The following account is from Sarnat et al. (2015) and figure numbers refer to that publication.
Pheidole obscurithorax is a member of the New World (and polyphyletic, see Moreau 2008) P. fallax species group defined by Wilson (2003). It is a large dark species over 6 mm in body length. The species is easily distinguished from P. megacephala by the much larger body size and relatively reduced postpetiole, in addition to the strongly sculptured head of the major worker (Fig. 12), and the much longer antennal scapes of the minor. It is separated from other New World species treated here, including those of the P. punctatissima clade and P. flavens complex, by the much larger size, prominence on the posterior slope of the promesonotum (Fig. 5, major; Fig. 50, minor), densely rugoreticulate face of the major (Fig. 12), and smooth head and long antennal scapes of the minor. The Old World species P. fervens, P. indica, and P. noda all have majors with strongly sculptured head and minors with smooth heads, and the reader is referred to the key for characters used to separate these from P. obscurithorax.
In its introduced range of the southeastern United States, P. obscurithorax is characterized by its large size, large nest mounds, very active foraging and fast recruitment to bait such as cookie crumbs (King and Tschinkel 2007). It nests in soil in open areas, where it produces conspicuous nests, each generally with a single large opening often covered by a leaf or other collected material (Storz and Tschinkel 2004). The species is an omnivorous scavenger of dead arthropods (possibly including dead fire ants), and less frequently of plant material such as flower petals (Storz and Tschinkel 2004). Studies in its introduced range found evidence that P. obscurithorax is monogynous and is spreading by natural dispersal of winged females in addition to human-mediated long-distance dispersal (King and Tschinkel 2007). The species was most often found associated with disturbed habitats such as lawns and roadsides, but there are also records of it occurring in natural areas such as hardwood forests (Wilson 2003). However, its steady expansion across the southeastern United States and co-occurrence with S. invicta suggest it is an important species to monitor.
Pheidole obscurithorax is not currently considered a pest in its introduced range, as it does not sting and is not known to infest dwellings or structures (King and Tschinkel 2007). However, the species is an aggressive predator (Deyrup et al. 2000) and may have the potential to become a pest or to negatively impact native species if its populations continue to grow and spread. Pheidole obscurithorax is thought to spread across the southeastern United States by mated queens (not colony fragments) that are being transported in substrates such as potted plants. It is possible that P. obscurithorax could become more widespread regionally and globally in the future.
Diagnosis of worker among Antkey species. Worker castes bimorphic. Head shape ovoid (minor workers) or subrectangular with posterolateral lobes (majors), but never triangular. Antenna 12-segmented. Antennal club 3-segmented. Antennal insertions at least partly covered by frontal lobes; not surrounded by a raised sharp-edged ridge. Frontal lobes do not obscure face outline between mandible and eye; relatively close together so that the posteromedian portion of the clypeus, where it projects between the frontal lobes, is at most only slightly broader than one of the lobes. Posterolateral corners of head unarmed, without spines. Mandibles triangular. Mesosoma with erect hairs. Pronotal spines absent. Propodeum armed with spines or teeth. Slope of mesosoma steep. Waist 2-segmented. Petiole pedunculate with a distinct and upright node; lacking large subpetiolar process. Postpetiole attached to lower surface of gaster; not swollen; in dorsal view not distinctly broader than long or distinctly wider than petiole. Minor worker characters. Head ovoid; distinctly longer than wide; smooth and shining, lacking punctation. Antennal scrobe lacking. Posterior head margin strongly convex. Antennal scapes extend beyond posterior margin of head. Postpetiole not swollen relative to petiole. Lateral portions of petiole and postpetiole strongly punctate. Color brown or reddish brown to black. Major worker characters. Antennal scrobe absent, no depression capable of receiving antennal scapes visible. Posterolateral lobes distinctly rugose; rugae above eye level strongly reticulated.
Diagnosis among introduced Pheidole. Medium reddish brown to dark brown. Major | HW 1.47–1.70, HL 1.49–1.84, SL 0.98–103, CI 92–99, SI 58–70 (n=3). Head subquadrate (Fig. 7); almost entirely covered by a network of intersecting rugae (Fig. 12a), lacking long, well-organized and parallel longitudinal rugae on the frons (Fig. 12b). Frontal carinae indistinct, quickly becoming integrated into dense rugoreticulum that covers the entire face. Antennal scrobes entirely lacking. Antennal insertions surrounded by deeply excavated pits (Fig. 12c). Head often a lighter reddish brown than the mesosoma. Promesonotum in profile with two convexities (Fig. 5), the large anterior dome in addition to a distinct mound or prominence on the posterior slope. Postpetiole not swollen relative to petiole (Fig. 3). Minor | HW 0.60–0.67, HL 0.78–0.85, SL 0.94–1.08, CI 76-82, SI 152–173 (n=5). Head predominantly glossy (Fig. 36), lacking punctation and or rugae above eye level. Posterior margin strongly convex in full-face view such that the head outline forms a single unbroken curve from eye to eye (Fig. 44). Antennal scapes extremely long, surpassing posterior head margin by more than 2x eye length (Fig. 39). Promesonotum in profile with two convexities, the large anterior dome (Fig. 43a) in addition to a distinct prominence on the posterior slope (Fig. 43b). Mesopleuron mostly sculptured. Postpetiole not swollen relative to petiole (Fig. 3). Petiole and postpetiole strongly sculptured laterally (Fig. 47).
Pheidole obscurithorax is presumed native to the South American region of Argentina, Paraguay and southern Brazil that includes the Paraguay, La Plata and Parana Rivers. This flood-prone area is the cradle of many other well-known invasive ants including fire ants (Solenopsis invicta Buren and S. richteri Forel), the Argentine ant (Linepithema humile), and many lesser-known species that were anthropogenically introduced (King and Tschinkel 2007; Storz and Tschinkel 2004; Suarez and Tsutsui 2008; Wilson 2003). Most of these species, including P. obscurithorax, were first introduced to North America via the Mobile, Alabama shipping port pathway. Pheidole obscurithorax was introduced to Mobile, Alabama around 1950 (Naves 1985) and subsequently expanded its range to include Florida, Georgia, Mississippi and Texas (Storz and Tschinkel 2004; Wilson 2003).