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About Fire Ants — History

About 75 years ago, the red imported fire ant, Solenopsis invicta Buren, was accidentally brought into Mobile, Alabama from South America. The black imported fire ant, Solenopsis richteri Forel, had also been accidently introduced into Mobile a few decades earlier, but the range of this species remains limited to northeastern Mississippi, northwestern Alabama and southern Tennessee. A large population of hybrid fire ants (S. richteri x S. invicta) exist in a band between the two parent species and can be found in southeastern Tennessee, northwestern Georgia, northern Alabama, and northern Mississippi. Imported fire ants now infest more than 340 million acres comprising most of nine southeastern states and Puerto Rico with smaller infestations in Virginia, Tennessee, Oklahoma, New Mexico and California. Fire ants can travel long distances when newly-mated queens land on cars, trucks or trains or when winged forms are carried by the wind. Shipments of nursery stock or soil from an infested area may relocate entire colonies or nests.

Why did eradication programs fail?

Attempts in the late 60's and early 70's at eradicating the red imported fire ant were not successful. The pesticides used, although effective, were no match against a species capable of re-invading previously treated areas. The reasons for failure are debatable, but it is now known that eradication is hindered by three obstacles: biological, economic, and pesticide treatment.

Biological obstacles to eradication

  • The ants infest 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 the mated females begin a colony wherever they land.
  • Queens can fly up to a mile on their own or further when assisted by favorable winds. 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.
  • The queen is protected from many poison baits, because she is fed only food eaten first by workers and larvae. 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.

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.

Pesticidal obstacles to eradication:Pesticide treatments are expensive and time-consuming, and there are only three basic approaches.

  • 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.
  • Individual Mound Treatment, which involves the application of a large volume of pesticide to reach the queen. However, it is nearly impossible to locate all of the colonies in an area, difficult to manipulate large volumes of liquid, and treatment is more expensive and time-consuming than other treatment options. Colonies not eliminated may move or split into several colonies.
  • 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.