Authors: Joe A. MacGown and Ryan J. Whitehouse
Solenopsis invicta, the scourge of the South, commonly referred to as the red imported fire ant (RIFA), has been the bane of many in the deep south. These ants are more than a nuisance with their large mounds that dot the landscape, but instead are aggressive antagonists in a pitched battle for dominion of the open landscapes. Their stings are quite potent individually, but they 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 also causes both medical and environmental harm resulting in a cost of many millions of dollars per year for southeastern states.
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)
Minor Worker: HL 0.68-.087mm, HW 0.57-0.70mm, SL 0.59-0.69mm, EL 0.13-0.18mm, MeSL 0.79-.097mm (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: HL 1.25-1.38mm, HW 1.29-1.38mm, SL 0.95-1.04mm, EL 0.45-0.48mm, MeSL 2.54-2.76mm (n=5) (MEM specimens). Head and body reddish brown with the anterior portion of the gaster slightly lighter in color, but not a distinct orangish spot. Head smooth and shining with erect setae; eyes large and situated laterally at the midpoint of the head; three ocelli present; mandibles with 4 teeth; clypeus tridentate, median tooth minute, with two anteriorly diverging carinae; antennae t\10-segmented with a 2-segmented club. Mesosoma smooth with numerous erect setae; dorsal surface relatively flat; 4 wings or wing scars present; propodeum unarmed. Waist 2-segmented; petiolar node narrower than the postpetiolar node in lateral view. Gaster shining with long erect setae; sting present.
Male: HL 0.83-0.85mm, HW 0.86-0.88mm, SL 0.17-0.18mm, EL 0.48-0.49mm, MeSL 2.60-2.72mm (n=2) (MEM Specimens). Head, body and gaster blackish with the legs a dark brown. Head small compared to the mesosoma with numerous erect setae; eyes large and well-developed located on the anterior half of the head; three ocelli present; mandibles small with 2 teeth; antennae 12-segmented with the scape subequal to antennae segments 3-12; second antennal segment is short and round. Mesosoma bulky and well-developed with dense erect setae; 4 wings present; pronotum with a distinct median indentation; propodeum unarmed. Waist is 2-segmented; petiolar node with a dorsal median notch. Gaster shining with erect setae; genitalia visible at the apex.
Solenopsis invicta can generally be recognized by their large mounds found dotting the landscape and they have polymorphic castes (varying sizes of workers). Because S. invicta and S. richeri form a hybrid, telling those species apart can be difficult, especially since the hybrid can have characteristics of both species. The most reliable method for identification of this group is a cuticular hydrocarbon test, which some labs are now equipped to do.
Biology and Economic Importance
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.
Solenopsis invicta is found in the southern halves of both AL and MS and also found in the western portion of MS following the Mississippi River northward, whereas S. richteri tends to be found in the northeastern part of MS and northwestern portions of AL, with the hybrid found in a band between the two populations. This is not a static situation and S. invicta appears to be on a continuous path northward, bounded only by temperature restraints (which it appears to be overcoming). As S. invicta moves northward, so also do the populations of S. richteri and their hybrid.
Most people’s interaction with this ant comes from getting stung. When their colony is disturbed, S. invicta will swarm out and attack the disturbance, aggressively stinging. Because of the voracity of attacks and the number of stings received, victims can often acquire an allergy to their venom and potentially require medical treatment. Stings often result inan itching red bump that often forms a hard pustule that will eventually go away over time
Australian: Australia, Hawaii (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).
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.
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.
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