Fire ants

Two distinct species of fire ants (genus Solenopsis ) from South America have been introduced into the United States this century. The South American black fire ant (S. richteri ) was first introduced into the United States in 1918. Its close relative, the red fire ant (S. wagneri ), was introduced in 1940, probably escaping from a South American freighter docked in Mobile, Alabama. Both species became established in the southeastern United States, spreading into nine states from Texas across to Florida and up into the Carolinas. It is estimated that they have infested over 320 million acres (130 million ha) covering 13 states as well as Puerto Rico.

Successful introduced species are often more aggressive than their native counterparts, and this is definitely true of fire ants. They are very small, averaging 0.2 in (5 mm) in length, but their aggressive, swarming behavior makes them a threat to livestock and pets as well as humans. These industrious, social insects build their nests in the ground—the location is easily detected by the elevated earthen mounds created from their excavations. The mounds are 18–36 in (46–91 cm) in diameter and may be up to 36 in (91 cm) high, although mounds are generally 6–10 in (15–25 cm) high. Each nest contains as many as 25,000 workers, and there may be over 100 nests on an acre of land.

If the nest is disturbed, fire ants swarm out of the mound by the thousands and attack with swift ferocity. As with other aspects of ant behavior, a chemical alarm pheromone is released that triggers the sudden onslaught. Each ant in the swarm uses its powerful jaws to bite and latch onto whatever disturbed the nest, while using the stinger on the tip of its abdomen to sting the victim repeatedly. The intruder may receive thousands of stings within a few seconds.

The toxin produced by the fire ant is extremely potent, and it immediately causes an intense burning pain that may continue for several minutes. After the pain subsides, the site of each sting develops a small bump which expands and becomes a tiny, fluid-filled blister. Each blister flattens out several hours later and fills with pus. These swollen pustules may persist for several days before they are absorbed and replaced by scar tissue. Fire ants obviously pose a problem for humans. Some people may become sensitized to fire ant venom, have a generalized systematic reaction, and go into anaphylactic shock. Fire-ant induced deaths have been reported. Because these species prefer open, grassy yards or fields, pets and livestock may fall prey to fire ant attacks as well.

Attempts to eradicate this pest involved the use of several different generalized pesticides, as well as the widespread use of gasoline either to burn the nest and its inhabitants or to kill the ants with strong toxic vapors. Another approach involved the use of specialized crystalline pesticides which were spread on or around the nest mound. The workers collected them and took them deep into the nest, where they were fed to the queen and other members of the colony, killing the inhabitants from within. A more recent method involves the release of a natural predator of the fire ant, the "phorid" fly. The fly releases an egg into the fire ant. The larva then eats the ant's brain while releasing an enzyme . The enzyme systematically destroys the joints causing the ant's head to fall off. The flies were released in 11 states as of 2001 and seem to be slowly inhibiting the growth of the fire ant population. As effective as some of these methods are, fire ants are probably too numerous and well established to be completely eradicated in North America.

[Eugene C. Beckham ]

RESOURCES

BOOKS

Holldobler, B. The Ants. Cambridge: Harvard University Press, 1990. Taber, Stephen Welton. Fire Ants. Texas A&M University Press, 2000.

PERIODICALS

Vergano, Dan. "Decapitator Flies will Fight Fire Ants." USA Today, November 20, 2000.

OTHER

"Imported Fire Ant." NAPIS Page for BioControl Host. April 2001 [cited May 2002]. <http://www.ceris.purdue.edu/napis/pests/ifa/ifabc.html>.
"Invasive Species and Pest Management: Imported Fire Ant." Animal and Plant Health Inspection Service. May 2002 [cited May 2002]. <http://www.aphis.usda.gov>.