Pheidole obscurithorax

From Storz and Tschinkel (2004): Pheidole obscurithorax is native to northern Argentina and Paraguay (Wilson 2003). It occurs 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 in the port city of Mobile, Alabama, in the early 1950s (Naves 1985; under the name, P. fallax obscurithorax). Several other ant species with ranges along the Paraguay River system (Brachymyrmex patagonicus Mayr, Linepithema humile, Solenopsis invicta, and Solenopsis richteri Forel) 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).

Identification
From Storz and Tschinkel (2004): Pheidole obscurithorax is a large, dark ant up to 6 mm in length, and its workers are dimorphic; the majors are larger than the minors and have enlarged heads. 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.

Also see the description in the nomenclature section.

Distribution
The species is native to Argentina, Brazil and Paraguay.

It has also found its way to the United States and was likely introduced in Mobile, AL or Pensacola, FL sometime around 1950. It is currently known to occur from eastern Texas to Tallahassee, FL.



'''Figure. US 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.

This taxon was described from Argentina.

Habitat
In its introduced range this species is associated with disturbed habitats that contain open areas of sandy soil.

USA
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.

South America
This species has not been studied in its native range.

Colony Attributes
As reported in King and Tschinkel 2007: large colonies of ~10,000 workers (WRT unpublished data).

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 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.

Nesting Biology
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).

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)

Additional References
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.

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 MA. 1985. A monograph of the genus Pheidole in Florida (Hymenoptera: Formicidae). Insecta Mundi. 1:53–90.

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.

Text and images from this publication used by permission of the author.