The Ant From Hell

They’re not picky: they’ll destroy either concrete or asphalt. But they have different methods for each.

At Camp Lejeune in North Carolina, fire ants carried out bits of soil from under asphalt roads, forming intricate tunnels. They crawled underneath to keep warm in late fall and early spring. Then, when traffic or a heavy truck came along: instant pothole. The ants caused 160 potholes, each costing North Carolina’s Department of Transportation $200 to fix.

On new concrete sections of Interstate 75 in Tampa, Fla., the ants had a different destructive approach: After entering naturally formed tunnels underneath the silicone sealant in joints between highway sections, they burrowed upwards. "Every so often, they’d feel the urge to come up," says William A. Banks, the entomologist who helped officials cope with the road destroyers.

Banks counted 226 holes in 3,085 yards of sealant. Repair costs range for $132 to $301 per highway mile, and fire ant control is about $90 per mile each year.

In both cases, Banks, an entomologist with the Agricultural Research Service (ARS) Insects Affecting Man and Animals Laboratory in Gainesville, Fla., recommended that officials kill the ants with a bait which contains the insecticide hydramethylnon.

Hydramethylnon, the insect growth regulator fenoxycarb, and the exotoxin abamectin (from the soil fungus Streptomyces avermitilis), were all discovered to be effective against fire ants by Banks and his colleagues in the late 1970s and early 1980s. Attractive baits that are widely used for fire ant control were developed as a result of their studies.

A REAL PAIN! Fire ants, accidentally imported in cargo ships from Brazil or Argentina, are serious picnic wreckers, ferociously stinging animals and people. The hapless softball player who disturbs a fire ant mound stands to acquire numerous stings - up to five or six per square inch in a serious attack. Each sting may leave a painful blister called a pustule.

If these insects weren’t such a pain - literally - it would be easier to be fascinated by their social structure and behavior. After a virgin queen mates in midair, she descends to the ground. No longer needing wings, she now chews or breaks them off. The remaining wing muscles are used as a nutritional source for her in establishing the new colony and in egg production.

The queen ant immediately searches for a place to hide herself under a stone or a piece of paper, or in a clump of grass - "anywhere that she won’t be obvious to birds or others that might feed on her," Banks says.

Once hidden, she digs a small burrow in the soil. Within 24 to 36 hour, she lays 50 to 60 eggs - some of them are fertile and will become ants; others, called trophic eggs, will serve as a sort of egg salad for the newly hatched larvae.

After hatching five to 10 days later, the white, legless larvae molt four times. They feed on the trophic eggs and on body fluids regurgitated by their queen. Finally, they enter a pupal, or dormant stage. The first pupae to emerge are minims, very tiny workers who begin to enlarge the nest and bring in bits of food.

Meanwhile, the queen has laid more eggs, which the minims tend.

Within 25 to 30 days after the queen mates and begins the new nest, a small worker forces is in operation. Thus begins a fire ant colony, unseen underground, that in a year’s time may have 75,000 to 100,000 workers. Before dying out in five to seven years, the colony may reach 200,000 to 300,000 ants. In a typical infestation by large single-queen colonies, there may be 40 or more conical soil mounds on one acre of land.

Minims initially remain close to the queen and developing young, feeding and tending them. With the hatching of the second group of eggs laid by the new queen, the minims begin to give way to slightly larger workers called minors. As the colony continues to grow and mature, still larger workers - medias and majors - appear. After about six months, the colony contains a wide range of worker sizes.

Worker ants care for and feed the queen and developing larvae, enlarge and maintain the mound, search for and gather food, and defend the colony against intruders. The workers divide these tasks according to their age.

The youngest workers are the nurses, who move the immobile young around, feed them, lick them, and spray them and the nest area with a fine, anti-microbial mist of venom from the poison sac - all to keep them microbe-free.

As the workers age, they move away from the queen and the young and become reserves. They transfer food from the foragers to the nurses, keep the tunnels cleared of soil, and when necessary, go out to do battle.

The oldest group of ants is the foragers. These are the workers who locate and bring in the food for the colony. These ants leave the nest through tunnels that radiate from the mound and have exit holes to the surface at various intervals. The foragers exit from these holes and search randomly for food.

ARS studies have shown that a forager who finds food returns to the hole, periodically touching its abdomen to the ground to leave a trail of chemical scents called pheromones. When the forager reaches the tunnel entrance, it emits a stronger dose of the chemicals, the recruitment pheromone.

After being recruited, the ants go marching one-by-one, following the pheromone trail, to find and bring back the hoard. If it’s a large chunk, they break it up with their jaw-like mandibles and carry it back in pieces.

"They are very good at breaking up stuff that seems extremely hard. They can even burrow into bone," Banks says.

Only one stage of ant - the fourth larval stage - eats solid food. All the others thrive on liquids, sucking the juice out of food particles and tossing the remaining solid in the "kitchen midden", which is basically a garbage heap.

Banks says that fire ants eat mostly other insects, "but will feed on almost anything, from small animals and plant tissue to discarded human food. They’re pretty opportunistic. If it’s edible, they take it."

A SPREADING NUISANCE. The red imported fire ant can infest and survive in roughly the southernmost third of the United States. Some officials have expressed concern, however, that the range could expand because of the cross-breeding between red and black fire ants.

"The hybrid may be better able to survive in the border states such as Tennessee, Kentucky, North Carolina, Oklahoma and others. Whether they will actually invade these areas remains to be seen," says ARS’ Richard S. Patterson.

Scientists had though the red and black fire ants were two different species. Because their hybrids produce viable off-spring, the scientists now question if they are simply two races of the same species that vary in color. ARS will study this possibility, Patterson says.

QUEENS GALORE. A comparatively recent discovery in the fire ant saga helps explain the insects’ reproductive ruthlessness: their habit of throne-sharing.

Multiple queen colonies, first discovered in Mississippi about 16 years ago, are now known to occur in eight infested states. Each may have 10 to 100 or more queens, each reproducing. Multiple queens can mean up to 10 times more mounds per acre, studies show.

With single-queen colonies, any newly mated queen that lands in an area already infested is likely to be attacked and killed by worker ants.

"The biggest destroyer of fertile fire ant queens is other fire ants," Patterson says. He calls the single-queen colony "a fortress unto itself."

But in polygyne colonies, instead of killing other ants, workers adopt them into their nest.

"If a newly mated queen comes down from the heavens," he reports, "instead of being killed, she is accepted by workers into a nest, where she sets up shop."

Studies by chemist Robert K Vander Meer show that the ability of worker ants to discriminate between nestmates and fire ants from other colonies has been lost on polygyne populations, so that foreign queens are now accepted rather than being killed. Thus, fire ant populations of many acres act as if they were a single colony.

The Gainesville research scientists are searching for more and better control methods. First they are developing more attractive baits.

The formulations now used for fenoxycarb and hydramethylnon baits are made of corn grits they also contain soybean oil, which attracts many types of insects - including other species of ants that are, in many cases, "fire ants’ greatest enemy," according to Patterson.

So not only are insects eliminated that would have naturally helped to control fire ants, but also some of the bait is removed that is meant for the fire ants.

"Our goal is to develop baits that are more specific to fire ants," Patterson says.

Entomologist David F. Williams, for example, found that house fly and eye gnat pupae attract fire ants, which carry the pupae back to the colony as food. The scientists are investigating practical ways to use the pupae in baits.

In other work, chemist Robert K. Vander Meer has identified the recruitment pheromone. He hopes to combine this chemical and/or other pheromones produced by the ants with insecticides or insect growth regulators in baits that will attract only fire ants.

In addition to trying to expand the label uses for fenoxycarb, the scientists screen new chemicals that private companies send to them.

THE SEARCH FOR ANSWERS. Scientists working in biological control at the lab are looking closely at a parasitic wasp called Orasema and a parasitic ant, Solenopsis (Labauchena) species. Entomologists Daniel P. Wojcik and Donald P. Jouvenaz, in work with Vander Meer, found that these colony wreckers sneak into the fire ant nests and actually acquire the colony odor so that they will not be attacked and thrown out.

Fire ants integrate the invaders into the colony, deluded by the familiar odor. The Labauchena queen finds the fire ant queen and becomes yoked onto her neck, stealing food as workers bring it to her. The starved and weakened fire ant queen produces few workers.

These organisms have never survived long in the lab after they were brought from Argentina or Brazil, but Juan Briano, who has reared them successfully in the ARS Hurlingham Laboratory in Argentina, will come north to tend them in Gainesville.

MAKING ANTS SICK. Like all other animals, fire ants are subject to infectious diseases. In their native South America, they are attacked by viruses, protozoa, fungi and nematodes. But when fire ants emigrated to the United States, their natural enemies were left behind.

A major goal of the Gainesville laboratory’s biological control research is to introduce and establish in the United States a complex of fire ant diseases and insect enemies that could help to maintain a more natural balance in fire ant populations.

Some microorganisms attack a wide variety of insects and can be used for temporary, local pest control in the same manner as chemical toxicants. These microbial insecticides do not work with fire ants, however, because of a multitude of defenses. Chief among these is a filtration system in the throat of the worker ant that removes even bacteria. The result is that the queen - the reproductive center of the colony - is fed only highly filtered, microbe-free food and this remains disease-free.

A second natural protective mechanism for both the queen and developing ant larvae is the anti-microbial aerosol emitted from the stinger of the worker that periodically sprayed onto the brood, queen and surrounding nest area.

Yet another defensive mechanism used by the fire ants is the tendency to move the nest to escape potentially damaging organisms. For example, when treated with such nonspecific organisms as nematodes, the fire ants will simply move the entire colony out of the treated nest and establish a new one nearby.

The establishment of a complex of natural enemies of fire ants in the United States may be effective enough to greatly reduce, if not eliminate, the dependence on chemical control.

Finally, Williams and entomologist Sandford Porter are investigating the reasons that fire ant populations may be extremely large in one field, yet very sparse or nonexistent in a seemingly identical field nearby.

They hope to discover what natural factors prevent fire ants from thriving in such areas.

ARS hopes its multi-pronged approach - chemical, biological and biochemical - can help towards getting fire ants off our golf courses, out of our back yards, and yes - out from under our highways.

Jessica Morrison Silva is with the Agricultural Research Service.