Invaders
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Red Imported Fire Ant (Solenopsis invicta)

What is it?

The red imported fire ant (RIFA) is a small reddish brown ant from South America . There are six known species of fire ants (Solenopsis spp.) in the United States, three of which are found in Arizona. These three species are the southern fire ant (S. xyloni), and two species of desert fire ant (S. aurea and S. amblychila). RIFA has not established in Arizona, but is present in the bordering states of New Mexico and California. It was discovered near Yuma, Arizona but was exterminated.


Adult red imported fire ants. Photo: USDA APHIS PPQ Archives, www.forestryimages.org .

RIFA workers marked with wire bands.
Photo: Bart Drees

RIFA are small but highly aggressive. They inject a necrotising, alkaloid venom when they sting. The stings result in painful, itchy, and persistent pustules, and sometimes in severe allergic reactions. Five million people are stung each year in the southeastern United States. About 25,000 of these people require medical consultation. When a fire ant mound is disturbed, workers boil to the surface, run up any legs, arms, etc. in the vicinity, grab the victim's skin in their mandibles and sting synchronously in response to the slightest movement. The attacks are coordinated and dozens or even hundreds of workers sting in unison.


Fire ants swarming boot after mound disturbance. Photo: Bart Drees.

Pustules from RIFA stings. Photo: Murray S. Blum, The University of Georgia, www.forestryimages.org

Fire ants live in colonies that may have 100,000 to 500,000 ants. The queen of the colony can lay from 1500 to 5000 eggs per day, never leaves the nest and can live for many years. Worker ants take care of the queen and her eggs, build the nest, defend the colony, and find food. Preferred food of fire ants consists of protein-rich sources such as insects and seeds. Winged male and female ants fly from the colony in the spring and summer to mate in the air. The males die and the females become queens that start new colonies.

Only the red imported fire ant has a median clypeal tooth and a striated mesepimeron; these may be difficult to see at first. RIFA also have an antennal scape that nearly reaches the vertex, a post-petiole that is constricted at the back half, and the petiolar process is small or absent. Of the native fire ant species, the southern fire ant (Solenopsis xyloni) looks the most like the red imported fire ant. It can be identified by its brown to black color, well-developed petiolar process, and no median clypeal tooth. Desert fire ants (Solenopsis aurea and S. amblychila) are both yellowish-red to reddish-yellow and have a well-developed petiolar process. RIFA can also be identified by the proportion of large to small workers in disturbed mounds. If half the workers in disturbed mounds are large and dark, it is RIFA. If only a few large ants appear relative to hundreds of small ants, it is non-RIFA.


Median clypeal tooth can be seen here on head above the mandibles. Mandible has 4 teeth. Photo: Carl Olsen

Side view of adult RIFA worker. Photo: USDA APHIS PPQ Archives, www.forestryimages.org.

Table: Characteristics of RIFA and RIFA mounds

Characteristic

Description

RIFA

 
# of node segments 2
# of sizes of workers Many (polymorphic)
Size of workers 3 7 mm
Shape of thorax Uneven
# of antennal segments

10 with two-segment club at tip

# pairs of spines in thorax

None

Color

Reddish brown, abdomen darker

Stinger

Present, inflicts pain, leaves white pustule (pustule is not an allergic reaction)

 

Mound

 
Material Formed from excavated soil
Size

Wider than a dinner plate at its base
Shape Amorphous, often oval shaped like a mountain cone
Visibility Above ground, 4 to 24 tall
Entrance None visible, ants access mound through subterranean tunnels that spoke out from the central mound
Texture Has a fresh-tilled appearance, especially after a rain

 


Size variation in RIFA worker ants and queen on the right. Photo: S. Porter.

RIFA mounds have a fresh tilled appearance. Photo: Bart Drees.

Why is it a Threat?

RIFA colonies are extremely destructive. They dominate their home ranges due to their large numbers and aggressiveness. The lack of natural enemies results in population booms in areas they invade.

RIFA alter the composition of the ecological communities in the areas they invade. They outcompete and frequently eliminate native fire ants. They also compete with other animals for food and alter abundance of prey species. RIFA attack eggs and young of many bird and reptile species. In areas of high infestation, RIFA have significantly reduced northern bobwhite quail populations (Allen et al. 1995) and may completely eliminate ground-nesting species from a given area (Vinson and Sorenson 1986). They also attack small mammals such as rodents and have been known to attack and sometimes kill newborn deer and cattle. Due to a 10-20 year lapse before bird population reductions are observed, it is suggested that actual effects of RIFA on animal populations may be underestimated (Mount 1981). Natural plant ecosystems could potentially be impacted as well. RIFA predates upon solitary bees that are pollinators of certain plants (Vinson 1997) and move and feed on large quantities of seeds.


Tricolor heron chick being attacked by fire ant workers. Photo: Bart Drees.

RIFA feeding on plant nectar. Photo: S. B. Vinson.

Stings from RIFA create health problems for many humans. Fire ants sting repeatedly and venom is injected from the poison sac with each sting. RIFA venom has a high concentration of toxins that cause an intense burning and itching that lasts for an hour and is followed by a blister that becomes a white pustule. Broken or scratched pustules can result in secondary bacterial infections and permanent scars. In some individuals, severe allergic reactions can occur resulting in anaphylactic shock and even death (Dowell et al. 1997).


RIFA worker biting and stinging human. Photo: Texas Department of agriculture file photo.

Secondary infection following RIFA sting on hand. Photo: Texas Department of Agriculture file photo.

RIFA cost the US billions of dollars a year in damage to agricultural crops and equipment, livestock, wildlife, public health, and electrical equipment such as air conditioners, traffic signal boxes, electrical and utility units, telephone junctions, airport landing lights, electric pumps for oil and water wells, computers, and even car electrical systems. Control methods for RIFA are extremely costly.


Fire ant mound in electrical utility housing. Photo: S. B. Vinson.

RIFA in traffic control relay switch box. Photo: Bart Drees.

Distribution

RIFA are native to South America and were brought to the US sometime around the 1930's. They now occupy more than 275 million acres of land in the US and are found in Florida, Georgia, Alabama, Mississippi, Arkansas, South Carolina, North Carolina, Texas, Louisiana, Tennessee, New Mexico , Oklahoma, and California. They invade via transported nursery stock, honeybee colonies, and on empty trailers and trucks. Cold temperatures may limit the northward spread of RIFA in the US and the westward spread may be limited by drier conditions. Natural dispersal occurs on flowing water. Areas with seasonal flooding are vulnerable to RIFA invasion.


RIFA colony floating in flood water. Photo: Bart Drees.

RIFA colony emerging from flood water. Photo: Bart Drees.

Habitat

In infested areas, colonies are common in lawns, gardens, school yards, parks, roadsides, and golf courses. Nests generally occur in sunny, open areas and are most common in disturbed and irrigated soil. RIFA mounds are 4 to 24 inches tall and have no visible surface entrance. Mounds are accessed through subterranean tunnels that spoke out from the central mound. Non-RIFA mounds rarely exceed an inch or 2 in height. RIFA mounds have a fresh-tilled appearance, especially after a rain.

Typical RIFA mound. Photo: USDA APHIS PPQ Archives, www.forestryimages.org .
Profile diagram of RIFA mound. Photo: Texas Cooperative Extension file photo.

History

RIFA are believed to have arrived in the southern United States around the 1930's on ships from South America as ballast waters were dumped or goods were unloaded. Their range expanded rapidly and today they occupy 13 US states and Puerto Rico .

What can be Done

RIFA is a regulated species in Arizona. To keep RIFA out of Arizona, the Arizona Department of Agriculture has been conducting surveys at high-risk sites such as nurseries, parks, truck stops, etc. In 2004, all samples collected were negative for RIFA. The drier climate in Arizona is a limitation for this species, however, as we irrigate more lawns, agricultural fields, and golf courses, we increase our chances of a successful RIFA invasion. Once RIFA has established in an area, the chances of eradicating it are slim and control becomes the primary means of fighting its spread. It is vital that we prevent the spread of this species. RIFA very easily travel in potted plants and soil and in our vehicles. If RIFA is detected, citizens should contact the Arizona Department of Agriculture for confirmation and eradication. Eradication methods are complex due to the life cycle of the species and should be conducted by trained individuals.

Links

Arizona Department of Agriculture Red Imported Fire Ant Update

Invasivespecies.gov a gateway to Federal efforts concerning invasive species

Texas Imported Fire Ant Research and Management Project

University of Florida Red Imported Fire Ant Site

US Department of Agriculture APHIS Imported Fire Ant Information

References

Allen, CR, Lutz RS, and Demarais S. 1995. Red imported fire ant impacts on northern bobwhite populations. Ecological Applications 5(3):632-638.

Dowell, RV, Gilbert, A, and Sorenson J. 1997. Red imported fire ant found in California . California Plant Pest and Disease Report 16(3-4) June-September: 50-55.

Mount RH. 1981. The red imported fire ant, Solenopsis invicta (Hymenoptera: Formicidae) as a possible serious predator on some southeastern vertebrates: direct observations and subjective impressions. Journal fo the Alabama Academy of Science 52:71-78.

Vinson SB. 1997. Invasion of the Red Imported Fire Ant (Hymenoptera: Formicidae) Spread, Biology, and Impact. American Entomologist 43(1):23-29.

Vinson SB, Sorenson, AA. 1986. Imported Fire Ants: Life History and Impact. The Texas Department of Agriculture. P.O. Box 12847 , Austin , Texas 78711 .