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Subfamily MYRMICINAE
Tribe SOLENOPSIDINI

Solenopsis invicta Buren, 1972
"red imported fire ant (RIFA)"

Authors: Joe A. MacGown and Ryan J. Whitehouse
Uploaded, 2009; last updated 29 August 2016


Solenopsis invicta, full face view of minor worker (MS, George Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, lateral view of minor worker (MS, George Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, dorsal view of minor worker (MS, George Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, full face view of major worker (MS, Holmes Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, lateral view of major worker (MS, Holmes Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, dorsal view of major worker (MS, Holmes Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, full face view of dealate queen (MS, George Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, lateral view of dealate queen (MS, George Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, dorsal view of dealate queen (MS, George Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, full face view of alate queen (AL, Bibb Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, lateral view of alate queen (AL, Bibb Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, dorsal view of alate queen (AL, Bibb Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, full face view of male (MS, Wayne Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, lateral view of male (MS, Wayne Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta, dorsal view of male (MS, Wayne Co.) (photo by Ryan Whitehouse and Joe A. MacGown)

Solenopsis invicta, angled view of major worker (MS, Holmes Co.) (photo by Ryan Whitehouse and Joe A. MacGown)
Solenopsis invicta queen head
Solenopsis invicta, full face view of queen (photo by Joe A. MacGown)
Solenopsis invicta side view of queen
Solenopsis invicta, profile view of queen (photo by Joe A. MacGown)

Solenopsis invicta profile view of male
Solenopsis invicta, profile view of male (photo by Joe A. MacGown)

Solenopsis invicta pen drawing by Joe MacGown

Solenopsis invicta, profile view of worker
(pen drawing by Joe A. MacGown)
Solenopsis invicta colored drawing by Joe MacGown

Solenopsis invicta, profile view of worker,
(pen colored with Adobe Photoshop by Joe A. MacGown)

Introduction
The genus Solenopsis includes both the "fire ants", known for their aggressive nature and potent sting, and the minute "thief ants", many of which are lestobiotic subterranean or arboreal species that are rarely collected. Many species may be polygynous. 

Solenopsis invicta (Buren) (Myrmicinae), the Red Imported Fire Ant (RIFA), is a small bicolored red and blackish species native to Brazil. This species is well established in the souteastern US and has spread to Texas west to California. Solenopsis invicta is considered to be a serious stinging pest. Colonies may be polygynous and workers are polymorphic (varying sizes of workers).

Solenopsis invicta, the scourge of the South, commonly referred to as the red imported fire ant (RIFA), has been the bane of many people in the deep south. These ants are more than a nuisance with their large mounds that dot the landscape, and are aggressive antagonists in a pitched battle for dominion of the open landscapes. Their stings are quite potent individually, but these ants attack en masse, and it is the lucky person who escapes with only one sting! The red imported fire ant is a major agricultural and urban pest throughout the southeastern states that causes both medical and environmental harm resulting in a cost of many millions of dollars per year for southeastern states.  The red imported fire ant, is thought to have been introduced into the U.S. through either Mobile, Alabama or Pensacola, Florida, from Brazil sometime between 1933 and 1945. The red imported fire ant together with Solenopsis richteri, the black imported fire ant, which was introduced sometime near 1918, have wreaked havoc on the economy of the South. As if these two species were not bad enough, they both can mate with one another producing a hybrid, which is as bad or worse than either the black or red fire ant. Two native fire ants, S. geminata and S. xyloni, have not been collected in either MS or AL in many years and it is thought that the two imported fire ants and their hybrid have out-competed them for resources and effectively driven them out from this area. 

Generic level identification of Solenopsis is relatively straight forward, although sizes are greatly variable ranging from approximately 1.0 mm to over 4.0 mm. The genus can be basically characterized by the following: mandible with four teeth (usually), bicarinate clypeus with 0-5 teeth, median part of clypeus with a pair of longitudinal carinae medially or at lateral edges, 10-segmented antennae that terminates in a distinctive 2-segmented club, overall shiny appearance and general lack of or reduced sculpture (when present usually restricted to rugulae or striae on the head, alitrunk, petiole, and postpetiole), lack of propodeal spines or other protuberances on the alitrunk, well developed petiole and postpetiole, and a well-developed sting. Workers are either polymorphic (especially in the fire ant group) or monomorphic (especially thief ants).

Hybridization is not uncommon among the larger fire ant group, which can make identification of some species difficult. Identification of thief ants is perhaps even more challenging due to their minute size, similar appearance of workers of one species to another, taxonomic problems, and lack of knowledge of all castes.

Taxonomic History (Bolton 2016)
Solenopsis invicta Buren, 1972: 9, fig. 2 (w.q.m.) BRAZIL. Neotropic. Wheeler & Wheeler, 1977: 588 (l.). Junior synonym of Solenopsis wagneri: Bolton, 1995: 388. [Trager, 1991: 173 incorrectly gave Solenopsis wagneri as an unavailable name; the name is available and has priority over Solenopsis invicta, see note under Solenopsis wagneri.]. Solenopsis invicta conserved over Solenopsis wagneri because of usage, in accord with ICZN (1999): Shattuck, Porter & Wojcik, 1999: 27. See also: Rhoades, 1977: 1; Smith, 1979: 1386.

Diagnosis
Workers of Solenopsis invicta are polymorphic ranging in size from about 1.0 mm to 4.0 mm in overall length, are bicolored with a reddish brown head and mesosoma and brownish black gaster, have a 10-segmented antenna that terminates in a two-segmented club, lack propodeal spines, have a two-segmented waist, and have a prominent stinger. The aforementioned characteristics will serve to easily separate this genus from other genera in our region; however species level identification is difficult. In Mississippi, S. invicta is most similar to S. richteri Forel, the Black Imported Fire Ant, which is generally darker in overall color, has black antennal scapes, and a strongly pronounced humeral area on the pronotum. These two closely related species produce hybrids, Solenopsis invicta X richtera, which have characteristics of both.The most reliable method for identification of this group is a cuticular hydrocarbon test, which some labs are now equipped to do.

Identification
Major Worker: HL 1.30–1.43 mm, HW 1.29–1.37 mm, SL 0.98-105mm, EL 0.24–0.26 mm, MeSL 1.52–1.56 mm (n=5) (MEM specimens). Reddish brown ant with a darker colored gaster. Sometime the anterior portion of the gaster is lighter, but not a large, reddish orange spot as seen in S. richteri. Head is shining with erect setae; occipital margin with a medial indentation; eyes located laterally at about the midpoint of the head; mandibles with 5 teeth; clypeus tridentate and with two anteriorly diverging, medial carinae; antennae 10-segmented with a 2-segmented club. Mesosoma mostly smooth and shining with numerous erect setae; lack of well-defined humeral process; distinct, well developed metanotal groove; propodeum unarmed. Waist 2-segmented with erect setae; petiolar node narrower than the postpetiolar node in lateral view. Gaster smooth and shining with erect setae; sting present.

Minor Worker: HL 0.68-–087 mm, HW 0.57–0.70 mm, SL 0.59–0.69 mm, EL 0.13–0.18 mm, MeSL 0.79–0.97 mm (n=5) (MEM specimens). Light brown to reddish brown ant, sometimes with a uniformly, darker colored gaster. Head oblong without a median indentation along the occipital border. The rest of the identification matches that of the major worker just with everything smaller in size.

Queen: Large (TL ≈ 7.0–7.5 mm, HL 1.25–1.38 mm, HW 1.29–1.38 mm, SL 0.95–1.04 mm, EL 0.45–0.48 mm, MeSL 2.54–2.76 mm) (n=5) (MEM specimens). Head, mesosoma, waist, antennae, and legs reddish brown; gaster brownish black, often  the anterior portion of first tergite with some light reddish brown coloration. Head about as wide as long, rounded square; smooth and shining with scatterred erect setae; eyes large and situated laterally at the midpoint of the head; three ocelli present; antennae 11-segmented with a 2-segmented club; clypeus tridentate, median tooth minute, with two anteriorly diverging carinae; mandibles with 4 teeth. Mesosoma rounded rectangular, almost square, with mesoscutum rounded anteriorly; mostly mooth and shining except some striae present on anterior portion of mesoscutellum and propodeum with numerous transverse striae; numerous erect setae present dorsally and on propodeum, but sparse on pronotum and mesopleura; dorsal surface relatively flat; four wings or wing scars present; propodeum unarmed; declivity straight, almost 90°. Wings clear with light amber colored viens; forewing with costal, basal, subbasal, submarginal, and discal cells presesnt, pterostigma present; hindwing with costal, basal, and anal cells present.  Waist 2-segmented, notes; petiolar node narrower than the postpetiolar node in lateral view. Gaster shining with long erect setae; sting present.

Male: Medium sized, larger than workers (TL ≈ 6.0 mm, HL 0.83–0.85 mm, HW 0.86–0.88 mm, SL 0.17–0.18 mm, EL 0.48–0.49 mm, MeSL 2.60–2.72mm) (n=2, MEM specimens). Head, mesosoma, waist and gaster blackish; scape and pedicel brown, remainder of funiculus yellow brown; legs  dark brown. Head small compared to the mesosoma, circular; shiny with course rugae anteriorly fading to dense punctation posteriorly, clypeus  mostly lacking sculpture; with numerous long, erect, whitesh colored setae present; eyes large, more than twice the head length, located on the anterior half of the head; three large ocelli present; frontal carina reduced; antennae 12-segmented, scape subequal to antennae segments 3-12; second antennal segment (pedicel) short, ovoid; mandibles small, with 2 teeth. Mesosoma bulky, elliptical in lateral view; mostly mooth and shining except some striae present on anterior portion of mesoscutellum and propodeum with dense punctation; entire mesosoma with numerous erect, elongate, whitesh setae present; propodeum unarmed. Wings clear with light amber colored viens; forewing with costal, basal, subbasal, submarginal, and discal cells presesnt, pterostigma present; hindwing with costal, basal, and anal cells present.  Waist 2-segmented, both nodes with dense, fine punctation; petiolar node with a dorsal median notch. Gaster shiniy, with  numerous erect, light colored setae, especially posterially genitalia visible at the apex.

Biology
Solenopsis invicta typically constructs a large, conical dirt mound above ground, which is an extensions of the remainder of the colony located below the surface. However, this species may also nest in rotting wood, in the soil beneath objects, in mulch, and numerous other situations. Colonies are almost always associated with open disturbed habitats. Colonies may be either polygynous, with multiple queens or monogynous colonies, with only one queen. Monogyne colonies may be founded with a single queen or a group of queens, which, in the latter case, only a single dominant single will eventually be present. Though queens have a life span of several years, workers typically only live for several months. Mature colonies may be large varying from 100,000 to more than 250,000 individuals. Fire ants are omnivorous and eat a wide variety of foods such as arthropods, dead mammals and other animals, seeds, and numerous sweet substances including hemipteran produced honeydew. Workers typically forage during the day during warm to hot days.  Both pheromones and various smiochemicals are used as a means of communication for defense, foraging, and recruitment. Nuptial flights occur during warm seasons, which in the Southeast is a long period of time.

Pest Status
The red imported fire ant is a notorious stinging pest species that causes billions of dollars annually. The red imported fire ant together with Solenopsis richteri, the black imported fire ant, which was introduced sometime near 1918, have wreaked havoc on the economy of the South. As if these two species were not bad enough, these species can mate with one another producing a vigorious hybrid.

Fire ants are most common in urban areas, where they nest in open, disturbed sites such as lawns, fields, roadsides, or other similar situations. Because fire ants thrive in urban areas, their presence may be a deterrent to outdoor activities. Most people’s interaction with fire ants comes from getting stung. When their colony is disturbed, S. invicta workers swarm out searching for the cause of the disturbance, then numerous individuals aggressively sting the intruder. Because of the voracity of attacks and the number of stings received, victims may develop an allergy to their venom and potentially require medical treatment. Stings often result in itching red bumps that in many individuals forms hardened pustules that eventually dissapate. Fire ants are capable of inflicting serious injury or killing various animals, especially weak or sick individuals. Nests are sometimes built under structures such as pavements and foundations, where they may cause structural problems, even occasionally causing structures to collapse. Red imported fire ants also are known to damage equipment and infrastructure, which may cause negative impacts on businesses and affect property values. Workers are apparently attracted to electricity and have been reported to short out electrical equipment. This species is a serious agricultural pest that invades and damages a variety of crops. Large mounds on farmland can damage machinery and affect harvesting.

Although fire ants are generally considered to be a serious pest species, they are beneficial at times as they consume large amounts of other insects, including many pest species.

Distribution
Native Range: Brazil

Australian: Australia, Hawaii (AntWiki.org).
Nearctic: United States (AntWiki.org and MEM specimens).
Neotropical: Anguilla, Antigua and Barbuda, Argentina, Aruba, Brazil, British Virgin Islands, Cayman Islands, Greater Antilles, Mexico, Montserrat, Netherlands Antilles, Paraguay, Puerto Rico, Saint Martin (French part), Trinidad and Tobago, Turks and Caicos Islands, United States Virgin Islands (AntWiki.org).
Palaearctic: China (AntWiki.org).

U.S. Distribution: AL, AR, CA, FL, GA, IL, LA, MO, MS, NC, NM, OK, SC, TN, TX, VA (AntWeb.org, AntWiki.org and MEM).
Southeastern U.S. Distribution: AL, AR, FL, GA, LA, MS, SC (MEM)

Acknowledgments
Funding for the ant work being done by the MEM in Alabama and Mississippi is from several sources including the National Institute of Food and Agriculture, United States Department of Agriculture, under Project No. MIS-012040, the Mississippi Agricultural and Forestry Experiment Station at Mississippi State University, with support from State Project MIS-311080, NSF Grants BSR-9024810 and DFB-9200856, the Tombigbee National Forest (U.S. Forest Service), the Noxubee Wildlife Refuge, Mississippi Natural Heritage Program Research Grant, USDA Forest Service Agreement No. 08-99-07-CCS-010, the William H. Cross Expedition Fund, and primarily by the USDA-ARS Areawide Management of Imported Fire Ant Project. Additionally, special cooperation has been provided by State Parks, National Forests, National Wildlife Refuges, the Natchez Trace Parkway, and from various private landowners in both Alabama and Mississippi.

Literature Cited

Bolton, B. 1995. A new general catalogue of the ants of the world. Cambridge, Mass.: Harvard University Press, 504 pp.

Bolton, B. 2016.  Bolton World Catalog Ants. Available online: http://www.antweb.org/world.jsp. Accessed 9 March 2016.

Buren, W. F. 1972. Revisionary studies on the taxonomy of the imported fire ants. Journal of the Georgia Entomological Society 7:1-26.

Global Invasive Species Database. 2009. Solenopsis invicta. Available online at: http://www.invasivespecies.net/database/species/distribution.asp?si=77&fr=1&sts=&lang=EN Accessed 25 February 2009. 
Rhoades, R. B. 1977. Medical aspects of the imported fire ant. Gainesville: University of Florida Press, 75 pp.

Shattuck, S. O.; Porter, S. D.; Wojcik, D. P. 1999. Case 3069. Solenopsis invicta Buren, 1972 (Insecta, Hymenoptera): proposed conservation of the specific name. Bulletin of Zoological Nomenclature 56:27-30.

Smith, D. R. 1979. Superfamily Formicoidea. Pp. 1323-1467 in: Krombein, K. V.; Hurd, P. D.; Smith, D. R.; Burks, B. D. (eds.) 1979. Catalog of Hymenoptera in America north of Mexico. Volume 2. Apocrita (Aculeata). Washington, D.C.: Smithsonian Institution Press, pp. i-xvi, 1199-2209.

Trager, J. C. 1991. A revision of the fire ants, Solenopsis geminata group (Hymenoptera: Formicidae: Myrmicinae). Journal of the New York Entomological Society 99:141-198.

Wheeler, G. C.; Wheeler, J. 1977. Supplementary studies on ant larvae: Myrmicinae. Transactions of the American Entomological Society 103:581-602.

LINKS
AntCat
AntWeb
AntWiki

Other Imported Fire Ant Links:
Extension Fire Ant Site- http://www.extension.org/fire+ants
Texas A & M Fire Ant Site- http://fireant.tamu.edu/
Imported Fire Ants in Tennessee - http://fireants.utk.edu/
Imported Fire Ant and Household Insects (IFAHI) research site-http://www.ars.usda.gov/Main/site_main.htm?modecode=66-15-10-15
LSU red imported fire ant research- http://www.lsu.edu/ants/index.shtml
Control of the Red Imported Fire Ant -http://www.ces.ncsu.edu/depts/ent/notes/Urban/ifa.htm