Thank goodness they aren’t as big as rabbits! Fire ants, tiny as they are, pack a powerful punch. Since their arrival, they have become a pervasive pest on the landscape of the South, pockmarking the countryside with mounds of dirt packed full of stinging ants. The stinging ants have had a predominantly negative impact on agriculture, resident insects, reptiles, people — and biologists are learning more about their impact on native wildlife.
• The Invasion: Mobile, Ala. is the most commonly listed point of entry for fire ants’ invasion of North America. The ants arrived around 1930, according to most literature, from South America in a cargo ship possibly stowed away in the dirt used as ballast. They have thrived in the South, essentially free from predators and diseases that control their population in South America.
According to the U.S. Department of Agriculture Agricultural Research Service (USDA), after 70 years flourishing in the South there are five times more fire ants per acre in the Southeast than in the places they came from in South America.
The rusty colored, aggressive ants are small, but in spectacular numbers per acre, they have had a big negative impact. According to USDA figures, fire ants cause upward of $5 billion in annual losses in medical costs, agricultural losses and chemical-treatment costs.
Fire ants are omnivorous, which means they will feed on nearly anything that has value as food, both plant and animal. They consume germinating seed from a long list of vegetables, they strip bark from seedling trees, they feed on developing fruit and flowers. They can make life miserable in a vegetable gar- den where they are commonly found on okra feeding on an oil produced by the okra plant. When you peel back an ear of corn, you may find that fire ants have beat you to the developing kernels — and they may sting you to boot.
Fire ants feed heavily on insects — everything from caterpillars to grasshoppers. On the positive side, they consume bugs considered harmful to agriculture, including army worms, boll weevils and sugar cane borers. They also consume ticks and cockroach larvae and flea larvae.
The ants live in colonies beneath mounds of soil. Mounds may be as tall as 2 feet above the ground and may extend twice that deep below the ground, depending on the soil type. The population of a single mound can range from 100,000 to 500,000 ants, and in heavily infested areas there may be more than 40 million ants per acre. The workers in a mound are sterile females charged with foraging for food and protecting the queen.
Fire-ant mounds seem to sprout in spring and early summer, especially after a rain, as the workers clear the tunnels and deposit the soil atop the mound. Within the mound, ants may move eggs and developing larvae to the areas with optimal temperature conditions during the day. During hot weather, a mound may appear to be inactive when the ants descend to escape the heat and to find moisture.
In ant-infested areas of the Southeast, the number of mounds may range from 100 per acre to more than 1,000 mounds per acre. The hardened soil mounds damage mowers, combines and other farm equipment. Farmers face higher costs due to equipment repair and the expense of baits or pesticides. Calves, pigs and other livestock born in the presence of fire ants may be injured by the stinging ants.
Fire ants have been called the ants from hell due to two memorable attributes: their aggressive behavior and their painful sting. If a mound is disturbed, the ants swarm out of the colony by the hundreds to sting any intruder, ranging from insects to browsing cows and horses to people.
Ants often swarm onto an intruder for several seconds before stinging in unison. Researchers debate the signaling mechanism; some say the ants release a pheromone that triggers the response to sting in unison. Another biologist at Auburn University suggests that the response is mechanical. When one ant stings, the victim jumps, signaling the other ants to sting.
When an ant stings, it first grips the skin of its victim with its pincher jaws. The grip gives it leverage to arch its body and use its hold to drive the hypodermic-needle-like stinger at the end of its abdomen into its victim. An ant may often pivot around its jaw-hold and sting repeatedly, injecting venom into each of a tiny circle of stings.
The venom injected is a combination of an alkaloid and protein which kills tissue and causes an immediate burning sensation, much like being poked with a red-hot needle. Within 24 hours a pustule — a raised blister — usually forms at the site of the sting. If the pustule is scratched and becomes infected, it can cause permanent scar- ring. In a small number of people, the venom can cause anaphylactic shock. Infants and the elderly are more prone to be stung repeatedly and face serious medical problems because they may not realize the danger or be able to quickly remove the ants.
Both winged males and females disperse from the mound during mating flights in the spring and summer. The flying fire ants may be carried for miles by the wind. Mating takes place while on the wing. The males die soon after mating and fall to earth. When the female alights, she burrows into the ground and lays about a dozen eggs which hatch within a week beginning a new colony.
Research into the impact fire ants have on wildlife is relatively new but is supports the assumption that ground- nesting birds, reptiles and small mammals are particularly vulnerable. Animals that hatch from eggs are a dinner awaiting a swarm of fire ants. Ants present when an egg is pipped (when the shell is first broken by the animal inside) enter the shell and attack the hatchling, whether it be turtle, bird or even alligator.
Some adult reptiles are also predated on by fire ants. Box turtles are particularly susceptible to fire-ant predation because of their defensive reaction to close their shell and wait for a threat to leave. The closed shell, however, still allows access by fire ants. A study at Florida Atlantic University found that fire ants could kill adult 3-toed box turtles by entering the closed shell.
Sea turtles that come ashore to nest also face the threat of fire ants when the eggs hatch. A 2001 study in Florida documented that fire ants monitored turtle eggs to be able to feed on them when they hatched.
Even alligators aren’t immune to fire ants. Fire ants readily inhabit the alligator’s nesting mound and feed on alligator hatchlings. They may irritate the adult gator enough that the gator may not uncover the nest when the eggs hatch. A Florida study in the late 1990s determined a 14.6 percent loss of alligator young to fire ants.
• Birds: Ground-nesting birds and birds that nest near the ground are high on the fire ant’s list of delicacies. Fire ants have been implicated in decline of the common ground dove in South Carolina. In Mississippi, studies have shown that the ants are predators of eastern towhees, indigo buntings, cardinals, and yellow-billed cuckoos, among other songbirds. The ants will climb fence posts to predate on nestlings in blue-bird and other bird-box-nesting species. The nesting bird may abandon a nest before the eggs hatch due to harassment by the ants.
A study by a graduate student at Texas A&M found ants caused a significant mortality of songbirds, killing as many as one-fifth of baby songbirds before they could leave the nest.
Another Texas study found a 92 percent reduction in survival of ground- nesting waterbirds that nested in June and July when the fire ants were most active. Birds affected included gulls, snowy egrets, black rails and great blue herons.
Brown pelicans nesting on the Georgia coast reportedly abandoned their nesting area due to fire ants but returned after the area was treated.
Bobwhite quail populations have plummeted in the South over the last 50 years. According to the U.S. Breeding Bird Survey estimates, the quail population in the Southeast has declined 3 per- cent per year since 1966 with a population reduction in some areas of more than 90 percent. Most biologists attribute the decline primarily to habitat loss and clean-farming practices that have eliminated the “edge” habitat that quail require, but fire ants have apparently also played a role.
A University of Florida study found a 38 percent mortality rate of bobwhite quail chicks due to fire-ant predation. The ants invade nests and sting and consume just-hatched young. Chicks that are stung and survive also have reduced survival rate coping with stings on their legs and eyes that impede their ability to move and feed.
A graduate thesis by a student at Louisiana State University documented that fire ants directly kill hatchling quail but also impact quail populations indirectly by competing for insects which are important for young quail. The study also found that the presence of fire ants change a quail chick’s behavior. A chick that is harassed by fire ants has less time to rest and feed because it is forced to spend the time avoiding ants.
Another Texas study in 1999 found bobwhite quail hatching success was equal in areas treated to suppress fire ants compared to an untreated area, but the survival rate was 50 percent less in the untreated area, the impact of ant predation being highest after hatching.
• Deer: Deer fawns are also susceptible to the presence of fire ants. A study done at Texas Tech found that fire ants didn’t kill deer fawns, but the ants may cause increased predation on fawns by increasing fawn movement. When a doe leaves her fawn(s) during the day, the fawn’s defensive response is to lie still relying on camouflage and lack of scent to avoid detection by predators. Fawns are often left in areas of early succession vegetation where fire ants are also present. When foraging fire ants discover a fawn, they tend to go for softer tissue around the eyes and nose where the venom may cause swelling, irritation or even blindness. When fire ants find an immobile fawn and sting, the fawn may finally be forced to stand and move, making it more apparent to predators. Chemical treatment of areas man- aged for deer can have positive impact. A Texas study showed fawn recruitment to be higher in areas treated for fire ants, compared to untreated areas.
• Small Mammals: Mice, rabbits and other small mammals are also impacted by fire ants that invade their nests while the newly born young are defenseless. An early study (1969) found that more than 25 percent of litters of cottontail rabbits in enclosures where fire ants were present were killed by the ants.
• Fish: The jury is still out on the impact of fire ants on fish. Much of the early information about fire ant’s impact on fish was reportedly based on observational studies — i.e. dead fish found and the cause was attributed to fire-ant stings from ingested ants.
There have been some dramatic incidents attributed to fire ants. In 1998, a fish kill of more than 22,000 stocked rainbow trout in the Guadalupe River in Texas was attributed to fire ants. According to Associated Press reports, the kill occurred when thousands of adult male ants, dying after their breeding flight, fell into the river and were eaten by the trout. The trout reportedly died from toxins in the dead ants rather than from stings.
Fish may develop the ability to avoid the stinging ants. At Clemson University, a study by C.R. Allen found that rainbow trout avoided feeding on fire ants after only one exposure.
According to the University of Arkansas Cooperative Extension Service, large numbers of stings may prove fatal to fish, but a few stings may have no impact.
• Biologic Control of Fire Ants: Biologists have looked for natural predators to help control red ants. Biologists with the USDA working with entomology departments at a number of universities have imported, raised and released phorid flies, also known as decapitating flies, from South America. These tiny flies pursue the workers and lay an egg on the worker fire ant’s thorax. The egg hatches into a larvae that migrates to the ant’s head where it develops, feeding on the head, which eventually falls off. The adult fly develops within the decapitated head. When it emerges, the attacks on ants continue. Each female fly may lay eggs on 200 to 300 fire ants.
The benefit of the decapitating flies is twofold. Besides killing ants, the tiny phorid flies are reportedly aggressive in their pursuit of fire ants, chasing the workers back to the mound and disrupting the ant’s foraging and thus the over- all health of the mound.
Phorid flies have been released in most Southeastern states and are also increasing their range. Overall, phorid flies have been released in 11 Southeast states and populations have been established in six states.
Decapitating flies aren’t considered a means to eradicate fire ants, but when used in combination with chemical agents, may lower populations.
A second biological effort is also underway to slow the fire ant’s efforts at conquering the South — the importation of fire-ant disease from South America.
Thelohania is a disease that afflicts fire ants in Brazil. In the South, biologists have used the disease to infect fire- ant mounds. Fire-ant larvae infected in the laboratory with thelohania are introduced into a fire-ant mound. The ants readily adopt the larvae and raise them, spreading the disease among the workers and to the queen.
• Chemical Treatment: Chemical treatment for fire ants fall into three categories: granular baits, mound treatments and broadcast insecticide.
Granular baits are scattered around fire-ant beds. The workers pick up the bait and carry it back to the mound. Interestingly, fire ants cannot eat solid food. When a worker finds food it brings it back to the mound where it is given to ant larvae. The mature larvae chew the food and convert the solid food into a liquid which is shared with other ants including the queen. The dilemma for fire-ant bait manufacturers was to create a bait that was slow-acting enough so that it didn’t kill the worker that found it before it could deliver the bait to the mound. If the bait isn’t dispersed within the mound and doesn’t kill the queen, the colony is unaffected.
Mounds can also be treated by flooding them with insecticide, which kills the colony quickly. This treatment may be most effective around highly used areas around homes. Broadcasting insecticide is an effective way to kill fire ants over a larger area, such as a pasture.
Any treatment that leaves the egg- laying queen alive is ineffective, which is why it’s recommend you do not disturb the mound ahead of treatment. If the mound is disturbed, worker ants may lead the queen away through underground tunnels to a safe haven. Because fire ants recolonize an area, any treatment is considered control rather than eradication, and must be ongoing. Fire-ant baits such as Amdro®, Logic® or Award® will control ant populations but only with regular re-application.
While research on the impact of fire ants is ongoing, biologists all agree that fire ants are expanding their range in North America, and they are ere to stay.