From Storz and Tschinkel (2004): Pheidole obscurithorax occurs as a native species in mature flood plain and open pasture along the Parana River (A.V. Suarez, personal communication) and presumably along the Paraguay River, which are traveled by ocean-going vessels. Its occurrence in North America was first observed by E.O. Wilson in the port city of Mobile, Alabama, in the early 1950's (Naves 1985; under the name, P. fallax obscurithorax). Several other ant species with ranges along the Paraguay River system (Brachymyrmex patagonicus, Linepithema humile, Solenopsis invicta, and Solenopsis richteri) have been transported to the southeastern United States via the ports of New Orleans, Mobile, and Pensacola (Naves 1985). Pheidole obscurithorax coexists in disturbed habitats with the fire ant Solenopsis invicta, which was introduced at Mobile in the late 1930s (Buren et al. 1974), but has been much slower to expand its range, the extent of which is unknown. It was first observed in Tallahassee, Florida (400 km east of Mobile), in 1998 (W.R. Tschinkel, personal observation).
- 1 Identification
- 2 Distribution
- 3 Biology
- 4 Castes
- 5 Nomenclature
- 6 References
Wetterer et al. (2015) - Santschi (1923) described Pheidole obscurithorax Naves from three sites in northern Argentina. Thirty years later, Kusnezov (1953) reported that P. obscurithorax had been collected on the bay front of Mobile, Alabama. Naves (1985) reported one additional record of P. obscurithorax from eastern Alabama, near the Florida border. Despite the scarcity of known records, Naves (1985) speculated that P. obscurithorax was “probably widely ranging in South America.” Until recently, however, there were very few South American records of P. obscurithorax available to assess Naves' (1985) speculation about its native range.
In the past decade, P. obscurithorax has been reported from many additional sites in both South and North America, where it has often found to be an ecologically dominant species (LeBrun et al. 2007, Calcaterra et al. 2008, 2010).
Wilson (2003) placed this species close to Pheidole fallax (from the Central America, northern South America, and the West Indies), Pheidole gigas (from Brazil), Pheidole jelskii (from the South America and the West Indies), Pheidole puttemansi (from Argentina and Brazil), Pheidole roushae (from the Central America), Pheidole tobini (from Peru), and Pheidole valens (from Brazil). Moreau (2008) constructed a phylogenetic tree of Pheidole that included 19 of the 103 species that Wilson (2003) placed in the fallax group, and found that the fallax species group was polyphyletic. Moreau (2008) placed P. obscurithorax closest to P. jelskii, the only other species in the analysis that was among the seven species that Wilson (2003) considered to be closely related to P. obscurithorax.
No doubt many P. obscurithorax specimen records remain hidden in the long lists of unidentified Pheidole species recorded in many published faunal studies, particularly from South America. In addition, it is likely that published studies have misidentified P. obscurithorax specimens as other species, notably P. fallax. Alex Wild (pers. comm.) wrote: “there are several species that are all quite similar: obscurithorax, jelskii, bergi, and at least two potentially undescribed species I've been calling cf. jelskii and cf. obscurithorax. Any of these might be going by "fallax" in the literature... In northern Argentina and Paraguay, Pheidole jelskii is the most commonly encountered member of the complex.” Rodrigo Feitosa (pers. comm.) wrote, concerning P. obscurithorax, jelskii, and fallax: “Minor workers are virtually identical and even examining the majors it can be almost impossible to recognize the limits between these three species, especially in southern Brazil... I would not be surprised if these three names were merged in the future.”
In North America, however, it is simple to distinguish major workers of P. obscurithorax from those of any other native or exotic species of Pheidole. Naves (1985) wrote, concerning the major workers, that P. obscurithorax “is a large, very dark species over 6 mm in body length. Its characteristics, such as the heavily sculptured head and thick scape which is curved mesally and more angular laterally are unique among the Pheidole of North America.” Minor workers of P. obscurithorax could be confused with those of Pheidole dentata Mayr, which may be found in the same habitats. However, P. obscurithorax minors are larger than P. dentata minors, have a more ovate-shaped head, and have the nuchal collar visible in full-face view.
True to Naves' (1985) speculation, the range of Pheidole obscurithorax in South America appears to be quite extensive, ranging 27.5 degrees of latitude, from 6.7°S to 34.2°S. If P. obscurithorax is so widespread in South America and often a dominant species, then it is clear that many South American Pheidole specimens previously identified in publications and in museums as other species, are probably P. obscurithorax. To more fully delineate the native range of P. obscurithorax, it would be valuable for researchers to re-examine South American Pheidole specimens, particularly specimens previously identified as Pheidole fallax. Despite Wilson's (2003) conclusion that P. fallax is limited to Caribbean area, with South American records limited to Colombia and Venezuela, numerous studies continue to record P. fallax from Argentina and Brazil (e.g., Ramos et al. 2004, Diehl et al. 2005, Martins et al. 2006, Delabie et al. 2007, Santos et al 2008, Vittar & Cuezzo 2008, Cuezzo and Campero 2009, Jaime 2010, Castro et al. 2012, Miranda et al. 2012, Ulysséa & Brandão 2013). While some of these records may be P. jelskii, others are probably P. obscurithorax.
Site records of P. obscurithorax from North America ranged only 3.5 degrees, from 28.0°N to 31.5°N. This may indicate that the North American populations of P. obscurithorax have a fairly narrow range of climatic tolerances. In South America, different P. obscurithorax populations may show physiological adaptations to the local climate. Thus, although the species as a whole is able to live under a great diversity of climatic conditions in South America, any one population may have a much narrower tolerance range. It is possible that the populations of P. obscurithorax now in North America are descended from colonists from a narrow geographic area and are not adapted to spread much beyond their current latitudinal range. In fact, Wild & Suarez (2009) conducted genetic analyses of native and exotic populations of P. obscurithorax and found that the North America populations appear to originate from a single introduction from a population most closely related to native study populations from a stretch along the Paraná River in northern Argentina from Resistencia (27.5°S) to Santa Fe (31.6°S). This latitudinal range is almost an exact match to the current latitudinal range of P. obscurithorax in North America. It is therefore possible that P. obscurithorax populations in North America will show little further latitudinal expansion.
Alternatively, the much greater latitudinal range of P. obscurithorax in South America could indicate that exotic populations of P. obscurithorax may show much additional expansion in North America and beyond. The range of P. obscurithorax in South America is similar to that of the fire ant Solenopsis invicta (Pitts 2002, Wetterer 2013). In North America, exotic populations of P. obscurithorax could potentially spread like those of S. invicta, across the southeastern US and into the West Indies (Wetterer 2013). Only time will tell.
MacGown (2015) - The combination of large size, dark coloration, heavily reticulate head, scapes strongly angled at base and thickened medially, and five distinct hypostomal teeth will separate queens of P. obscurithorax from any other North American Pheidole species.
Pheidole fallax, Pheidole gigas, Pheidole jelskii, Pheidole puttemansi, Pheidole roushae, Pheidole tobini, and Pheidole valens are similar. These related species are all Neotropical in distribution. Thus far, P. obscurithorax is the only member of this group of eight species known to have made its way into the US, although P. fallax and P. jelskii have been reported from the West Indies (Wilson 2003 and MEM specimen data). Of these species, P. fallax and P. jelskii are the most similar in appearance to P. obscurithorax. With the identification keys in Wilson's monograph of the Pheidole in the New World (Wilson 2003), it is relatively easy to separate the major workers of these related species; however, the minor workers of all three are very similar to one another making identifications based on the minor worker caste alone difficult, especially in Central America, northern South America, and the West Indies where their respective ranges all overlap. Of the eight related species, P. jelskii is the only one for which the male and queen have been described (Forel 1893). Identifications of males and queens unassociated with a colony in this group of eight species from outside of the US may be unreliable. The current situation in the US is quite different, as P. obscurithorax is the only exotic member of the fallax group known to occur there. Only two native species, Pheidole dentata and Pheidole morrisii, in the fallax species group occur in the southeastern US, including eastern Texas where P. obscurithorax has been reported. Major workers of both native species are easily distinguished from P. obscurithorax by their lack of strong sculpture. Minor workers of P. dentata may be confused with P. obscurithorax, but can be separated by their smaller size and the nuchal collar not being visible in full-face view. Males and queens of both of these native species in the fallax group have been described (Forel 1901, Mayr 1886) and both lack the strong sculpture of P. obscurithorax. With the description of the queen of P. obscurithorax provided here, reliable identification of queens of P. obscurithorax collected in the US is now possible.
Keys including this Species
See the extensive discussion by Wetterer et al. (2015) regarding the range of this species in the identification section above.
Figure. United States distribution of Pheidole obscurithorax. Storz and Tschinkel 2004 
From Wilson (2003): Kempf (1972b) reports obscurithorax from Córdoba, Formosa, and Santa Fé in northern Argentina. I have also seen series from the Parana River and Canindeyú Province in Paraguay. In 1950 I discovered a colony nesting in Mobile, Alabama, within a kilometer of the ship docking area, an ideal entry point for exotic species, such as obscurithorax, that occur in or near potential embarkation areas along or close to the banks of the Paraguay and La Plata Rivers of southwestern Brazil, Paraguay and Brazil. Shipping has occurred regularly over decades between these areas and the ports of the southern United States.
In Florida Deyrup, Davis & Cover (2000) found this species to be established in Escambia, Santa Rosa, Okaloosa, Walton and Leon Counties. Pest status: none. No previous published records from Florida; earlier specimens: 1992. Naves (1985) reported this species from Alabama, close to the Florida border.
Distribution based on Regional Taxon Lists
Distribution based on AntMaps
Distribution based on AntWeb specimens
Check data from AntWeb
In its introduced range this species is associated with disturbed habitats that contain open areas of sandy soil.
Also see the extensive discussion by Wetterer et al. (2015) in the identification section above.
Wild and Suarez (2009) - Genetic analysis shows the North American populations of this species appear to originate from a single introduction from a native population found along a stretch of the Paraná River in northern Argentina from Resistencia (27.5°S) to Santa Fe (31.6°S).
From Wilson (2003): A Mobile, Alabama, colony occupied a large crater nest in open, sandy soil. Stefan Cover, Lloyd Davis Jr., and Mark Deyrup (Cover, personal communication), found the species in western Florida mostly in disturbed areas, but once in secondary forest in a creek valley. On several instances the nests were located close to those of the imported fire ant, Solenopsis invicta, suggesting superior resistance to this formidable invasive.
This species has not been studied in its native range.
As reported in King and Tschinkel 2007: large colonies of ~10,000 workers (WRT unpublished data).
Storz and Tschinkel 2004: Pheidole obscurithorax is omnivorous and collects a variety of arthropod prey, including other ants, and less frequently plant material such as flower petals. Its midden piles are often littered with heterospecific ant body parts, especially those of Solenopsis invicta, an abundant ant often found nesting near P. obscurithorax nests. Pheidole obscurithorax uses a combination of foraging tactics; if a prey item is small enough, scouts carry it unaided back to the nest. If the prey item is too large, teams of workers carry the prey to the nest whole. The size of prey handled and the speed at which it is carried to the nest are remarkable. The workers defend their prey items from other ants, including S. invicta, and P. obscurithorax both wins battles and loses them.
Pheidole obscurithorax nests in soil in open areas, where it produces conspicuous nests, each generally with a single large (1- to 5-cm-wide) opening often covered by a leaf or other collected material (Storz and Tschinkel 2004). They prefer well-drained open areas and are an aggressive predator and scavenger, even able to maintain colonies near large colonies of Solenopsis invicta (Deyrup, Davis & Cover, 2000).
The following information is derived from Barry Bolton's New General Catalogue, a catalogue of the world's ants.
- obscurithorax. Pheidole fallax subsp. obscurithorax Naves, 1985: 61 (s.w.) ARGENTINA. [First available use of Pheidole fallax st. arenicola var. obscurithorax Santschi, 1923d: 58; unavailable name.] Raised to species: Wilson, 2003: 331.
Unless otherwise noted the text for the remainder of this section is reported from the publication that includes the original description.
From Wilson (2003): A member of the fallax group close to Pheidole fallax, Pheidole gigas, Pheidole jelskii, Pheidole puttemansi, Pheidole roushae, Pheidole tobini and Pheidole valens, distinguished as follows.
Major: large (Head Width 1.70–1.80 mm); medium to dark reddish brown; robust; with relatively short scapes (Scape Length/Head Width 0.50–0.59 mm); posterior half of head heavily rugoreticulate; anterior half of first gastral tergite heavily shagreened and opaque; petiolar node thick from the side and from above, as illustrated.
Minor: dorsa of petiolar and postpetiolar nodes foveolate and opaque; anterior fringe of median strip of first gastral tergite shagreened.
MEASUREMENTS (mm) Lectotype major: HW 1.70, HL 1.84, SL 0.98, EL 0.24, PW 0.86. Paralectotype minor: HW 0.62, HL 0.80, SL 0.94, EL 0.18, PW 0.48.
COLOR Major: head light reddish brown; mesosoma, waist, and appendages medium reddish brown; gaster dark reddish brown.
Minor: body medium reddish brown to plain dark brown; appendages light reddish to yellowish or light plain brown.
Figure. Upper: lectotype, major. Lower: paralectotype minor. Scale bars = 1 mm.
MacGown (2015) - HL 1.36–1.42, HW 1.58–1.64, SL 0.98–1.00, EL 0.32–0.40, MOL 0.15–0.16, IHTD 0.46–0.49, OHTD 0.63–0.65, ML 2.53–2.55, PRW 1.30–1.60 PSL 0.12–0.15, PTW 0.53–0.55, PPW 0.75–0.80, FWL 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 with 12 antennomeres (including scape and pedicel), apical three antennomeres forming a loose club; scape short, strongly angled basally and thickened medially. Eyes large, located at approximately the midpoint of head on sides of head. 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. Mesoscutum and mesoscutellum with longitudinal striae; sides of pronotum, mesopleura (anepisternum and katepisternum), metapleura, sides and dorsum of propodeum, and sides and dorsum of petiolar and postpetiolar nodes with transverse striae with interstitial punctation dulling the surface (except on propodeal dorsum where punctures are reduced). First gastral tergite with fine longitudinal striae anteriorly, becoming shagreened and shiny posteriorly. Wings pale translucent amber color with veins light brown; forewing venation of typical myrmicine type with pterostigma and costal, basal, subbasal, discal, and two submarginal cells closed; hindwing lacking jugal lobe and with costal, basal, and subbasal cells closed.
ARGENTINA: Alta Gracia, Córdoba. Naturhistorisches Museum, Basel - as reported in Wilson (2003)
L obscurithorax, dark thorax. (Wilson 2003)
- Buren WF, Allen GE, Whitcomb WH, Lennartz FE, Williams RN. 1974. Zoogeography of the imported fire ants. Journal of the New York Entomological Society. 82:113–124.
- Deyrup, M., Davis, L. & Cover, S. 2000. Exotic ants in Florida. Transactions of the American Entomological Society 126, 293-325.
- Kempf, W. W. 1972b. Catálogo abreviado das formigas da região Neotropical. Stud. Entomol. 15: 3–344.
- King, J.R. and W.R. Tschinkel. Range expansion and local population increase of the exotic ant, Pheidole obscurithorax, in the southeastern United States. Florida Entomologist 90:435-439 (2007).
- Naves, M. A. 1985. A monograph of the genus Pheidole in Florida, USA (Hymenoptera: Formicidae). Insecta Mundi 1: 53-90 (page 61, soldier, worker described)
- Santschi, F. 1923d. Pheidole et quelques autres fourmis néotropiques. Ann. Soc. Entomol. Belg. 63: 45-69 (page 58, First available use of Pheidole fallax st. arenicola var. obscurithorax; unavailable name)
- Sarnat, E. M., G. Fischer, B. Guenard, and E. P. Economo. 2015. Introduced Pheidole of the world: taxonomy, biology and distribution. Zookeys. 1-109. doi:10.3897/zookeys.543.6050
- Storz SR, Tschinkel WR. 2004. Distribution, spread, and ecological associations of the introduced ant Pheidole obscurithorax in the southeastern United States. 11pp. Journal of Insect Science, 4:12.
- Wetterer, J. K., J. A. MacGown, and L. A. Calcaterra. 2015. Geographic spread of Pheidole obscurithorax (Hymenoptera: Formicidae). Transactions of the American Entomological Society. 141:222-232. doi:10.3157/061.141.0113
- Wild, A. and A. Suarez. 2009. Mitochondrial DNA implicates the Paraná “invasion cradle” in another ant introduction - the South American big-headed ant, Pheidole obscurithorax. Abstract. ESA Annual Meeting, December 13 – 16, 2009.
- Wilson, E. O. 2003. Pheidole in the New World: A dominant, hyperdiverse ant genus. Harvard University Press, Cambridge, MA. (page 331, Raised to species; new status, fig. major, minor described)