Pheidole obscurithorax

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. (Wilson 2003)

Identification
See the description in the nomenclature section.

Distribution
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. P. obscurithorax has since been found in Baldwin County, Alabama, across the bay from Mobile, and in adjacent, westernmost Florida (Naves 1985). Walter Tschinkel (personal communication) reports its appearance around Tallahassee, Florida, in the late 1990s. (Wilson 2003)

This taxon was described from Argentina. It is also found in the United States and Paraguay.

Nesting Biology
It 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).

Foraging/Diet
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 S. 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 Solenopsis invicta, and P. obscurithorax both wins battles and loses them.

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.'''

Type Material
- as reported in Wilson (2003)

Type Locality Information
ARGENTINA: Alta Gracia, Córdoba. (Wilson 2003)

Etymology
L obscurithorax, dark thorax. (Wilson 2003)