Red imported fire ants (RIFA), Solenopsis invicta, are an invasive insect that was introduced into the United States in the 1930’s from South America. They have since spread throughout the southeastern states. Currently, fire ants infest over 336 million acres and are now found in much of North Carolina’s piedmont and coastal plain. Caldwell County is just outside the official US Department of Agriculture RIFA quarantine zone. However, Catawba & Burke counties were both included in the quarantine zone recently. Caldwell County is falling victim to the natural spread of these insects. Fire ant mounds are randomly showing up across the County. Everyone reading this article will likely have to deal with fire ants in the near future.

Fire ants have a painful sting and are aggressive in nature. Fire ants like sunshine. These insects can be a pest in gardens, lawns, athletic fields, parks, pastures, and other open spaces. Utility companies are also plagued by fire ants. These insects like to construct nests/mounds near electrical transformers, junction boxes, power lines, and in ground meter boxes.

Fire ants build mounds. They vary in size but are usually in direct proportion to the size of the colony. The mound allows the ants to regulate the internal temperature of the colony. A mound that is 2 feet in diameter and 18 inches tall may contain over 100,000 ants. Mounds in clay soil will usually be symmetrical and dome-shaped, and mounds constructed in sandy soils will be irregular in shape.

Red imported fire ant mound. This was part of a fire ant control study being done in a pasture in Rutherford County. (credit: Seth Nagy)

Individual mounds may be treated with a liquid or dust insecticide formulation or with an insecticidal bait. Liquid treatments may be done by rodding the chemical deep into the mound or by drenching the mound. To be effective, the drench must penetrate throughout the mound and contact most of the fire ants in the colony. Ants coming into contact with the drench die soon after. Drenches are the preferred treatment when the risk of human contact with fire ants is high and the fire ant infestation must be eliminated immediately because of the health risks of someone getting stung. High-risk areas include home lawns, school and child care properties, parks, and other areas frequented by the public. Best control results are usually obtained in spring and fall when temperatures are between 70° and 85°F. Control with mound drenches can be more difficult during very hot summer months because the ants often remain deep within their mounds and are hard to reach with liquid insecticides. In the summer, drenches are best done in the morning or evening.

Granular baits can be used to treat individual mounds. These baits are a mixture of an insecticide and a food that is attractive to fire ants. Worker ants carry particles of the bait back to the mound and feed them to the “brood” (larvae or immature ants) and the queen. Even when the insecticide kills the queen, workers may be active inside the mound for several weeks before the colony finally disappears. Baits are somewhat slow acting but easier to apply than mound drenches. Therefore, they are best used in situations where many mounds must be treated, or when water for mixing mound drenches is difficult to obtain, or when the risk of human or non-target animal contact is low and there is no urgent need to eliminate the infestation. The active ingredients in ant baits are rapidly degraded by high temperature, high humidity, and intense sunlight.

Newer baits, like Amdro Fire Strike and Extinguish Plus, contain both the synthetic insecticide hydramethylnon, as well as an insect growth regulator. Insect growth regulators work differently than chemical insecticides by preventing insects from developing rather than killing them immediately. Insect growth regulators, which include (S)-methoprene and abamectin, are extremely safe and are one of the few types of ant control products that can be used in agricultural lands, including vegetable gardens and pastures. They are very effective in the long term but take three to four months to reduce ant populations. Since brand names change constantly, be sure to check the active ingredient listings before you purchase any pesticide to make sure you are getting the product you want. Please note: Any recommendations of brand names or listing of commercial products in this article are included solely as a convenience to the reader and do not imply endorsement by N.C. Cooperative Extension nor discrimination against similar products. Before applying any chemical, always obtain current information about its use, and read the product label carefully.