By ELEANOR SPICER
They’ve hit shore, creeping with stealth onto the lawns of unwary Outer Banks residents, with their populations of hundreds, or hundreds of thousands, or even millions of individuals, all hungry. Soon, the lawn is not enough. They trickle into coastal homes, a steady stream growing stronger by the day. People begin finding them in sinks and cereal boxes, trespassing on car seats and creeping across crepe myrtle.
Meet the Argentine ant. This invader is here to do more than ruin a Sunday picnic.
The Argentine ant’s scientific name is Linepithema humile, which refers to its humble size and the recruitment lines the ant forms while marching to and from food sources. Small in stature and reddish-brown in color, the Argentine ant does not bear the appearance of a troublemaker. But looks can be deceiving.
“These ants are ranked among the world’s 100 worst invaders,” says Jules Silverman, an entomology professor at North Carolina State University.
The ants may have been introduced to the United States as long as 100 years ago in the soil ballasts of ships coming to port in New Orleans. Since then, they’ve been introduced to several other ports, wreaking havoc wherever they land.
Beating the Competition
Like most ant species, each Argentine ant worker is able to smell other ants to distinguish if they are from her colony or not. Ants do not fight members of their own colonies.
Unlike other ants, however, Argentine ant colonies often span long distances. Because a colony is at peace, it can keep growing and spreading. A large Argentine ant colony is called a “supercolony.” One along the Mediterranean coastline stretches 3,700 miles. Another in California spans nearly 375 miles.
North Carolina has its own supercolony in progress. Nobody knows how they are moving across the state, but Argentine ants are found from Wilmington to all the way up through the Outer Banks, and at least one supercolony has been identified in Research Triangle Park. Even though the ants are not poisonous and do not sting, their ecological influence among invaded territories packs a pretty mean punch.
“They work well in groups,” explains John Brightwell, a postdoctoral researcher at NC State who is studying the Argentine ant population on Ocracoke Island. “This particular species of ant has many queens in each nest. Each queen can crank out tens of thousands — if not hundreds of thousands — of workers each year,” he says.
“They pretty much wipe out all other ant species in their introduced range,” Brightwell adds. “Each type of native ant is here for a reason — each ant has its own niche to fill.”
For example, some ants in North Carolina find seeds and plant them. Other ants might be food for birds, lizards or other insects. “When these native ants are gone, the whole balance of the ecosystem can get thrown out of whack,” Brightwell explains.
In California, where Argentine ants have been established for nearly a century, they threaten the existence of the endangered coastal horned lizard. The South American invaders drive away or kill all of the area’s native ants, which the lizard depends on for food.
“There is no telling how many species of plant and animals in North Carolina depend on native ants, but the removal of native ants can have cascading effects on our environment,” Brightwell says.
As if wiping out native species weren’t enough, Argentine ants bring along an entourage of shady characters — tiny insects called aphids that live in the tree canopy.
“The ants will tend the aphids like a farmer will tend cattle,” Brightwell says. “They herd them along the branches and leaves in a tree, protect them from predators, and ‘milk’ them for a sweet substance the aphids produce from plant juices called honeydew.”
But aphids have mouthparts like tiny hypodermic needles, injecting plants with diseases wherever they feed. “This can cause your plants to get sick or even die,” Brightwell notes.
Despite the food from their aphid-tending business, Argentine ants are always looking for more, which is why people find them invading homes in force.
Fighting the Invaders
Because an Argentinean supercolony can span miles of territory, getting rid of it is tricky. Unlike nests of other ant species, which are treated by spraying, Argentine ants have no single nest to treat.
“In invaded areas, people seem to be treating their houses for ants constantly, only to find the ants keep returning,” says Brightwell.
On an island, such as Ocracoke, it is possible the entire ant population could be explored and managed. But on the mainland, the extent of the ants’ spread can seem endless. By observing the ant populations in the smaller, isolated environment of an island, researchers are able to make conclusions they hope can be applied to larger areas.
Brightwell has traversed Ocracoke Island, crawling under houses and knocking on doors to find the ants. After locating some, he tests different methods of controlling them to see what is most effective. Brightwell hopes his observations on Ocracoke are a first step to learning more about this growing mass of unwelcome insects.
“There are so many unknowns right now,” Brightwell says, “We’re not sure how they’re impacting this coastal environment, the best way to control them, or the extent of the invasion. But one thing we do know is that they are here and they could be spreading.”
History of Invasions
North Carolina has some history with ecological invaders. Terri Kirby Hathaway, North Carolina Sea Grant’s marine education specialist, points to the large rat-like nutria and fire ants as examples.
“Nutria were introduced to the United States from South America in the 1880s for use in the fur industry,” Hathaway says. Their herbivorous nature causes problems for the marshlands they inhabit, and these critters have proven their staying power.
Nutria’s habit of burrowing and chewing destroys roads and other man-made objects, and their penchant for snacking on plant life has destroyed thousands of acres of marshland habitat since their introduction.
“Now they’re pretty much an accepted part of the U.S. marsh ecology in the states that have them,” Hathaway says.
Fire ants literally marched their way into the United States from South America. These ants thrive on the edges of disturbed habitat, such as roadways. Added to their annoying habit of digging up lawns and building conspicuous mounds, fire ants have a nasty sting. They bite to hold on, and then sting in a circle using stingers in their abdomen. New research has also shown that fire ants can have negative effects on seabird nesting, driving adult birds from their nests and attacking the young. Some homeowners have reported Argentine ants attacking nesting birds as well.
One of the best ways to avoid an invasion is to prevent it Hathaway says.
“Never release plants, fish, or animals into a body of water unless they came out of that body of water!”
And be careful about accidentally transporting organisms to new places. Carefully inspect things like gardening mulch or plants to make sure tricky trespassers aren’t hitching a ride. Aromatic cedar mulch has been shown to repel Argentine ants, and — if placed around a house — could help prevent them from coming inside.
Know Your Neighbors
One way to prevent an ant invasion is by getting to know ants a little better.
“A lot of people think they may have Argentine ants invading their kitchen,” Brightwell says. “What they may actually have is the odorous house ant, the Argentine ant’s native counterpart.”
Although the odorous house ants can be just as worrisome in the kitchen, they usually don’t have the negative environmental impacts that Argentine ants do. The two species look alike to the untrained eye, and both are commonly referred to as “sugar ants.” But it is easy to tell them apart by smell.
“All you have to do is squish one of the ants in your kitchen and sniff it. If it has a fruity smell, it’s an odorous house ant,” Brightwell explains.
No fruity odor? It’s probably an Argentine ant.
Some people may not mind sharing their neighborhood or home with what looks like harmless little insects tottering across a sidewalk or a kitchen windowsill. However, entomologists see each Argentine ant as a soldier in disguise — each one representing a vast, underground army that is growing in ranks and spreading. With careful public attention, these miniscule marauders can be halted in their tracks, Brightwell says.
“When we quickly change parts of nature, like bringing in Argentine ants into a habitat, sometimes nature doesn’t have the defenses to catch up. Sometimes, it’s up to us to help out where we can.”
Other Coastal Invasions
Argentine ants are not the first invaders along the North Carolina coast. Take a look at some other coastal pillagers:
- Lionfish, native to the Indo-Pacific, have been found lurking around the reef systems in North Carolina waters. In addition to depleting fish populations, they have dangerous, venomous spines.
- Large, rat-like mammals known as nutria scurry through North Carolina marshlands. They have been here so long that they now are accepted as part of the state’s marsh ecology.
- The African clawed frog Xenopus laevis — invasive in many states and known to prey on native fish populations — was found in a North Carolina fish hatchery. It was successfully eliminated when the hatchery was drained.
- No one knows how the butterfinger nudibranch, an exotic sea slug from western Africa, made it to North Carolina. It has been photographed on shipwrecks 25 miles off the coast and may be competing with native mollusks for resources.
- Brought in accidentally on ships, the Australian spotted jellyfish negatively affects the shrimp industry along the East Coast. These jellyfish accumulate in such large numbers that they clog shrimp nets and reduce yield.
This article was published in the Holiday 2009 issue of Coastwatch.
For contact information and reprint requests, visit ncseagrant.ncsu.edu/coastwatch/contact/.