Introduction
The genus Pheidole has been described as 'hyperdiverse" by Wilson (2003) in large part by the abundance and diversity of the genus, especially in the Neotropics. Workers in this genus have discrete major and minor castes, with the major workers having greatly enlarged heads. In the southeastern US, 21 named species are known to occur and at least one undescribed species is known from the region. Species in the Southeast can be found in all terrestrial habitats from fine sandy localities bordering creeks to hard-pan dirt in open area beside roadways to inside rotting wood in natural forested habitats. Both major and minor workers have 12-merous antennae (including scape and pedicel) that terminates in a 3-merous club; waist with both petiole and postpetiole, and atrophied sting. Many species possess propodeal spines, although a few do not. Majors are distinctive with their hugely enlarged heads (proportional to the body), which may be as wide or as long as the mesosomal length, and by the large mostly edentate mandible. Minor workers are much smaller than majors and often have dramatic physical differences. Queens are similar to major workers, except the heads are not proportionally enlarged as compared to the body, eyes are proportionally larger to the head than with workers, ocelli are present, overall are typically larger than both minor and major workers, and have enlarged thoracic regions to accommodate wings. Males are quite different in appearance from both workers and queens being somewhat wasp-like with beaded 13-merous antennae, greatly enlarged eyes (eye length may be half of more of the head length), reduced mandibles, enlarged ocelli that bulge from posterior margin of head, and with proportionally large wings.

Pheidole obscurithorax (Naves) (Formicinae), the obscure big-headed ant is a large, dark colored species native to South America that is now common along the US gulf coast from northern Florida to eastern Texas. Based on the steady expansion of its range in the US and its ability to successfully compete with the red imported fire ant (Solenopsis invicta Buren) and native species nesting in the same habitat, this species shows invasive potential in the US.

Taxonomic History (from Bolton 2016)
Pheidole fallax subsp. obscurithorax Naves, 1985: 61 (s.w.) ARGENTINA. Neotropic. [First available use of Pheidole fallax arenicola obscurithorax Santschi, 1923: 58; unavailable name.] Raised to species: Wilson, 2003: 331.

Diagnosis
In North America, it is relatively easy to distinguish major workers of P. obscurithorax from any other native or exotic species of Pheidole. The large size, heavily sculptured head, and thick scape which is curved mesally and more angular laterally are unique among the Pheidole of North America." This is the only Pheidole species in the southeastern US of this size. However, in the US, minor workers could easily be confused with those of Pheidole dentata Mayr, 1886, both of which may be found in the same habitats. Minor workers of P. obscurithorax differ from P. dentata by being larger, having a more ovate head, and by having the nuchal collar visible in full face view. Queens of P. obscurithorax are similar to major workers but larger, have ocelli, and have an enlarged mesosoma for flight muscles.

Identification
Minor Worker: Small to medium sized (HL 0.67–0.73 mm, HW 0.53–0.62 mm, SL 0.82–0.93 mm, EL 0.16–0.18 mm, MeSL (mesosomal length) 0.95–1.06 mm, PRW 0.40–0.47 mm, PSL 0.03–0.04 mm, PTW 0.11–0.14 mm, PPW 0.16–0.19 mm) (MEM specimens) (n=5). Concolorous dark reddish-brown with mandibles and tarsi a lighter yellowish-brown; entire body with numerous flexuous, elongate setae present. Head longer than wide, ovate; nuchal collar visible; strongly shining, mostly lacking sculpture except for striae present between eyes and frontal lobes, striae not extending posteriorly beyond eyes;. antenna 12-segmented, apical three antenomeres forming a loose club, scape straight, longer than head length; clypeus evenly rounded anteriorly; mandibles elongate triangular with 9 small teeth followed by two large apical teeth; eyes large, located on sides of head at approximately the midpoint of head. Mesosoma with "stepped" appearance in profile, promesonotal and metanotal sutures deep, pronotum raised above mesonotum, and mesonotum raised above propodeum; pronotum strongly rounded, lacking sculpture, shiny; remainder of mesosoma and waist strongly foveolate; propodeal dorsum longer than declivity, short spines present.  Waist two-segmented, with rough punctate sculpture; petiolar peduncle thickened, node conical in lateral view, subquadrate dorsally; postpetiole circular in lateral view, rounded square in dorsal view, about the same width as petiole. Gaster lacking sculpture, shiny; sting absent.

Major Worker: Large, more than twice the size of minor workers (HL 1.62–1.84 mm, HW 1.56–1.72 mm, SL 0.92–1.00mm, EL 0.20–0.25mm, IHT 0.38–0.44mm, OHT 0.57–0.65mm, MeSL 1.47–1.62mm, PRW 0.86–0.96mm, PSL 0.08–0.12mm, PTW 0.24–0.32mm, PPW 0.34–0.47mm) (MEM specimens) (n=6). Head medium to dark reddish-brown, mandibles and antennal scapes dark reddish-brown to black: mesosoma, waist, and legs medium reddish brown to dark reddish brown; gaster dark reddish-brown to black; entire body with numerous flexuous, elongate setae present, setae on head shorter and stiffer. Head about as long as wide to slightly longer than wide; posterior corners strongly rounded; posterior margin deeply cleft; front and sides of head with strong rugoreticulation; anterior margin of clypeus with deep notch in middle; eyes small with numerous tiny facets; located on sides of head (in full face view) approximately one-third of head length from anterior edge of clypeus; antenna 12-segmented, apical three antenomeres forming a loose club; scape short, strongly angled basally and thickened mesally; mandibles large, mostly edentate, but with short, stout basal tooth, and two large apical teeth; hypostomal border with short, blunt tooth in middle, two short, triangular inner teeth (reduced in some specimens), and two widely spaced thickly triangular outer teeth.  Mesosoma with "stepped" appearance in profile, promesonotal and metanotal sutures deep, pronotum raised above mesonotum, and mesonotum raised above propodeum; pronotum roughly rounded with transverse striae dorsally and semi-circular concentric striae laterally; mesopleurae and sides of propodeum mostly foveolate, but with some transverse striae, especially basally; mesonotal, metanotal, and propodeal dorsa with transverse striae, shiny; propodeal dorsum longer than declivity, short spines present. Waist two-segmented, with rough punctate sculpture; petiolar peduncle thickened, node conical in lateral view; postpetiole subquadrate in lateral view, ovate in dorsal view, wider that petiole. Gaster with strong shagreening on anterior half of first tergite; remainder of gaster mostly shining; sting absent.

Queen: Large, larger than major worker  (HL 1.36–1.42 mm, HW 1.58–1.64 mm, SL 0.98–1.00 mm, EL 0.32–0.40 mm, MeOL (median ocellar length) 0.15–0.16 mm, IHT 0.46–0.49 mm, OHT 0.63–0.65 mm, MeSL 2.53–2.55 mm, PRW 1.30–1.60 mm, PSL 0.12–0.15 mm PTW 0.53–0.55 mm, PPW 0.75–0.80 mm, WL 7.40–7.80 mm) (MEM specimens) (n=4). Concolorous dark reddish-brown; entire body with numerous semi-erect to erect setae present, with setae on head and dorsum of mesosoma shorter and stiff, those on gaster longer and more flexuous. Head quadrate, slightly wider than long; posterior border straight to weakly convex; front and sides of head with deep rugoreticulation present; anterior margin of clypeus with deep notch in middle; eyes large, placed at approximately the midpoint of head on sides of head in full face view; three ocelli present, arranged in roughly an equilateral triangle; Antenna 12-merous, apical three antenomeres forming a loose club; scape short, strongly angled basally and thickened mesally;  mandibles large, mostly edentate, but with large basal tooth, four minute denticles, and two large apical teeth; hypostomal border with short, blunt tooth in middle, two short, triangular inner teeth, and two widely spaced thickly triangular outer teeth. Mesosoma enlarged for wings, dorsally flattened; dorsum of pronotum, mesonotum, and metanotum with longitudinal striae; sides of pronotum, mesopleurae, sides and dorsum of propodeum, and sides and dorsum of petiolar and postpetiolar nodes with transverse striae with punctation between striae dulling the surface; propodeal spines short, denticle-like. Wings, when present, with light yellow brown tint; pterostigma brown; forewing with closed costal, basal, subbasal,discal, and two submarginal cells; hingwing with costal, basal, and subbasal cells. Waist two-segmented, with rough punctate sculpture; petiolar peduncle thickened, node conical; postpetiole subquadrate in lateral view, oblong-ovate in dorsal view. First gastral tergite with fine striae anterior, becoming shagreened and finally shiny posteriorly. 

Biology and Economic Importance
In the US, Pheidole obscurithorax typically nests in open, disturbed habitats where populations may be abundant. Nests are medium to large sized and marked by an obvious crater. In its native range in South America and invasive range in North America, P. obscurithorax is a major competitor of the invasive red imported fire ant Solenopsis invicta (Calcaterra et al. 2008, 2010). MacGown (unpublished data) has observed P. obscurithorax to quickly find baits and then outcompete S. invicta at those baits. Although P. obscurithorax is an omnivorous species,  it is an aggressive predator as well. MacGown has observed minor and major workers cooperating together to attack and carry much larger prey such as lepidopteran larvae into a colony. This species has been observed to defend prey from S. invicta (Storz 2003, Storz and Tschinkel 2004), albeit, not always successfully. It is unclear what effect this species may have on native arthropods or ground nesting vertebrates. Hill (2006) reported this species attacking a hatchling chicken, which illustrates its potential to impact ground nesting birds. As with other Pheidole species, the sting is atrophied, and thus, this species cannot sting.

Pest Status

Distribution
Native Range: Thought to be Argentina

Nearctic: United States (MEM and Wetterer et al. 2015).
Neotropical: Argentina, Brazil, Bolivia, Cuba, Paraguay (AntWeb.org and Wetterer et al. 2015).

U.S. Distribution: AL, FL, GA, LA, MS, TX (MEM and Wetterer et al. 2015).  
Southeastern U.S. Distribution: AL, FL, GA, LA, MS (MEM and Wetterer et al. 2015).

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

Calcaterra, L.A., Cuezzo, F., Cabrera, S.M., and Briano, J.A. 2010. Ground ant diversity (Hymenoptera: Formicidae) in the Iberá Nature Reserve, the largest wetland of Argentina. Ann. Entomol. Soc. Am. 103: 71-83. 

Calcaterra, L.A., Livore, J.P., Delgado, A., and Briano, J.A. 2008. Ecological dominance of the red imported fire ant, Solenopsis invicta, in its native range. Oecologia 156: 411–421.

Hill, J.G. 2006. First report of the exotic ant, Pheidole obscurithorax Naves (Hymenoptera: Formicidae), attacking a hatchling chicken. J. Entomol. Sci. 41: 397-398.

Naves, M. A. 1985. A monograph of the genus Pheidole in Florida, USA (Hymenoptera: Formicidae). Insecta Mundi 1:53-90. 

Santschi, F. 1923. Pheidole et quelques autres fourmis néotropiques. Annales de la Société Entomologique de Belgique 63:45-69.

Storz, S.R. 2003. Distribution, spread, activity patterns, and foraging behaviors of the introduced ant Pheidole obscurithorax in the southeastern United States. M.S.Thesis, The Florida State University, Tallahassee; FL.

Storz, S. R. and W. R. Tschinkel. 2004. Distribution, spread, and ecological associations of the introduced ant Pheidole obscurithorax in the southeastern United States. The Journal of Insect Science, 4: 12, 1-11.

Wetterer, J., J. A. MacGown, and L. A. Calcaterra. 2015. Geographic spread of Pheidole obscurithorax(Hymenoptera: Formicidae). Transactions of the American Entomological Society 141: 222-231.

Wilson, E. O. 2003. Pheidole in the New World. A dominant, hyperdiverse ant genus. Cambridge, Mass.: Harvard University Press, [ix] + 794 pp.

Links

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