Subfamily MYRMICINAE
Tribe ATTINI

Pheidole obscurithorax Naves, 1985

Uploaded on 4 August 2009; last updated on 2 December 2014

Pheidole obscurithorax, full face view of a minor worker (Baldwin Co., AL) (photo by Joe A. MacGown)
Pheidole obscurithorax, lateral view of a minor worker (Baldwin Co., AL) (photo by Joe A. MacGown)
Pheidole obscurithorax, dorsal view of a minor worker (Baldwin Co., AL) (photo by Joe A. MacGown)
Pheidole obscurithorax, full face view of a major worker (Baldwin Co., AL) (photo by Joe A. MacGown)
Pheidole obscurithorax, lateral view of a major worker (Baldwin Co., AL) (photo by Joe A. MacGown)
Pheidole obscurithorax, lateral view of a major worker (Baldwin Co., AL) (photo by Joe A. MacGown)
Pheidole obscurithorax, full face view of a major worker (Jackson Co., MS) (photo by Joe A. MacGown)
Pheidole obscurithorax, lateral view of a major worker(Jackson Co., MS) (photo by Joe A. MacGown)
Pheidole obscurithorax, lateral view of a major worker (Jackson Co., MS) (photo by Joe A. MacGown)
Pheidole obscurithorax, full face view of a dealate queen (Baldwin Co., AL) (photo by Joe A. MacGown)
Pheidole obscurithorax, lateral view of a dealate queen (Baldwin Co., AL) (photo by Joe A. MacGown)
Pheidole obscurithorax, lateral view of a dealate queen (Baldwin Co., AL) (photo by Joe A. MacGown)

Introduction
Pheidole obscurithorax is a large, dark colored species native to South America that is now common along the US gulf coast from northern FL 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, 2014)
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.

Identification
Minor Worker: (from MEM specimens). HL 0.67–0.73, HW 0.53–0.62, SL 0.82–0.93, EL 0.16–0.18, MeSL (mesosomal length) 0.95–1.06, PRW 0.40–0.47, PSL 0.03–0.04, PTW 0.11–0.14, PPW 0.16–0.19, (n=5). Concolorous dark reddish-brown with mandibles and tarsi a lighter yellowish-brown. Entire body with numerous flexuous, elongate setae present. Head ovate, nuchal collar visible; strongly shining, mostly lacking sculpture except for striae present between eyes and frontal lobes, not extending posteriorly beyond eyes; clypeus evenly rounded anteriorly. Mandibles elongate triangular with 9 small teeth followed by two large apical teeth. Antenna 12-merous, apical three antenomeres forming a loose club, scape straight,  longer than head length. Eyes prominent, placed on sides of head at approximately the midpoint of head. Pronotum strongly rounded, lacking sculpture, shiny. Remainder of mesosoma and waist strongly foveolate. Promesonotal and metanotal sutures deep, with mesonotum in profile forming an angular projection between pronotum and propodeum.  Propodeal spines short.  Gaster lacking sculpture, shiny.

Major Worker: (from MEM specimens). HL 1.62–1.84, HW 1.56–1.72, SL 0.92–1.00, EL 0.20–0.25, IHT 0.38–0.44, OHT 0.57–0.65, MeSL 1.47–1.62, PRW 0.86–0.96, PSL 0.08–0.12, PTW 0.24–0.32, PPW 0.34–0.47 (n=6). Large. 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. Mandibles large, mostly edentate, but with short, stout basal tooth, and two large apical teeth.  Antenna 12-merous, apical three antenomeres forming a loose club; scape short, strongly angled basally and thickened mesally. Eyes placed on sides of head (in full face view) approximately one-third of head length from anterior edge of clypeus. 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.  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 transverve striae, shiny. Promesonotal and metanotal sutures deep, with mesonotum in profile forming an angular projection between pronotum and propodeum.  Propodeal spines short. Gaster with strong shagreening on anterior half of first tergite; remainder of gaster mostly shining.

Queen: (from MEM specimens). HL 1.36–1.42, HW 1.58–1.64, SL 0.98–1.00, EL 0.32–0.40, MeOL (median ocellar length) 0.15–0.16, IHT 0.46–0.49, OHT 0.63–0.65, MeSL 2.53–2.55, PRW 1.30–1.60 PSL 0.12–0.15, PTW 0.53–0.55, PPW 0.75–0.80, WL 7.40–7.80 (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.  Mandibles large, mostly edentate, but with large basal tooth, four minute denticles, and two large apical teeth. Antenna 12-merous, apical three antenomeres forming a loose club; scape short, strongly angled basally and thickened mesally.  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. 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. 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. First gastral tergite with fine striae anterior, becoming shagreened and finally shiny posteriorly. 

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.

Biology and Economic Importance
In the US, Pheildole 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.

Distribution

US Distribution: Alabama, Florida, Georgia, Louisiana, Mississippi, and Texas.

Literature Cited

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. Distibution, spread, and ecological associations o fthe intorduced ant Pheidole obscurithorax in the southeastern United States. The Journal of Insect Science, 4: 12, 1-11.

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

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