Red Imported Fire Ants in Central Texas Clay Soils

The Ant Behind the Mound

Solenopsis invicta Buren is a social insect that lives in colonies built around one or more queens, worker ants, and, periodically, winged males and females that leave the nest on mating flights. Texas A&M AgriLife Extension describes two colony structures found in Texas: single queen colonies and multiple queen colonies. In multiple queen colonies, worker ants are not territorial and move freely between mounds, which is part of why infested properties can carry so many mounds at once.

A queen can live seven years or more. A newly mated queen starts small, laying roughly a dozen eggs on her own, but once workers begin tending her she can produce up to 800 eggs a day, according to the AgriLife Extension biology page for the Texas Imported Fire Ant Research and Management Project. Eggs develop into adults in about 30 days, passing through four larval stages and a pupal stage. A mature colony can hold 100,000 to 500,000 workers, and it takes as few as half a dozen workers to start a brand new colony after a mated queen lands and begins digging.

Why Central Texas Clay Soil Favors Fire Ants

Much of Austin sits on the Blackland Prairie, where Texas A&M AgriLife Extension's Travis County office describes the native soil as deep, dense black and reddish clay that packs into a heavy mass and absorbs water very slowly, with little air space between particles. Once that clay does take on moisture, it holds it well. As the same clay dries out, it shrinks and cracks, forming the wide surface cracks familiar to anyone who has watched an Austin yard through a dry summer.

That drainage pattern matters to fire ants because mound building depends on workable, moist soil rather than hard, dry ground. AgriLife Extension's fire ant program notes that during drought, colonies stop building visible mounds in hard, dry soil and instead nest deeper underground, foraging during cooler periods instead. Clay that stays damp below a dry crust, and that turns soft again quickly after rain, gives fire ants a substrate they can move through and mound in for more of the year than a fast draining, sandy soil would. Foraging itself is also temperature dependent: AgriLife Extension's fire ant program puts the effective range for bait uptake at 65 to 95 degrees Fahrenheit, which lines up with the long Central Texas warm season.

Reading a Fire Ant Mound

A fire ant mound is a dome of loose, disturbed soil built up by workers rather than a simple hole in the ground. AgriLife Extension's ENTO-019 publication notes that mounds commonly form in open, sunny areas and can rise to more than 18 inches tall. Because multiple queen colonies are not territorial, AgriLife Extension reports that areas infested with multiple queen colonies can harbor 200 to 800 or more mounds per acre, since workers move between mounds instead of defending a single nest.

Colonies also spread on their own. Mated queens can fly for miles before landing to start a new nest, and during floods, colonies will mass together and float on the surface of the water until they reach dry ground, where they establish again. Because infested soil, hay, sod, and nursery stock can carry whole colonies to new areas, AgriLife Extension notes that quarantine regulations enforced by the Texas Department of Agriculture restrict moving those materials out of infested counties into noninfested ones.

The Two-Step Method

AgriLife Extension calls its recommended approach the two-step method, and recommends it for yards that are fully infested, meaning five or more mounds for every quarter acre. The method combines a broadcast bait treatment across the whole yard with spot treatment of individual mounds that remain afterward.

Fire ant baits are built around processed corn grit coated in soybean oil, carrying a slow acting active ingredient. Foraging workers carry the bait back to the colony and share it with the queen, who then dies or becomes infertile. Because the mechanism runs through the whole colony rather than just the ants on the surface, AgriLife Extension reports that baits typically take weeks to months to reach 80 to 90 percent control, in exchange for treating a large area with a very small amount of active ingredient.

1. Count mounds per quarter acre of yard. Five or more means the broadcast bait step is worth doing; fewer than that, treat mounds individually instead.
2. Buy a fresh bait product, ideally from a container that has not been opened before, since bait attractiveness fades with age.
3. Apply the bait with a hand held seed spreader. Do not mix it with fertilizer or grass seed.
4. Broadcast only when the ground and grass are dry and no rain is expected for 24 to 48 hours.
5. Apply in late afternoon or evening, when workers are actively foraging. Test this by setting a small pile of bait next to a mound and checking whether ants have found it within 30 minutes.
6. Time the broadcast for late August through mid-October where possible, since AgriLife Extension identifies this as the window that lets bait take effect before ants slow down for winter. Reapply once or twice a year.
7. About a week after broadcasting, treat any mounds that are still active with a labeled mound drench, granule, bait, or dust, following the product label exactly.
8. For a nonchemical option on a single mound, pour 2 to 3 gallons of very hot or boiling water directly onto it. AgriLife Extension reports this kills the colony about 60 percent of the time, and it will also kill the grass the water touches.

Health Risk and When to Get Help

The Centers for Disease Control and Prevention's National Institute for Occupational Safety and Health groups fire ants with bees, wasps, and hornets as stinging insects that pose a risk to anyone working outdoors. Per CDC, while most stings cause only mild discomfort, some stings result in severe allergic reactions that require immediate medical care, and in some cases stings can cause death.

Because mounds are easy to miss in tall grass or mulch, it is worth checking play areas, garden beds, and paths before letting children or pets run through a yard, and treating any mound that sits along a walkway or near an outdoor gathering space as a priority for the individual mound treatment step. Anyone who develops symptoms beyond localized swelling and the sterile pustules that follow a typical sting, such as difficulty breathing, dizziness, or hives spreading well beyond the sting site, should get medical attention right away rather than waiting it out.