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Red Imported Fire Ant Control

By Leslie McGuire, regional editor

RIFA Colonies may contain 250,000 workers reaching infestation rates of 500 ant mounds per acre with millions of ants. Photo courtesy of University of California Cooperative Extension

The red imported fire ant (RIFA) is a highly aggressive invasive species known for its fiery sting. Also known as Solenopsis invicta, it is a formidable enemy wherever it establishes a colony. It is one of the few pests that is dangerous in urban settings, on agricultural lands and in natural habitats. They are showing up in greater numbers throughout the south and the southwest in sports fields, lawns, parkland, school yards–anyplace where the soil temperature is consistently warm and there's plenty of water from either irrigation or natural rainy conditions.

Imported fire ants are attracted by electrical currents and have caused considerable damage to heat pumps, air conditioners, telephone junction boxes, transformers, traffic lights and gasoline pumps. Photo courtesy of Orange County Vector Control

Photo courtesy of Texas A & M University

The red imported fire ant has caused billions of dollars in damage to agriculture and poses a significant threat to public safety and the environment. They attack humans, animals, ground nesting birds, plants and seeds as well as anything electrical. Since invading the southeastern United States in the l930s the pest has become widespread in Alabama, Arkansas, California, Florida, Georgia, Kansas, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee and Texas. Now established on 310 million acres in 16 southern states stretching from the Atlantic to the Pacific oceans, they appear to be capable of steadily extending their range.

First arriving in the United States from South America, they entered through Mobile, Alabama, probably in soil used as ship's ballast, and they've been spreading dramatically ever since through the actions of man. Since the 1950s they have spread northward, westward and southward. Their northward spread depends on temperature. Cold winters tend to push them back. Their western spread is largely dependent on water. This species was also recently discovered in Australia and New Zealand, and many areas around the globe are largely at risk for infestation including large portions of Europe, Asia, Africa and numerous island nations. They are most likely to be found in urban areas, creek bottoms and irrigated land.

Because of the rapid housing boom of the 1950s, the pest spread throughout the rural and urban south largely because of the movement of grass sod and woody ornamental plants used in landscaping.

The entire Pacific coast is fertile ground for infestation. In Southern California, state and federal officials have placed Orange County and portions of Los Angeles County and Riverside County under quarantine. It is believed that the infestations in Southern California may stem from a shipment of infested nursery stock from the southeastern states. Fruit orchard infestations in the agricultural regions have been traced back to colonies that hitchhiked on beehives shipped to California from Texas. The quarantine limits the movement of articles including plants and soil, and requires commercial nursery growers to take steps to ensure their products are free of red imported fire ants. The ants are presently well distributed in Orange County, Calif. where they have been found on sports fields, parks, and large irrigated lawns. According to Mike Hurst of the Orange County Vector Control, because of the budget cuts, the state will stop funding fire ant control in October of 2004. However, recently, 67 percent of voters in Orange County (notorious for voting down tax increases) approved an additional tax that would pay for fighting red imported fire ants and mosquito-borne West Nile disease. It is now the only part of the state with a countywide fire ant program.

When red imported fire ants swarm, the first bites cause the person to jerk, which triggers all the other ants to sting in response. That's why they appear to all sting at once. Photo courtesy of Texas A & M University. Photo courtesy of Texas A & M University

Red imported fire ants also nest within urban structures such as the walls of homes and offices. They establish colonies under sidewalks and roadways. When the nest is abandoned the ground subsides, cracks appear and may occasionally result in the complete collapse of sections of these structures. The presence of fire ants can deter outdoor activities in yards, parks and school grounds. Home invasions can threaten small children and the elderly. House invasions are especially prevalent during periods of heavy rain and flooding. The ants are often found in places near water such as utility rooms, bathrooms, near water heaters or in laundry because the folds resemble the tunnels of their mounds. Fire ant colonies have also been found inside automobiles, trucks and recreational vehicles. Traffic accidents have been caused by fire ants stinging the drivers. Accident victims have been attacked by fire ants when thrown from their vehicles.

Drawing courtesy of Orange County Vector Control

Fire ants are most notorious for their stinging behavior. They respond rapidly and aggressively to any movement or disturbance to the colony or a food source. A single fire ant can sting repeatedly and will continue to sting even after its venom sac has been depleted. After an initial intense burning sensation, a pustule forms 24 to 48 hours later that can become the site of a secondary infection leading to permanent scarring if not kept clean. A minority of those stung by red imported fire ants have an allergic reaction, suffering chest pains, nausea, dizziness, shock or, in rare cases, falling into a coma. Some deaths have occurred, but these are rare.

After mating, the new queen excavates a brood cell underground. Within 30 days, large workers emerge daily and within six months several thousand workers can occupy a colony or mound. The mound is a cone-shaped dome of excavated soil that has a hard, rain-resistant crust. The mound averages 0.40 m in diameter and 0.25 m in height. In heavier soils, the mound can be bigger.

As the colony matures, the polymorphic nature of the worker becomes more apparent. The largest workers in the colony (majors) can be as much as 10 times the size of the smallest workers (media). The Queen lives up to seven years and produces and average of 1600 eggs per day throughout her life. Photo courtesy of Orange County Vector Control

Red imported fire ants feed on almost any plant or animal material. In rural areas they've had a huge impact on local ecosystems. Studies have shown a minimum two-fold reduction among populations of field mice, deer, song birds, lizards, toads, snakes, turtles, quail and other vertebrates, with, in some cases, entire species being eliminated. There are equally intense repercussions for other species because the food chain is being disrupted. In addition to reducing animal populations they also feed on plants such as young saplings and trees by stripping the bark and girdling them. They destroy buds and developing fruits as well as the seeds of 139 other species including peanuts, pecans, corn, sorghum and soybeans. In addition, red imported fire ants also raise and nurse plant pests such as aphids and scale insects. Their activity on plants may reduce the ability of pollinators to pollinate flowers. In drip irrigated fields they have been known to build their mounds over emitters, reducing or blocking the flow of water.

A recent study prepared by Clemson University estimates the average home in South Carolina spends $200 annually for red imported fire ant control, with total state expenditures ranging between $111 million and $166 million annually. In 1998, a survey of South Carolina physicians estimated 660,000 stings, 33,000 requiring medical treatment and two reported deaths from fire ants. The USDA estimates the pest causes billions of dollars in agricultural losses, ecological damage and chemical control costs.

The Australian government feels that red imported fire ants pose such a serious threat to their economy, environment and life style, they've been declared a "notifiable pest" under the Plant Protection Act of l989. That means landholders who think they may have fire ants on their property are legally obliged to inform the DPI Fire Ant Control Center or be liable for large fines.

Fire Ant Control

At present, any attempts to control fire ants over large areas are impractical. Red imported fire ants cannot be eliminated completely with methods available today, but some methods can successfully control them within limited areas including individual mound treatments and broadcast treatments. However, no method is permanent, and they will most likely return within a year.

Given a dark, protected site with enough moisture and food supply, fire ants will nest in rotten logs, walls of buildings, under sidewalks and roads, in automobiles and in cow manure.

Late August through October is an ideal time to apply RIFA control. The ants are still foraging and weather patterns are more predictable so bait can be applied when no rain is expected for several days after treatment. Baits are best applied with crank-type seeders or spreaders. Baits are slow acting and can take weeks or months to reduce mound numbers, however with regular maintenance applications they are effective. Be careful and only use insecticides when and where they're needed

Baits and Insecticides

Baits are safest for use around children, pets and animals. Baits when broadcast have a very low toxicity. Scattered thin, they sink down into the grass. Granular and dust products will remain on the soil surface where potential contact can occur unless the material is watered.

There are usually no external openings on the RIFA mound, and workers emerge from underground tunnels that radiate from the mound. While mounds are important to a colony, they are not essential to its survival. Photo courtesy of USDA APHIS PPQ

Insecticidal mound drenches are generally effective, however great care must be taken to avoid disturbing the mound prior to the application. If the mound is disturbed in any way, the workers will carry the queen deep into the tunnels where the insecticide will not reach. Injectants are usually more effective, however they are expensive and dangerous if not handled properly. One proven method of reducing RIFA populations in heavily infested home lawns and ornamental turf is the two step method consisting of (1) once or twice a year broadcast an application of a bait product (e.g., Amdro(R), Logic(R), Award(R) or Ascend(R)), and then waiting several days to a week before (2) using an individual mound treatment such as dust, granule, bait or drench insecticide such as Siege(R), Raid(R) Fire Ant Killer, or use Orthene(R), Dursban(R), Green Light(R), Spectracide(R) or Bayer's TopChoice(TM). Bayer Environmental Science is now offering a reduced product price to school districts for fall applications

Biological Controls

Phorid flies, known as decapitating flies, attack red imported fire ants by dive bombing them and laying eggs in their heads. Two species of the flies are now permanently established in hybrid populations in Alabama and possibly Mississippi. P. tricuspis the larger of the two species is permanently established at more than a dozen sites and populations are beginning to expand at the rate of 10 to 20 miles a year. Both were extensively tested for environmental safety prior to applying for release permits. In order to avoid them, the worker ants retreat into the mound and stop foragitng. The smaller of the two species, P. curvatus is the smallest of the decapitating flies which means that it can parasitize small worker ants, the most abundant workers in the colony.

There are about 20 species of phorid flies in South America that specifically attack fire ants. Pseudacteon curvatus and Pseudacteon tricuspis are the only ones known to be established here. Researchers at the Agricultural Research Service at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, Fla. report that they have collected P. curvatus flies from the research site where they were released by Sanford Porter, acting research leader of the ARS Imported Fire Ant and Household Insects Research Unit in 2003 after collection in Argentina. It is very exciting that these phorid flies managed to survive over the winter and establish a permanent foothold in the United States.

P. curvatus is a phorid fly that is the smallest of the decapitating flies which means it can parasitize small worker ants, the ones that are the hardest workers in the colony. Phorid fly maggots live in the head capsules of their fire ant hosts, eventually decapitating them and pupating inside their heads. Phorid flies only attack fire ants. Other biocontrol agents used in addition to the parasitic phorid fly are the microsporidian protozoan disease Thelohania solenopsae, and Solenopsis dagerrei (Hymenopter: Formicidae), a workerless ant that is a social parasite.

The Australian government uses all methods possible to spray and drench mounds including helicopters, trucks and off road vehicles.

Orange County is now actively looking for bio-rational materials that are growth regulators and metabolic inhibitors. By eating baits that mimic growth hormones, the ants will eventually die out. "Orange Country is too dry for phorid flies," says Mike Hurst of OC Vector Control. "Furthermore, the flies inhibit foraging, so that means the ants won't take up the baits. Using bio-rational materials takes awhile, but we do see preliminary results in two weeks. We will continue to treat three to four times over the course of the next year."

Integrated Pest Management

The weapon of choice is an integrated pest management strategy that combines monitoring, thresholds, precision targeting, bio-controls and reduced-risk pesticides. Dr. David Williams, research entomologist and internationally recognized RIFA expert from CMAVE's Imported Fire Ant and Household Insects Research Unit (IFAHI), is working on a project at the Fort Jackson Army Base and the MacIntyre Airforce Base in South Carolina to control the spread. His integrated strategy combines computerized Arc View GIS/GPS precision-targeting technology to plot and monitor RIFA mounds and classic biocontrol agents specific to the red imported fire ant. The strategy also makes use of the pesticide fipronil, a reduced risk pesticide registered as an alternative to organophosphates. Fipronil is an ingredient in many commercial pest control products and should be identified by checking the contents.

Amazingly enough, fire ant populations are now much greater in the United States than they are in South America where they originated. The reason for this is that in South America, their natural enemies keep them from becoming the dominant species. Here in the United States they have moved into residential populated areas with a vengeance–even into houseplants.Photo courtesy of Orange County Vector Control

The demonstration, which will continue for three years, has preliminary findings showing that RIFA populations were reduced by 96 percent in the fipronil plus biocontrol test plot and reduced by 87 percent in the fipronil only test plot. Populations increased by 44 percent in the untreated control plot. Samples of fire ant workers in the biocontrol test plot also indicated successful inoculation of the colonies with the disease T. solenopsae.

Photo courtesy of Texas A & M University

It is important to note that as a result of the research demonstration at Fort Jackson Army base, the USDA's Animal Health and Inspection Service (APHIS) decided to raise phorid flies for controlled release as RIFA biocontrol agents. APHIS's Gulfport, Miss. Facilities will oversee releases and assist states in monitoring field sites. The released phorid flies cannot handle the red imported fire ant alone, however an integrated approach will serve as the treatment strategy of choice until additional biocontrol agents are identified and evaluated for release.


Potential United States Range Expansion of
the Invasive Fire Ant, Solenopsis invicta

Potential Red Imported Fire Ant Range Expansion.

This map predicts areas in the United States that are susceptible to invasion by the red imported fire ant, Solenopsis invicta. Predictions are based on climate and current northernmost distributions of red imported fire ants. To generate these predictions, a dynamic model of colony growth was developed that depended on daily minimum and maximum soil temperatures. An annual precipitation limit was selected to indicate regions where arid conditions may prohibit growth in nonirrigated areas. Results of the model predict that red imported fire ants will likely move 50 to 100 miles north in Oklahoma and Arkansas. They will also likely continue expanding into portions of Virginia, Maryland and Delaware in the east, and New Mexico, Arizona, California, Oregon, Nevada and maybe even Washington and Utah in the west. This study was a joint project of the University of Arkansas at Monticello and the USDA-ARS lab in Gainesville, Florida headed by S.D. Porter, L.C. Thompson, Korzukhin, M.D. and S. Wiley.

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June 26, 2019, 12:03 pm PDT

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