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More than 75 years ago, the red imported fire ant, Solenopsis invicta Buren, was accidentally brought into Mobile, Alabama from South America. It now infests more than 325 million acres, comprising most of eleven southern states and Puerto Rico, with infestations also in New Mexico and California. It has recently been reported in northern Mexico, Australia, Taiwan and China. Another species—the black imported fire ant, Solenopsis richteri Forel—also was introduced, but this species is limited to northeastern Mississippi, northwestern Alabama and southern Tennessee. A large population of hybrids between S. richteri and S. invicta occurs in northern Mississippi, Alabama and Georgia and southern Tennessee (see map). Fire ants can travel long distances when newly mated queens land on cars, trucks, trains or aircraft cargo containers, or when winged forms are carried by the wind. Shipments of nursery stock or soil from an infested area may relocate entire colonies.
 A typical fire ant mound
Why Early Eradication Programs Failed
Attempts in the late 1960s and early 1970s to eradicate the red imported fire ant were not successful. The pesticides used, although effective, were no match against a species capable of re-invading treated areas. The reasons for failure are debatable, but it is now known that eradication is hindered by the ant’s biology and by problems with treatment methods. Recent attempts to eradicate S. invicta from parts of California and Australia using new products and treatment methods have shown very promising results. Successful eradication of the imported fire ant in large areas has yet to be documented.
Biological Obstacles to Eradication
In the southeastern U.S., S. invicta infests such an extensive area that a single treatment would take years and massive resources to apply. Fire ants have a high reproductive rate and disperse easily. Thousands of reproductive females are produced per colony, and mated females begin a colony wherever they land. Queens can fly up to a mile on their own or even farther when assisted by the wind. Once colonies are established, the ants eliminate competing insects and then rapidly overwhelm an area. Whole colonies can move, and in the multiple-queen form, the colonies can split into many new colonies. If a poison works too rapidly, the worker is killed before the poison is passed to the queen. Finally, worker ants from well-fed colonies may not forage on a bait product, or a bait may not be as attractive as some abundant natural food.
Colonies move vertically and horizontally in the soil profile to escape floods, droughts and extreme temperatures. When new colonies are not actively foraging, they may be un-affected by baits or other pesticides applied to the soil surface.
Technological Obstacles to Eradication
Pesticide treatments are expensive and time-consuming, and there are only three basic approaches. The first is surface treatment using a residual contact poison. This approach is the least environmentally sound because the treated surface remains toxic for a long time. The ants may survive by foraging underground. The second is individual mound treatment, which involves applying a large volume of pesticide to reach the queen. However, it is nearly impossible to locate all of the colonies in an area and difficult to manipulate large volumes of liquid. Also, mound treatment is more expensive and time-consuming than broadcast treatments. Colonies not eliminated may move or split into several colonies. The third method is bait treatment, which uses some sort of attractive substance the ants like to eat. Unfortunately, baits are not always consumed, and the bait’s attractiveness is short-lived. The bait must be slow-acting and effective over a range of doses, since the dose the ants get cannot be controlled. Baits may also be attractive to and kill some native ant species that compete with fire ants.
Economic, Regulatory and Environmental Obstacles to Eradication
The best way to treat large areas (hundreds of acres) is by an aerial application of bait. However, not all areas can be treated because of label restrictions and application limitations. Even with a bait product, it is not feasible to treat the entire infested area or even a large part of a single state, and untreated areas are sources for reinfestation. The larger the treatment area the more slowly reinfestation occurs. If periodic treatments are discontinued, the area may become more infested than it originally was within a year or two because treatments may have eliminated competing ant species.
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