Linepithema humile

The Argentine ant is one of the most well known invasive ants and has been introduced in many parts of the world.

Identification
Worker Eyes large (OI > 30); antennal scapes long (SI > 105); pronotum and first two gastric tergites lacking erect setae; mesopleura and metapleura densely pubescent.

Workers of the sister species Linepithema oblongum, from the high Andes of Bolivia and northern Argentina, normally have at least some members of each series with dilute pubescence on gastric tergites 2–4. These ants also have, on average, smaller eyes (OI 28–38) and longer antennal scapes (SI 120–139) than L. humile. Workers of Linepithema anathema, a rarely-collected Brazilian species, have a more produced propodeum, a narrow head (CI < 86), and usually bear short standing setae on gastric tergites 1–2. Workers of other Humile-group species have shorter antennal scapes and often bear erect setae on the pronotum and basal gastric tergites. Males of related species are much smaller than L. humile and lack the greatly swollen mesosoma.

Male Forewing with single submarginal cell; mesosoma robust (MML > 1.3), mesoscutum greatly enlarged and overhanging pronotum; wings short relative to mesosomal length (WI < 21).

Distribution
Native to the Paraná river drainage of Brazil, Paraguay, Argentina, and Uruguay. Introduced worldwide.

This introduced species occurs sporadically throughout Florida, in places forming massive populations. It occurs in both moist and dry open habitats, usually in heavily disturbed sites. Pest status: can become a nuisance by sheer numbers, trailing long distances to outdoor eating areas and into buildings. First published Florida record: Wheeler 1932; earlier specimens: 1914. (Deyrup, Davis & Cover, 2000.)

Linepithema humile was previously reported in Colombia (Sanabria & Chacón de Ulloa 2009; Wild 2004, 2007). However, we found that the specimens identified as L. humile in Sanabria & Chacón de Ulloa (2009), and many other “Linepithema humile” specimens in Colombian entomological collections, were misidentified. In most cases these specimens were Linepithema piliferum or Linepithema neotropicum, and in no cases were L. humile. In order to confirm the occurrence of L. humile in Colombia (Wild 2007), we studied the L. humile specimens reported in Wild (2007) and deposited in WPMC. We confirm that the specimens are L. humile, and this remains the only known collection of the species in Colombia. These ants were collected in the Colombian coffee zone (Armenia, Quindio) in 1973, but despite subsequent intensive sampling done in that area by the program Paisajes Rurales (Instituto Alexander von Humboldt), there are no more L. humile records from Armenia or elsewhere. Thus it is possible that this was an introduction that did not persist and the species no longer occurs in Colombia. (Escarraga & Guerrero, 2016)

Distribution based on Regional Taxon Lists
Afrotropical Region: Lesotho, Mozambique, Namibia, Saint Helena, United Arab Emirates. Australasian Region: Australia, New Zealand, Norfolk Island. Indo-Australian Region: Hawaii, Vanuatu. Nearctic Region: United States. Neotropical Region: Argentina, Bermuda, Brazil, Ecuador, French Guiana, Mexico, Paraguay, Uruguay. Palaearctic Region: Balearic Islands, Belgium, Bulgaria, Canary Islands, Channel Islands, Democratic Peoples Republic of Korea, France, Germany, Gibraltar, Greece, Iberian Peninsula, Iran, Japan, Malta, Monaco, Montenegro, Morocco, Poland, Portugal, Spain, United Kingdom of Great Britain and Northern Ireland.



Biology
Wild (2007) - This important pest species has a literature too extensive to be covered in depth here. An early general review of the biology of this ant is given by Newell and Barber (1913). The spread of Argentine ants around the world is documented by Roura-Pascual et al. (2004), Wild (2004), Giraud et al. (2002), and Suarez et al. (2001). Ecological impacts of Argentine ant invasion have been detailed in numerous studies, including Suarez and Case (2003), Touyama et al. (2003), Christian (2001), and Human and Gordon (1997). Colony structure has also received considerable attention, and relevant papers include Holway and Suarez (2004), Tsutsui and Case (2001), Reuter et al. (2001), and Kreiger and Keller (2000). A series of studies by Cavill and colleagues (Cavill and Houghton 1973, Cavill and Houghton 1974, Cavill et al. 1980) describe some of the glandular and cuticular chemistry of L. humile. Chemical and biological control options are reviewed by Harris (2002).

Of the L. humile material examined, more than 90% of native range records are within 10 kilometers of a major river in the Paraná drainage. Contrary to some reports (Buczkowski et al. 2004), L. humile can reach high densities in urban areas in Argentina and Paraguay (Wild 2004) as well as in less disturbed habitats (Heller 2004). Where nest information was recorded in the native range, 24 nests are from soil, five from under covering objects such as stones or garbage, one from an old termite mound, and one from under bark. This species is polygynous and polydomous, and many nests are recorded as having numerous dealate queens. In contrast to introduced populations, alate queens are not uncommon in nests in Argentina (Wild 2004). One observation in Victoria, Argentina, notes a live lycaenid larva in the brood nest (Wild, pers. obs.).

This ant forms what has been called supercolonies: a large number of nests spread over large areas where the individuals from one nest can be brought to any other nest and accepted as nestmates. Linepithema humile do not have nuptial flights (Passera and Keller 1990). Queens mate with related males in their natal nests (Markin 1970).

Inou et al. (2015) suggested the results of a genetic study of four supercolonies in Kobe Japan showed that these colonies replaced their queens during the reproductive season. Genetic differentiation among workers varied significantly in comparing May samples to September samples. They feel this provides evidence for queen execution, which has been reported in two introduced populations (USA, Markin 1970 and France Keller et al. 1989).

Bertelsmeier et al. (2015a, b) examined elements of interspecific aggression, and food resource discovery and dominance, between this species and several other highly invasive ants. In laboratory assays Linepithema humile was highly aggressive when confronted with workers of other invasive ants. Of the group of four species that were found to be aggressive, L. humile was found to be fairly adept at finding and recruiting to food in a laboratory arena experiment.

Regional Notes
DaRocha et al. (2015, Brazil) studied the diversity of ants found in bromeliads of a single large tree of Erythrina, a common cocoa shade tree, at an agricultural research center in Ilhéus, Brazil. Forty-seven species of ants were found in 36 of 52 the bromeliads examined. Bromeliads with suspended soil and those that were larger had higher ant diversity. Linepithema humile was found in a single bromeliad and was associated with the suspended soil and litter of the plant.

Espadaler (2007) - Canary Islands: The Argentine ant is known from all the Canary Islands (Espadaler & Bernal, 2003). At El Hierro it occupies habitats from next to sea level to one thousand meters, in pine forests. Confronted with the two populations known to exist in North Mediterranean Europe (Giraud et al., 2002), the Argentine ants from El Hierro showed aggressiveness towards the “Catalan” population and reacted peacefully towards the “Main” population from mainland Europe. Aggression tests (one to one worker; five replicates) were run with two samples from El Hierro (La Frontera; Mirador de las Playas). I conclude that both samples from El Hierro belong to the genotypic profile of the “Main” population, the more abundant in Western Mediterranean Europe.

Collingwood (1979) - Europe: This species was introduced into Europe from South America and has become an established and notorious pest in the Mediterranean area, developing very populous multi-queened colonies along the coast. It is sometimes brought into North Europe with plant materials and occasionally colonises heated premises. It does not appear to be able to establish outside in northern latitudes but is present and said to be increasing in the Channel Islands.

Other Insects
This ant has been observed tending larvae of Lampides boeticus (Obregon et al. 2015).

Genetics
Linepithema humile has had their entire genome sequenced.

Palomeque et al. (2015) found class II mariner elements, a form of transposable elements, in the genome of this ant.

Castes
Queens differ in their Cuticular Hydrocarbons according to ovarian activity. Whereas the cuticular profile of non-laying queens is similar to that of sterile workers, it gradually changes both qualitatively and quantitatively once queens start to lay eggs. These changes are independent of mating status, since virgin egg-laying queens show a CH profile similar to that of mated egg-laying queens (de Biseau et al. 2004).

Nomenclature

 *  humile. Hypoclinea humilis Mayr, 1868b: 164 (w.) ARGENTINA. Forel, 1908c: 395 (m.); Newell, 1908: 28 (q.); Wheeler, G.C. & Wheeler, J. 1951: 186 (l.); Crozier, 1969: 250 (k.). Combination in H. (Iridomyrmex): Mayr, 1870b: 959; in Iridomyrmex: Emery, 1888d: 386; in Linepithema: Shattuck, 1992a: 16. Senior synonym of arrogans, riograndensis: Wild, 2004: 1207. See also: Gallardo, 1916a: 97; Bernard, 1967: 251; Collingwood, 1979: 33; Smith, D.R. 1979: 1418; Ward, 1987: 1; Wheeler, G.C. & Wheeler, J. 1990a: 465; Shattuck, 1994: 123; Wild, 2007a: 61; Solis, Fox, Rossi & Bueno, 2010: 19.
 * arrogans. Iridomyrmex humilis var. arrogans Chopard, 1921: 237 (footnote),:241, figs. 1-31 (w.q.m.l.) FRANCE. [Also described as new by Santschi, 1929d: 306 (in key).] Combination in Linepithema: Shattuck, 1992a: 16. Junior synonym of humile: Bernard, 1967: 251. Revived from synonymy as subspecies of humile: Shattuck, 1992a: 16. Junior synonym of humile: Wild, 2004: 1207.
 * riograndensis. Iridomyrmex riograndensis Borgmeier, 1928b: 64 (w.) BRAZIL. Combination in Linepithema: Shattuck, 1992a: 16. Junior synonym of humile: Wild, 2004: 1207.

The taxonomy and distribution of L. humile was reviewed in depth by Wild (2004).

Wild (2007):

Worker
Holotype: HL 0.74, HW 0.66, MFC 0.16, SL 0.76, FL 0.65, LHT 0.68, PW 0.45, ES 2.93, SI 115, CI 89, CDI 24, OI 40.

Worker: (n = 81) HL 0.62–0.78, HW 0.53–0.72, MFC 0.14–0.18, SL 0.62–0.80, FL 0.52–0.68, LHT 0.57–0.76, PW 0.35–0.47, ES 1.98–3.82, SI 108–126, CI 84–93, CDI 23–28, OI 32–49.

Head in full face view longer than broad (CI 84–93), narrowed anteriorly and reaching its widest point just posterior to compound eyes. Lateral margins broadly convex, grading smoothly into posterior margin. Posterior margin straight in smaller workers to weakly concave in larger workers. Compound eyes large (OI 32–49), comprising 82–110 ommatidia (normally around 100). Antennal scapes long (SI 108–126), as long or slightly longer than HL and easily surpassing posterior margin of the head in full face view. Frontal carinae narrowly to moderately spaced (CDI 23–28). Maxillary palps relatively short, shorter than ½ HL, ultimate segment (segment six) noticeably shorter than segment 2.

Pronotum and mesonotum forming a continuous convexity in lateral view, mesonotal dorsum nearly straight, not angular or strongly impressed, although sometimes with a slight impression in anterior portion. Metanotal groove moderately impressed. Propodeum in lateral view inclined anteriad. In lateral view, dorsal propodeal face meeting declivity in a distinct though obtuse angle, from which the declivity descends in a straight line to the level of the propodeal spiracle.

Petiolar scale sharp and inclined anteriorly, in lateral view falling short of the propodeal spiracle.

Dorsum of head (excluding clypeus), mesosoma, petiole, and gastric tergites 1–2 ( = abdominal tergites 3–4) devoid of erect setae (very rarely with a pair of small setae on gastric tergite 2). Gastric tergites 3–4 each bearing a pair of long, erect setae. Venter of metasoma with scattered erect setae.

Integument shagreened and lightly shining. Body and appendages including gula, entire mesopleura, metapleura, and abdominal tergites covered in dense pubescence.

Body and appendages concolorous, most commonly a medium reddish or yellowish brown but ranging in some populations from testaceous to dark brown, never yellow or piceous.

Queen
(n = 13) HL 0.83–0.92, HW 0.83–0.93, SL 0.81–0.89, FL 0.78–0.90, LHT 0.88–0.97, EL 0.31–0.36, MML 1.67–2.09, WL 4.42–4.51, CI 93–101, SI 96–102, OI 36–39, WI 24–27, FI 40–48.

Moderately large species (MML 1.67–2.09). Head slightly longer than broad to as broad as long in full face view (CI 93–101), posterior margin slightly concave to slightly convex. Eyes of moderate size (OI 36–39). Ocelli small. Antennal scapes relatively long (SI 96–102), in full face view scapes in repose surpassing posterior margin by a length greater than length of first funicular segment.

Forewings short relative to mesosomal length (WI 24–27). Forewings with Rs+M at least three times longer than M.f2. Legs of moderate length relative to mesosomal length (FI 40–48).

Dorsum of mesosoma and metasoma with scattered standing setae. Mesoscutum bearing 2–11 standing setae. Body color medium reddish brown. Antennal scapes, legs, and mandibles concolorous with body.

Male
(n = 12) HL 0.56–0.70, HW 0.56–0.74, SL 0.13–0.16, FL 0.60–0.77, LHT 0.51–0.66, EL 0.31–0.34, MML 1.40–1.96, WL 2.55–3.26, PH 0.25–0.34, CI 99–106, SI 22–27, OI 51–55, WI 17–20, FI 37–45.

Head about as broad as long in full face view (CI 99–106). Eyes large (OI 51–55), occupying much of anterolateral surface of head and separated from posterolateral clypeal margin by a length less than width of antennal scape. Ocelli large and in full frontal view set above adjoining posterolateral margins. Antennal scape of moderate length (SI 22–27), about 2/3 length of 3rd antennal segment. Anterior clypeal margin straight to broadly convex. Mandibles small, bearing a single apical tooth and 4-8 denticles along masticatory margin and rounding into inner margin. Masticatory margin relatively short, subequal in length to inner margin. Inner margin roughly parallel to, or converging distally with, exterior lateral margin.

Mesosoma unusually well developed, considerably wider than head width, and larger in bulk and in length than metasoma. Mesoscutum greatly enlarged, projecting forward in a convexity overhanging pronotum. Scutellum large, convex, nearly as tall as mesoscutum and projecting well above level of propodeum. Propodeum well developed and overhanging petiolar node, posterior propodeal face strongly concave. Forewings short relative to mesosomal length (WI 17–20) and bearing a single submarginal cell. Wing color whitish or yellowish with dark brown veins and stigma. Legs short relative to mesosoma length (FI 37–45).

Petiolar scale taller than node length and bearing a broad crest. Ventral process well developed. Gaster oval in dorsal view, nearly twice as long as broad. Gonostylus produced as a bluntly rounded pilose lobe. Volsella with cuspis present, digitus short and downturned distally.

Dorsal surfaces of body largely devoid of erect setae, occasionally with a few fine, short setae scattered on mesoscutum, scutellum, and posterior abdominal tergites. Venter of gaster with scattered setae. Pubescence dense on body and appendages, becoming sparse only on medial propodeal dorsum.

Color as for worker.