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Imported Fire Ant
Order: Hymenoptera
Family: Formicidae

Scientific name: Solenopsis invicta Buren
Common name: imported fire ant (IFA)
Adults: There are two kinds of IFA, Black Imported Fire Ants (BIFA), Solenopsis richteri, and Red Imported Fire Ants (RIFA), Solenopsis invicta. Both are small ants, mostly about 1/8-inch long. Males and queens are larger. BIFA are all dark-brown to black, while RIFA have the forebody reddish-brown and the abdomen dark-brown to black.There are many ants that appear similar to IFA. Two other aggressive mound-building ants in Oregon are harvester ants and thatch ants. Harvester ants are found in eastern Oregon in areas that are too dry for IFA, which require sources of water. Thatch ants are found in western Oregon, but they are larger than IFA. Both ants have mounds with visible entrances. Neither of these ants sting as fiercely or are as aggressive as IFA. A common non-aggressive mound building ant in western Oregon is the mud ant, which makes somewhat columnar mounds in seasonally wet areas. Microscopic examination by entomologists is normally necessary to confirm the identity of suspected IFA.
Mounds: Mounds vary in size depending on the size of the colony. Usually they are about 18 inches across and 8 to 12 inches tall. The ants tend to build mounds in open, sunlit, grassy areas near irrigation or other water sources. Mounds in sandy soil tend to be irregularly shaped and mounds in clay soils tend to be symmetrical and dome shaped.
imported fire ant adult
imported fire ant mound
red imported fire ants
Gardens and Fields - Occasionally they feed on vegetable plants - the worst damage usually occuring during hot, dry weather. The ants seek out compost piles and mulched flower beds for warmth and moisture during cooler months.
Homes and Other Structures - Fire ants can form colonies close to homes and other buildings. During the hot, summer months they sometimes search indoors for food and moisture. Entire colonies may nest in wall voids or rafters, sometimes moving into buildings during floods. IFA may form colonies in potted plants forcing quarantines on shipments of plants from greenhouses and nurseries to other non-infested areas. They also nest under cracked pavement, removing dirt from underneath sidewalks and roadways and aggravating structural problems. Such colonies may be difficult to locate.

Electrical Equipment - Like many other ants, fire ants frequently infest electrical equipment. They chew on insulation causing short circuits and even fires. They interfere with switching mechanisms and can damage air conditioners, traffic signal boxes and other devices. Fire ants sometimes nest in housings around electrical and utility units. The ants move soil into these structures, which causes shorting and other problems.

Impacts: Imported Fire Ants or IFA have been an agricultural and health problem in the U.S. for almost 80 years. Invasive pest insects, such as fire ants, cost Americans nearly $20 billion annually. These pests greatly affect farmers, homeowners, and others involved in outdoor activities. IFA infest greenhouses and cattle pastures, stinging people and livestock. They are a nuisance in crop and nursery fields where the hardened mounds cause problems for machinery, and the ants damage plants and trees. IFA mounds detract from the aesthetic value of landscaping, and homeowners may face lower property values as a result. IFA pose serious health risks to pets and children. Imported fire ants interfere with recreation by invading parks, golf courses, wildlands, and open space. They can seriously harm fish and wildlife, causing considerable damage to sensitive habitats. IFA are now well established pests in 11 southeastern states. The threat of an Oregon infestation is greater than ever before because IFA has now spread to California.
Health Risks: Over 10,000 people a year seek medical attention for IFA stings. Fire ants are aggressive and will repeatedly sting anything that disturbs them. Characteristics of a sting include intense burning and itching which usually subsides within 1 hour. However, the itching may return periodically over the next several days or more. Within the next 4 hours, a blister forms at the site of a sting. A white pustule forms in a day or so. Treatment is aimed at preventing secondary bacterial infection, which occurs easily if the pustule is scratched or broken. On rare occasions, anaphylaxis, or a generalized, systemic allergic reaction to the fire ant stings, can occur. This is usually in persons who have been sensitized by previous stings. Symptoms include flushing, general hives, swelling of the face, eyes or throat, chest pains, nausea, severe sweating, loss of breath or slurred speech. In very sensitive victims, reactions can include heart attack and occasionally death. If an extreme reaction occurs from this or any insect sting or bite, a person should get immediate emergency medical assistance.
Distribution: IFA were originally introduced into Mobile, Alabama around 1920. There are two kinds of IFA, Black Imported Fire Ants (BIFA), Solenopsis richteri, and Red Imported Fire Ants (RIFA), Solenopsis invicta. Both species of imported fire ants are native to South America. IFA were originally introduced into Mobile, Alabama around 1920. At this time, BIFA only infests Alabama and Mississippi. The rest of the U.S. infestation is due to RIFA.

IFA is firmly established in the southeastern United States, extending north to North Carolina, west to eastern Texas, and two southern California counties. Isolated infestations, which have been eradicated, have occurred in several eastern and southwestern states. Recent outbreaks have been found in New Mexico, California, and Nevada. One of these outbreaks in California was associated with honey bees from Texas that were brought in for almond orchard pollination in Stanislaus County. Efforts are underway to eradicate infestations in those states. Fire ants have now spread to eleven California counties spanning the southern part of the state northward to Sacramento County. Some of these infested counties have nurseries that are known to ship their plants to Oregon.

Some entomologists question whether IFA could tolerate the cool and wet winters typical of western Oregon. However, the U.S. Department of Agriculture predicts that IFA could survive along a western strip (e.g. the Willamette Valley) as far north as Washington. New infestations often occur when people bring infested materials from federally quarantined areas across state lines.

distribution maps
Biology and Life Cycle: Like other ants, IFA form colonies. Winged males and queens make mating flights following warm weather rains. A fertilized queen then sheds her wings and digs a chamber in which to begin a new colony. Colonies normally have domed mounds up to 18 inches high, often near stumps or trees. Unlike many ants, the mounds generally lack visible openings, except when the winged individuals emerge. Also in contrast to many ants, IFA do not extensively forage above ground. Instead, most workers travel through a network of shallow, subterranean tunnels.

Consequently, IFA may not be noticeable until they are numerous. A colony can be a single mound or, in some cases, hundreds of mounds extending over several acres and having thousands of queens. The average colony contains 100,000 - 500,000 workers. Colonies can begin producing new queens within six months of founding. IFA largely feed upon insects and other invertebrates, but they also prefer oil-rich seeds and may eat agricultural crops and other plants.

Although they spread most easily with human assistance, IFA can spread over modest distances under their own power and colonies can migrate up to several hundred feet away, requiring as little as a single queen and a few workers to start anew. Flooding or heavy irrigation causes IFA to form rafts of workers protecting the queen and brood within, enabling a colony to become established wherever it is deposited by the water. On average, queens fly no more than about 1/2 mile from their original nest, although these flights can exceed 12 miles on occasion.

The most important means by which IFA spreads is with human help. IFA was first introduced into this country with cargo from South America. Commerce continues to spread IFA throughout the U.S., in trucks, trains, or other vehicles. IFA-infested cargos have included soil, sod, building materials, cotton seed, nursery stock, roof tiles, paint, and honeybee hives. Cars and recreational vehicles have also been found harboring IFA.

How you can help:
  • If traveling through or visiting an area known to have IFA, don´t bring back nursery stock, plants, sod, firewood, hay or straw, or other items in which IFA could hitchhike. Inspect all vehicles, including cars or RV´s, to ensure no IFA are inside.
  • If engaged in commercial activities, observe federal quarantines and make sure that any trucks, trailers, machinery, equipment, supplies, products, or cargo are free of soil and ants BEFORE you return.

If you think you have found IFA here in Oregon:

  • Make a note of the exact location of the suspect nest or mound.
  • Immediately contact the Oregon Department of Agriculture.
What ODA is doing: Extensive surveys in areas attractive to IFA in Oregon´s counties were conducted in 1999, 2000, 2001, and 2002 by ODA survey technicians. Greenhouse/ Nursery sites are regularly monitored by ODA personnel. In 1992, a railroad box car was intercepted in Salem which contained IFA. The car was treated and the ants were eradicated.

UC IPM Key to Identifying Common Household Ants
Texas A&M University imported fire ant research and management
Photo credits:
rifa adult:
USDA APHIS PPQ Archives, USDA Animal and Plant Health Inspection Service, www.forestryimages.org

rifa mound:
USDA APHIS PPQ Archives, USDA Animal and Plant Health Inspection Service, www.forestryimages.org
ants on wood:
Scott Bauer, USDA Agricultural Research Service, www.forestryimages.org