fire ants nest near frequently flooded river beds. Most ordinary ant species would drown in these floods, but not our fire ant (Note, as a southerner, despite not always liking fire ants, I do think of them, a bit, as “mine”). When the water rises, these ladies hook their legs together to make massive living rafts for their entire colony to float along and ride out the flood. Workers take turns riding the waves so no ant is left under water for too long. Covered in small, water-repellent hairs, fire ant bodies stay coated in a silvery sheen of air even if you try to dunk them underwater.

Young stick-toting naturalists aren’t the only observers captivated by fire ants’ curious lifestyle. In fact, the fire ant is the most studied of all ant species by scientists who spend years tracking its behavior and ecology. Scientists and citizens alike try to prevent fire ants from marching forward across the United States. We use an array of poisons and home remedies like boiling salt water. We release enemies like fire ant-eating flies. We make laws to prevent moving them from state to state. But, despite these efforts, fire ants remain, turning soil, laying trails, tending aphids, making rafts. Fire ants live up to their scientific name—invicta, or “unconquered.”

Find out more about this species at Antweb and see more photos at Alex Wild's photography site.