Central America, Jamaica, Puerto Rico, the Lesser Antilles, and the northern coast of South America south along the Andes to Paraguay and southeastern Brazil. Incidental worldwide.
From Wild 2007:
Linepithema iniquum is primarily an arboreal ant. Thirteen nest records are from dead branches or dead wood inside living trees, four are from dead twigs or vines, two records are from the base of a bromeliad, one from a bamboo sheath, two from rotting wood on the ground, and nine nests intercepted in ports-of-entry around the world have been in orchids. Wheeler (1908: 153) reports the ant in Puerto Rico (as Iridomyrmex melleus) nesting in hollow twigs, in leaf petioles of plantain, and in Òfriable carton (earth mixed with vegetable debris) on the under sides of the huge reniform leaves [of Coccoloba rugosa (Ortegon)], and in Dominica under and in dead stalks of bananas and plantains (Wheeler 1913: 242). On St. John Island, Pressick and Herbst (1973) report L. iniquum nesting in twigs and in logs in moist forest and grassy field habitats. Leuderwaldt (1926) reports two nests in bamboo (as I. iniquus), one under bark (as I. iniquus var. succinea), and one in a dry branch (as I. iniquus var. succinea) in southern Brazil. Linepithema iniquum has been collected rarely in leaf-litter surveys, but these likely reflect incidental ground foragers.
Linepithema iniquum has been collected from sea level to over 2000 meters in elevation. This species is exclusively montane in Central America and northern South America, and is found at varying elevations in the Caribbean, southern Brazil, and Paraguay. 25 museum records are from human disturbed habitats such as 2nd growth forest edges, roadsides, orchards, and pastures. Four are from tropical humid montane forest, seven from primary Atlantic forest in Brazil and Paraguay, three Paraguayan records are Malaise trap samples from low inundated forest, and one Paraguayan record is from an inundated grassland. In Ecuador this species can be locally abundant along roadsides, in pastures, and in Psidium guayaba orchards, and in Puerto Rico L. iniquum is among the most abundant ants in the mountains (Wheeler 1908, Wild, pers obs).
Linepithema iniquum is probably polydomous, as individual arboreal nests frequently contain no queens (Wild, pers obs). Of five full nest excavations conducted by the author in Ecuador and Puerto Rico that found dealate queens, four colonies had a single dealate queen and one had two dealate queens. This pattern suggests that L. iniquum is monogynous to weakly polygynous, although confirmation will require molecular genetic data. Male and female alates have been observed in nests in Brazil from October to April, in Ecuador in August and December, in Costa Rica in November, and in Puerto Rico year round. Both male and female alates have been attracted to lights in November in Puerto Rico.
In Puerto Rico this species has been observed tending Coccus and Saissetia scale on coffee (Smith 1942: 22), tending pseudococcids on Cecropia (Wild, pers. obs), and visiting extra floral nectaries (Wild, pers. obs). There is one observation of aphid-tending in Ecuador (Wild, pers. obs). Smith (1929) and Wheeler (1929) describe the behavior of introduced L. iniquum in North American greenhouses in Illinois and Massachusetts, respectively, and observe the ant tending scale, visiting flowers, and nesting in and under soil pots.
Like its better-known congener L. humile, L. iniquum is carried around the world with human commerce. More than a dozen museum records of this species are intercepts at various ports-of-entry and quarantine in the United States and Europe, usually carried with epiphytes. Unlike L. humile, this species apparently has not been successful in establishing outdoor populations in spite of the opportunity to do so, although there are a few records of this ant persisting in greenhouses in the temperate zone (Wheeler 1929, Creighton 1950).
Wild (2007) reports several collections from the Central Valley area, all from high elevation. Kenji Nishida collected the species at the 2000m site on the Barva Transect in Braulio Carrillo National Park.
Diagnosis among workers of introduced and commonly intercepted ants in the United States. Antenna 12-segmented. Antennal scape length less than 1.5x head length. Eyes medium to large (greater than 5 facets); do not break outline of head; placed distinctly below midline of face. Antennal sockets and posterior clypeal margin separated by a distance less than the minimum width of antennal scape. Anterior clypeal margin variously produced, but never with one median and two lateral rounded projections. Mandible lacking distinct basal angle. Profile of mesosomal dorsum with three distinct convexities. Dorsum of mesosoma lacking a deep and broad concavity; lacking erect hairs. Promesonotum separated from propodeum by metanotal groove. Propodeum with dorsal surface not distinctly shorter than posterior face; globular, strongly and uniformly convex. Propodeum and petiolar node both lacking a pair of short teeth. Mesopleura and metapleural bulla bulla lacking pubescence; surface glabrous. Propodeal spiracle distinctly anterior to posterior margin of propodeal profile. Waist 1-segmented. Petiole upright and not appearing flattened. Gaster armed with ventral slit. Erect hairs present on cephalic dorsum (above eye level), gastral tergites 1 and 2; often present on pronotum. Dull, not shining, and color uniformly light to dark brown. Measurements: head length (HL) 0.55–0.74 mm, head width (HW) 0.46–0.70 mm.
Among introduced and commonly intercepted ants in the United States, Linepithema iniquum is most easily confused with the Argentine Ant, L. humile. It can be separated from that species by the following characters: (1) the mesopleura and metapleural bulla lack pubescence and have a glabrous (versus dull) surface; (2) erect hairs are present (versus lacking) on cephalic dorsum (above eye level), gastral tergites 1 and 2, and often present on pronotum; (3) the profile of mesosomal dorsum has three (versus two) distinct convexities; (4) the propodeum is globular (versus angular) and is strongly and uniformly convex (versus flat or weakly convex dorsal and posterior faces); and (5) the propodeal spiracle distinctly anterior to (versus bordering) the posterior margin of propodeal profile.
Creighton, W. S. 1950. The ants of North America. Bull. Mus. Comp. Zool. 104:1-585.
Luederwaldt, H. 1926. Observa›es biologicas sobre formigas brasileiras especialmente do estado de S‹o Paulo. Revista do Museu Paulista 14:185-303.
Pressick, M. L., and Herbst, E. 1973. Distribution of ants on St. John, Virgin Islands. Caribb. J. Sci. 13:187-197.
Smith, M. R. 1929. Two introduced ants not previously known to occur in the United States. J. Econ. Entomol. 22:241-243.
Smith, M. R. 1942. The relationship of ants and other organisms to certain scale insects on coffee in Puerto Rico. J. Agri. Univ. Puerto Rico 26:21-27.
Wheeler, W. M. 1908. The ants of Porto Rico and the Virgin Islands. Bull. Am. Mus. Nat. Hist. 24:117-158.
Wheeler, W. M. 1913. Ants collected in the West Indies. Bull. Am. Mus. Nat. Hist. 32:239-244.
Wheeler, W. M. 1929. Two Neotropical ants established in the United States. Psyche (Camb.) 36:89-90.
Wild, A. L. 2007. Taxonomic Revision of the Ant Genus Linepithema (Hymenoptera: Formicidae). University of California Press, Berkeley, CA, USA.
Found most commonly in these habitats: 4 times found in psidium guayaba orchard, 5 times found in Port of entry, 3 times found in cloud forest, 3 times found in 2nd growth forest next to road mixed in with living fence, 3 times found in bosque bajo inundado, 1 times found in edge of 2nd growth rainforest, 2 times found in garden with native ferns, Psidium guayaba, bromeliads, citrus, 2 times found in in subtropical wet forest, 1 times found in riparian wet forest, 1 times found in tropical humid montane forest, ...
Collected most commonly using these methods or in the following microhabitats: 5 times Search, 3 times malaise 3, 1 times 1100hr., 1 times pitfall, 1 times 1730-1900hr., 1 times light trap., 1 times malaise 1, 1 times malaise 4
Elevations: collected from 40 - 2100 meters, 1095 meters average