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How to identify it
- Two sharply different worker castes, as in all Pheidole: minor workers with a head about 0.7 mm long and a body uniformly dark reddish-brown with lighter mandibles and feet, and major workers (soldiers) more than twice that size, with a head 1.6 to 1.8 mm long.[1]
- Majors are the easiest big-headed ants to name in North America: the large size, a head covered in strong net-like sculpture with a deeply notched rear margin, and a short, thick antennal scape that is strongly angled at its base are unique among the continent's Pheidole. No other big-headed ant in the Southeast is this large.[1]
- Major mandibles are big and mostly toothless apart from a basal tooth and two large tips, built for crushing; minor mandibles are elongate triangles with nine small teeth and two large apical teeth.[1]
- Both castes have 12-segmented antennae ending in a loose three-segmented club, a two-segmented waist, short propodeal spines, a shiny gaster and a thorax that looks stepped in profile, with the pronotum raised above the mesonotum and the mesonotum above the propodeum.[1]
- Nests in open ground are medium to large and marked by an obvious crater of excavated soil.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Toothed big-headed ant (Pheidole dentata) | The native big-headed ant found in the same habitats. Its minor workers are smaller than those of P. obscurithorax, with a less oval head and no nuchal collar visible in full face view; its majors are far smaller than the imported ant's soldiers.[1] |
| Bigheaded ant (Pheidole megacephala) | Another exotic Pheidole in the region. P. obscurithorax majors are told from every other native or exotic big-headed ant in North America by their large size, heavily sculptured head and thick, basally angled scape.[1] |
| Red imported fire ant | The fire ant has a working sting and is the species P. obscurithorax competes with at baits and for prey; big-headed ants have an atrophied sting and cannot sting at all.[1] |
Where it lives
Native to South America (thought to be Argentina), Pheidole obscurithorax is now recorded in Alabama, Florida, Georgia, Louisiana, Mississippi and Texas, and is common along the Gulf Coast from northern Florida to eastern Texas. A field survey found it occupying an 80-kilometer-wide coastal band between Mobile, Alabama, and Tallahassee, Florida, and still extending its range.[1][2]
Biology and life cycle
In the United States the ant typically nests in open, disturbed habitats, where populations may be abundant; it belongs to a largely exotic community of ants that tolerate highly disturbed ground. Nests are medium to large and topped by an obvious crater.[1][2]
Pheidole obscurithorax is omnivorous but also an aggressive predator. Minor and major workers cooperate to attack and carry much larger prey, such as caterpillars, into the colony, and the ant has been seen defending prey from red imported fire ants, though not always successfully. One report describes it attacking a hatchling chicken.[1]
It is a major competitor of the red imported fire ant in both its native and introduced ranges. In the field it quickly finds baits and then outcompetes Solenopsis invicta at them, yet it coexists with the fire ant and seems to have little negative effect on the ant communities it invades.[1][2]
The spread has been gradual and apparently by natural means. In Tallahassee the ant was spreading rapidly, with nest density increasing 6.4-fold over a two-year survey period.[2]
Queens are larger than major workers, carry three ocelli, and have the thorax enlarged for flight muscles; their wings are 7.4 to 7.8 mm long and lightly tinted yellow-brown.[1]
Damage and signs
Health risk
Like other Pheidole species, Pheidole obscurithorax has an atrophied sting and cannot sting.[1]
Control
Prevention
Neither source gives control recommendations. The ant favors open, disturbed ground and cannot sting, and the museum notes it quickly finds baits, where it outcompetes fire ants.[1]
Common questions
- Does the large imported big-headed ant sting?
- No. The sting is atrophied, as in other Pheidole species, so it cannot sting.[1]
- How is it different from fire ants?
- Fire ants sting; this ant cannot. In the field it coexists with the red imported fire ant, competes with it at baits and for prey, and often wins those contests, though the two share the same disturbed habitats.[1][2]
- How far has it spread?
- It is established in Alabama, Florida, Georgia, Louisiana, Mississippi and Texas and is common along the Gulf Coast from northern Florida to eastern Texas. A survey found the range still increasing, with spread occurring gradually by natural means.[1][2]
Sources
- Mississippi Entomological Museum, Mississippi State University, “Ants of the Southeastern United States: Pheidole obscurithorax Naves, 1985 (J. A. MacGown, updated 2016).” mississippientomologicalmuseum.org.msstate.edu/Researchtaxapages/Formicidaepages/genericpages/Pheidole.obscurithorax.htm (accessed 2026-11-10)
- Journal of Insect Science, abstract via PubMed, “Distribution, spread, and ecological associations of the introduced ant Pheidole obscurithorax in the southeastern United States (Storz and Tschinkel 2004, Journal of Insect Science 4:12; PubMed 15861228).” doi.org/10.1093/jis/4.1.12 (accessed 2026-11-10)
- iNaturalist, “Pheidole obscurithorax taxon page (order and family).” www.inaturalist.org/taxa/710657 (accessed 2026-11-10)