Pheidole obscurithorax Naves

Sarnat, Eli M., Fischer, Georg, Guenard, Benoit & Economo, Evan P., 2015, Introduced Pheidole of the world: taxonomy, biology and distribution, ZooKeys 543, pp. 1-109 : 43

Taxon classification Animalia Hymenoptera Formicidae

Pheidole obscurithorax Naves Figs 82, 88j

Pheidole obscurithorax . Pheidole fallax subsp. obscurithorax Naves 1985: 61 (s.w.) ARGENTINA, Alta Gracia, Córdoba (Bruch). [First available use of Pheidole fallax st. arenicola var. obscurithorax Santschi 1923: 58; unavailable name.] Raised to species; lectotype (s.) (CASENT0913311, NHMB) designated: Wilson 2003: 331.

Diagnosis among introduced Pheidole .

Medium reddish brown to dark brown. MajorHW 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). MinorHW 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 2 × 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).

Identification, taxonomy and systematics.

Pheidole obscurithorax is a member of the New World (and polyphyletic, see Moreau 2008) Pheidole 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 Pheidole 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 Pheidole punctatissima clade and Pheidole 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 Pheidole fervens , Pheidole indica , and Pheidole 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 Pheidole obscurithorax .

Biology.

In its introduced range of the southeastern United States, Pheidole 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 Pheidole 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 Solenopsis invicta suggest it is an important species to monitor.

Distribution.

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 Solenopsis 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 Pheidole 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). Additional occurence records, including the first record for Bolivia, were published ( Wetterer et al. 2015) just as this manuscript was going to press, and were not included in the present study.

Risk statement.

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 Pheidole obscurithorax could become more widespread regionally and globally in the future.