What to do about those Fall fire ants

By By Gail Barton / horticulture columnist
November 7, 2004
When our weather is rainy, they say it is fine weather for frogs. Our recent heat wave has been fine weather for fire ants.
Normally by this time of year, fire ants are on the decline. Daily temperatures in the 80s have caused an increase in mound activity. The ants have invaded the sidewalk by our greenhouse and have taken residence in any pansy flat sitting on a sidewalk crack. We've had to initiate control measures that are normally not needed in fall.
Like kudzu and Japanese honeysuckle, fire ants are invasive exotics. The Red Imported Fire Ants arrived in Mobile from South America in the 1920s. Despite a federal quarantine, nine Southern states and more than 275 million acres have become infested with imported fire ants since then.
The figures are impressive, but most of us detest fire ants because we have experienced their venom first hand. When disturbed, fire ants become quite aggressive. They attack by gripping the skin in powerful jaws or mandibles and stinging the victim repeatedly.
Generally it doesn't take much to provoke fire ants. Simply walking near a fire ant mound can trigger an attack.
A typical fire ant nest is 12 inches to 18 inches across and 3 feet to 5 feet deep in the ground. The nest consists of a network of underground tunnels and chambers. After cool rainy weather in spring, the ants expand the nest to make a mound of loose soil above ground. The ants dwell in this mound during warm weather when temperatures are optimal for raising a brood. Since fire ants love the heat, they seek warm place to make their mounds. Normally fire ant mounds are in sunny spots. They also prefer building mounds on or near concrete like our sidewalk.
Within the mound, the brood varies greatly in appearance. The eggs laid by the queen hatch into worm-like larva. The larva feed hungrily until they metamorphose into an immobile pupa. The pupa eventually transform into one of three types of adults.
Most mounds have one fertile female or queen. The queen is the brain of the mound. If the queen lives, the mound will continue to grow and if she dies, the mound dies.
All the other ants in a mound except for a few winged fertile males are wingless sterile female workers. A nest that is a year or more old can contain 100,000 to 300,000 workers. The workers excavate tunnels that extend out from the mound for several yards. Tunnels allow the workers to exit in all directions to search for food like insects, spiders, centipedes and earthworms. When a foraging worker finds a new food source, she returns to the mound leaving a chemical pheromone trail for other workers to follow.
For dessert, fire ants feed on sugary honeydew that is given off by sap feeding insects like aphids. Most fire ant colonies keep a "herd" of aphids. If you've ever been stung by fire ants while picking peas or okra, you've experienced the anger of fire ant workers protecting their herd of aphids.
A mature fire ant colony can produce many queens each year. During warm spring weather (usually after a rain-storm), excess queens leave the mound with the males to establish new colonies. Since the queen and males are winged, they fly from the nest and mate during flight. The male dies after mating and the queen flies on in search of an open sunny nesting site.
Only a small percent of queens successfully start new mounds. If the queen is lucky, she finds an appropriate site, lands, sheds her wings and begins digging underground tunnels. She will lay her eggs in these tunnels within one or two days after mating. These eggs produce the first generation of workers. After that, the queen lays eggs continuously and the workers forage to feed the brood. New fire ant colonies get off to a slow start. Generally they aren't even visible to us until about six months after the queen lays her first eggs.
The most effective fire ant control tactics disturb the mound as little as possible. When the ants realize there is a threat, they take the queen deep into the ground to safety. If the queen is not killed, the mound can regenerate. I normally use low toxicity baits like Amdro to control fire ants. Bait should be scattered around the edge of the mound in late afternoon where foraging workers will find it and take it in. This late in the season, the Mississippi Cooperative Extension Service advises that mound drenches are more effective. Drenches should be applied in mid morning after sun has warmed the mound.