Solenopsis invicta

Stefan Cover (EOL) - The Red Imported Fire Ant (Solenopsis invicta) is a native of tropical and subtropical South America that has achieved international notoriety by becoming an enormously successful invasive ant throughout much of the southern United States. S. invicta is now spreading rapidly in parts of the Caribbean, and new infestations have been detected and exterminated in Arizona, California, Australia, New Zealand, and southern China. The probability of new invasions is therefore quite high and S. invicta must be considered a potential threat worldwide in all areas where climates are suitable.

Invasive populations of Solenopsis invicta are by no means easy to ignore. They have been linked to a multitude of destructive effects, including stinging humans, agricultural and horticultural damages, and substantial negative impacts on native faunas and floras. This has resulted in social and political pressure on governments to "solve" the fire ant problem. Government involvement in fire ant research developed first in the United States, which has an 80 year history of Imported Fire Ant infestation, but other governments have more recently begun research and control efforts of their own.

In the US, federal and state governments have responded primarily by funding research and by developing detection and infestation prevention programs. There have also been expensive and ecologically disasterous attempts to exterminate entire Imported Fire Ant populations. On the brighter side, over the course of fifty years, federal and state funding agencies have underwritten a plethora of research programs that have examined in detail the behavior, ecology, life-history, genetics, and potential controls for Solenopsis invicta. As a result, S. invicta has become in some sense the Drosophila melanogaster of the ant world. We probably know more about its biology than is known for any other species of ant. Despite this, our ability to control large-scale infestations remains limited.

Identification
Pitts et. al. (2018) - A member of the Solenopsis saevissima species-group. As noted with the workers (Trager 1991; J. Pitts and K. R. Ross, pers. obs.), the gynes are highly variable in color. The darker variants of gynes are found in southeastern Brazil to Uruguay and Argentina and are associated with the darker workers. A lighter variant occurs in the northern area of the species’ range. The gynes and workers of these colonies are also similar in coloration. A third variant, a light orange form, is found in the Pantanal region of Brazil. This form has larger workers and gynes. No morphological differences could be found between these forms, other than size and coloration.

The darker colored gynes of S. invicta look most similar to Solenopsis megergates and Solenopsis quinquecuspis. However, the CI and OI of S. invicta gynes are normally smaller than those of S. megergates and the OI of S. invicta is normally smaller than that of S. quinquecuspis. The lighter gynes of S. invicta look similar to Solenopsis richteri and Solenopsis interrupta. The gynes of S. interrupta normally are lighter in coloration than S. invicta, have larger cephalic foveolae, and sometimes have distinct striations between the foveolae. The OOI of S. invicta gynes is normally greater than that of S. richteri, and the postpetiole of S. invicta gynes has straight sides, unlike the concave sides of S. richteri. In many cases, the sculpture of the mandible and postpetiole can help separate S. invicta gynes from similar species. The gynes of S. invicta normally have the most densely sculptured postpetiole compared to the other species.

The male of S. invicta is dark in coloration and is similar to most of the other darker species. The pubescence of the S. invicta male is longer and denser than that of Solenopsis saevissima. Sometimes the head of the S. invicta male is shagreened as in S. interrupta. The gena of the male is much more sculptured than in other species. In moderate to extreme forms, this feature is easily recognized and it may be autapomorphic for S. invicta.

Buren (1972) - Similar to Solenopsis richteri. The best and easiest method of distinguishing invicta from richteri is by the morphological characters given in the descriptions, particularly those concerning the shapes of the head, thorax, and postpetiole. Reiterated, in richteri the sides of the head are usually broadly elliptical in shape and lack the weakly cordate shape seen in invicta; the peaks of the occipital lobes nearer the midline and the occipital excision more creaselike in richteri than in invicta; scapes longer in richteri than in invicta in relation to their ability to reach toward the occipital peaks; pronotum with strong and rather angulate shoulders in richteri, this character nearly absent in invicta; a shallow but distinctly sunken area on posterior median dorsum of the pronotum of large workers in richteri, absent in large workers of invicta; the promesonotum strongly convex in profile in invicta, more weakly so in richteri; in profile the base of propodeum elongate and straight in richteri, convex and shorter in proportion to the declivity in invicta; the postpetiole wide and with straight or diverging sides posteriorly in invicta, narrower and usually with converging sides in richteri; transverse impression on posterodorsal face of postpetiole usually apparent and strong in richteri, usually weak or absent in invicta.

Distribution
Solenopsis invicta is a native of South America, centered on the Pantanal region found at the headwaters of the Paraguay River. This area is comprised of seasonally flooded savannas and wetlands. Solenopsis invicta first became noticed as pest species after its introduction in Mobile, Alabama in the 1930's. This introduction was thought to have its origins from a cargo ship from somewhere in NE Argentina. It is also possible there were numerous introductions. The fire ant is now found throughout the SE United States and California. It has subsequently been introduced and spread to other parts of the world as well (as detailed below).

Pitts et. al. (2018) - The range of Solenopsis invicta in South America currently extends from as far north as Porto Velho, Rondonia State, Brazil and eastward from Peru and Bolivia to Cuiaba, Mato Grosso State, Brazil, southward to Santiago del Estero Province of Argentina, through Uruguay to Sao Paulo State, Brazil. Its range in North America includes the Gulf States west to Texas. It is found sporadically in New Mexico and Arizona and apparently is well established in California. It recently has been introduced to Australia, Taiwan, China, and Japan (Henshaw et al. 2005; Ascunce et al. 2011). A comprehensive assessment of genetic variation for colonies sampled from 75 geographic sites worldwide revealed that at least nine separate introductions of S. invicta occurred into newly invaded areas and that the main southern U.S. population is probably the source of these secondary introductions (Ascunce et al. 2011).

Distribution based on Regional Taxon Lists
Australasian Region: Australia. Indo-Australian Region: Hawaii. Nearctic Region: United States. Neotropical Region: Anguilla, Antigua and Barbuda, Argentina, Aruba, Brazil, British Virgin Islands, Cayman Islands, Greater Antilles, Mexico, Montserrat, Netherlands Antilles, Paraguay, Puerto Rico, Saint Martin (French part), Trinidad and Tobago, Turks and Caicos Islands, United States Virgin Islands. Palaearctic Region: China.

Biology
There is an Antwiki webpage with a list of some recent publications about the imported fire ant.

Roeder et al. (2018) in an Oklahoma study found the CTmax (critical thermal maximum) for this species was 49.0 ± 0.4 C. They also found the imported fire ant was able to forage for 18 hours per summer day and occupied baits at a higher total biomass than 4 co-occurring native species.

Buren (1972) - The sting venon constituents of invicta (MacConnell et af. 1971) appear to consist largely of the following alkylated piperidines; trans-2-methyl-6-nundecylpiperidine, trans-2-methyl-6-n-tridecylpiperidine, trans-2-methyl-6-(cis-4-tridecenyl) piperidine, trans-2-methyl-6-n-pentadecylpiperidine, and trans-2- methyl-6- (cil'-6-pentadecenyl) piperidine, of which the last four predominate. These constituents are different from those of richteri, in which the first 3 listed compounds predominate, and the last two are essentially lacking. (Unpublished data of Dr. John Brand and Dr. Murray Blum, Department of Entomology, University of Georgia).

Genetics
Solenopsis invicta 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.

Florida
Deyrup, Davis & Cover (2000): Solenopsis invicta, although a major ecological and economic problem through out Florida, does not seem to live up to its potential as a pest, especially compared to the situation in Texas. Heavy outbreaks of S. invicta in Florida seem to require constant maintenance by humans in the form of perennial habitat disturbance and, on upland sites, frequent irrigation. The sand soils characteristic of most of Florida maybe suboptimal for S. invicta because sand drains quickly and is also permeable to a great variety of burrowing animals, some of which might be enemies of ants. Tschinkel has suggested (1988) that subterranean Solenopsis of the subgenus Diplorhoptrum might reduce populations of S. invicta in sandy uplands. Since S. invicta originates in habitats where the soil is periodically saturated, forcing the fire ants themselves to build elevated nests, it seems logical that this ant would better adapted to defend itself against enemies from above rather than enemies from below.

Nomenclature

 * wagneri. Solenopsis saevissima var. wagneri Santschi, 1916e: 380 (w.) ARGENTINA. Junior synonym of saevissima: Wilson, 1952b: 55. Revived from synonymy, raised to species and senior synonym of invicta: Bolton, 1995b: 391. [Trager, 1991: 173 wrongly gave wagneri as an unavailable name, incorrectly citing its original notation as S. saevissima subsp. electra var. wagneri, and referring its material to invicta. But wagneri is available, with the original notation cited above, and has priority over invicta.] S. invicta conserved over wagneri because of usage, in accord with ICZN (1999): Shattuck, Porter & Wojcik, 1999: 27.
 *  invicta. Solenopsis invicta Buren, 1972: 9, fig. 2 (w.q.m.) BRAZIL. Wheeler, G.C. & Wheeler, J. 1977: 588 (l.). Junior synonym of wagneri: Bolton, 1995b: 388. [Trager, 1991: 173 incorrectly gave wagneri as an unavailable name; the name is available and has priority over invicta, see note under wagneri.] S. invicta conserved over wagneri because of usage, in accord with ICZN (1999): Shattuck, Porter & Wojcik, 1999: 27. See also: Rhoades, 1977: 1; Smith, D.R. 1979: 1386.

Worker
Buren (1972) - Head length .77 to 1.41 mm, about 1.35 to 1.40 in majors; width .65 to 1.43 mm, about 1.39 to 1.42 mm in majors. Scape length .96 to 1.02 mm. in majors. Thoracic length 1.70 to 1.73 mm. in majors.

Head wider behind eyes, with rounded occipital lobes, lobe peaks further from the midline than in Solenopsis richteri, but occipital excision not as creaselike. Scapes in majors failing to reach occipital peaks in full face view by 1 or 2 scape diameters, a more noticeable space than in richteri. Scapes meeting occipital border in medium sized workers, slightly exceeding rear border in small workers. Head with more elliptical sides in medium or small medium workers. Only the small workers have the head slightly wider in front than behind.

Thorax of majors; pronotum without angular shoulders or a sunken posteromedian area. In profile the promesonotum evenly and strongly convex, and the base of the propodeum also usually convex and rounded rather evenly into the declivity; base and declivity in profile about equal in length in very large workers. Promesonotal suture moderately strong to rather weak centrally in large workers.

Petiole with thick, blunt scale; seen from behind the scale is usually not as evenly rounded above as in richteri and may be subtruncate, but this character variable. Postpetiole large and broad, in very large workers much broader than long; seen posterodorsally, sides parallel or nearly so, in very large workers often broader behind than in front; transverse impression on rear dorsal surface present or very feeble, usually noticeably weaker than in richteri.

Sculpture similar to richteri; punctures from which the pilosity arises often shallowly elongate on dorsal and ventral sides of head; sculptured areas on cheeks in frorit of eyes less striate and more irregularly rugose than in richteri. Striae on sides of thorax less deeply etched and with fewer intercalated punctures than in richteri. Mesopleura with anteroventral portion of striate area usually obliterated and nearly smooth and shining in major workers. Petiole punctate on the sides. Postpetiole from above with strong shagreen anteriorly, medially and posteriorly with distinct transverse punctostriae, sides covered with fine, deep punctures, these appearing to be individually smaller but deeper than those in richteri, giving a more opaque appearance to this surface; some punctostriae may be present toward the rear.

Pilosity very similar to that of richteri; erect hairs numerous and of various lengths; some very long hairs always present on each side of pronotum and mesonotum and in longitudinal rows on head; appressed pubescent hairs on anterior face of the petiolar scale moderately numerous, apparently always sparse in richteri.

Gastric spot present only in some of the large workers, never as brightly colored as in richteri, usually occupying a smaller area on first gastric tergite, and with rather indistinct posterior border. Remainder of gaster very dark brown, in some large workers nearly black. Thorax concolorously light reddish brown to darker brown; legs, including coxae, usually of a lighter shade. Head with rather constant color pattern in large workers; occiput and vertex brownish as in the thorax, but the larger portion of head, including front, genae, and central body of the clypeus, yellowish or light yellowish brown; venter of head also usually light yellowish brown. Mandibles and anterior border portions of the genae dark brown or of about the same shade as the occiput; a small dark brown arrowshaped or “rocket”-shaped mark centrally on front. Scapes and funiculi varying from matching the light colored area of the head to the same shade as occiput. In minors and medium sized workers, light colored area of the head restricted to frontal area, with dark arrow or rocket-shaped mark nearly always present. An occasional nest series with colors very much darker than described, large workers without trace of gastric spot and nearly concolorously very dark brown. Even in these specimens, the head similar to or approaching color patterns described above.

Pitts et. al. (2018) - Head subquadrate to weakly cordate. Head of largest specimens cordate. Sculpture of head and mesosomal dorsum with small piligerous foveolae, <0.01 mm in diameter. Median frontal streak present. Median ocellus in largest major workers absent. Mandibular costulae absent medially, distinct apically and basally along outer border. Mesonotum with 20–25 setae. Mesonotum with anteromedian margin in largest major workers gently curved. Mesonotum in lateral view convex. Propodeum sculpture glabrous posteroventral to spiracle. Postpetiole shape in posterior view width greater than height. Postpetiole in posterior view with lower 0.66 or greater transversely rugose to punctate-rugose, extreme dorsum nitid, granulate. Color generally with head and mesosoma yellow red to dark red brown, gaster brown, T1 with maculation yellow red to concolorous with surrounding integument.

Queen
Buren (1972) - Head length 1.27 to 1.29 mm; width 1.32 to 1.33 mm; scapes .95 to .98mm. and thorax 2.60 to 2.63 mm. in length.

Head nearly indistinguishable in shape from richteri, except that the occipital excision is not as crease-like. The scapes appear slightly shorter in general than in richteri. Petiolar scale much as in richteri, very convex above, seen from behind. Postpetiole with rather straight sides, seen from above, the sides never concave, unlike richteri. Sculpture of thorax not appreciably different from richteri, clear space between metapleural striate area and propodeal spiracles absent or reduced to a narrow crease. Sides of petiole punctate. Sides of postpetiole opaque with fine punctures, without much of the irregular roughening seen in richteri; anterior portion of dorsum strongly shagreend; middle and rear portion with distinct, tranverse puncto-striae. Erect hairs present on all surfaces. Anterior faces of petiole and pqstpetiole with dense matts of appressed pubescence, similar pubescent matts usually present on rear surface of propodeum.

Colors similar to worker. Gaster very dark brown. Thorax, legs, and scapes light brown, often with three longitudinal very dark streaks on mesoscutum. Head yellowish or yellowish brown centrally, occiput and mandibles approximately matching thorax. Wing veins very pale brown.

Pitts et. al. (2018) - Head. Slightly broader than long, quadrate, wider dorsal to eyes than ventral to them, sides of head convex from eyes to occipital angles, straight to nearly straight ventral to eyes. Eyes sometimes with 2–10 setae protruding from between ommatidia, setal length ≤ 3X width of ommatidium. Median ocellus large, prominent, circular. Lateral ocelli moderate to large, slightly ovate. Clypeus projecting, carinal teeth stout and sharp, carinae well defined, less so dorsally, slightly divergent ventrally. Paracarinal teeth small, sometimes poorly defined. Median clypeal tooth well developed. Approximately 0.50 of eye dorsal to midpoint of head. Antennal scape in repose surpasses lateral ocellus.

Mesosoma. Parapsidal lines present on posterior 0.50 of disk. Mesonotum with indistinct, median furrow on posterior one-sixth or less. Bidentate median process present on metasternum.Wing venation as in Fig.

Metasoma. Lateral faces of postpetiole slightly concave to wider ventrally. Petiolar and postpetiolar spiracles slightly tuberculate to not tuberculate.

Coloration, Sculpturing, and Pilosity. Piligerous foveolae small, sparse, width <0.01 mm in diameter, larger on head than on thorax and abdomen. Pubescence simple, golden and erect, longer and denser on head than elsewhere, longest on anterior edge of clypeus. Mesosoma with longest pubescence (length ≤ 0.25 mm) 2X longer than shortest pubescence. Mandible with 9–11 fine, distinct, costulae, sometimes costulae obsolescent medially. Propodeum with fine striae throughout. Petiolar nodes with lower 0.75 finely striate; granulate throughout. Postpetiole usually with 12–18 striations, often transverse, other times appearing to create swirling or circular patterns. Remaining integument smooth and polished. Color varies from red brown to brown red on dorsum of head, dorsum of thorax, and katepisternum of mesopleuron. Gaster brown. Sometimes on lighter colored individuals, bases of T1 and S1 are somewhat orange blending to brown apically. Brown maculations sometimes present anteromedially and on parapsidal lines. Median streak present, weak, sometimes indistinct or absent. Internal margins of ocelli often dark brown.

L ~5.9–8.3, HW 1.30–1.46, VW 0.66–0.88, HL 1.18–1.43, EL 0.38–0.49, OD 0.10–0.18, OOD 0.19–0.26, LOW 0.08–0.15, MOW 0.16–0.24, CD 0.15–0.22, MFC 0.15–0.25, EW 0.25–0.48, SL 0.78–1.11, PDL 0.13–0.25, LF1 0.08–0.14, LF2 0.07–0.10, LF3 0.07–0.12, WF1 0.06–0.11, FL 0.94–1.26, FW 0.22–0.36, MW 1.14–1.48, DLM 2.42–2.73, PRH 0.88–1.19, PL 0.72–0.83, PND 0.56–0.84, PH 0.57–0.78, PPL 0.24–0.42, DPW 0.51–0.74, PPW 0.71–0.77, PHB 0.26–0.48, N=25.

Male
Buren (1972) - Not appreciably different from male of richteri; upper border of petiolar scale appearing more strongly concave, seen from behind, but there probably is overlap in this character. As in both species, the spiracles of the petiole and post petiole are strongly projecting.

Concolorous black expect for the whitish antennae. Wing veins colorless to very pale brown.

Pitts et. al. (2018) - Head. Eyes sometimes with 2–10 setae protruding from between ommatidia, setal length ≤ 3X width of ommatidium. Ocelli large and prominent, elliptical.

Mesosoma. Propodeum rounded, declivous face perpendicular, flat except with distinct to indistinct median longitudinal depression, basal face strongly convex transversely and longitudinally. Metapleuron not broad, ~0.33 as wide as high, sometimes with transverse posterior carina. Wing venation as in Fig.

Metasoma. In cephalic view, dorsum of node with shallow to deep median impression and weakly to strongly bilobate. Petiolar and postpetiolar spiracles distinctly tuberculate to not tuberculate.

Coloration, Sculpturing, and Pilosity. Pubescence short, sparse, yellow to brown, erect to suberect (0.20–0.30 mm), longest on gena and vertex. Mesonotal pubescence dense. Propodeum with base striato-granulate, medially finely granulate. Area between eye and insertion of antenna, posterior portion of metapleuron, lateral faces of scutellum, and base of petiolar node granulate to striatogranulate. Posterior surface of postpetiolar node granulate throughout, rugae present dorsomedially. Area between ocelli weakly to coarsely striato-granulate. Gena coarsely rugose to coarsely striato-granulate. Head often completely granulate, shagreened, dull. Remaining integument smooth and polished. Color red brown to black with antenna completely yellow, sometimes scape and pedicel brown. Mandibles brown to light brown.

L ~5.4–6.3, HW 0.91–1.08, VW 0.30–0.40, HL 0.67–0.83, EL 0.37–0.53, OD 0.06–0.11, OOD 0.18–0.26, LOW 0.09–0.18, MOW 0.10–0.17, CD 0.16–0.24, MFC 0.13–0.18, EW 0.28–0.39, SL 0.16–0.20, SW 0.09–0.14, PDL 0.06–0.10, PEW 0.10–0.15, LF1 0.10–0.17, LF2 0.13–0.15, LF3 0.12–0.16, WF1 0.07–0.10, FL 1.00–1.15, FW 0.15–0.20, MW 1.20–1.68, DLM 2.27–2.64, PRH 0.78–1.04, PL 0.62–0.69, PND 0.54–0.61, PH 0.44–0.56, PPL 0.20–0.28, DPW 0.55–0.71, PPW 0.58–0.69, PHB 0.14–0.28, N=25.

Larva
Pitts et. al. (2018) - Fourth instar worker larva.—Head. Large, subpyriform in anterior view (height 0.50 mm, width 0.54 mm). Cranium slightly wider than long. Antenna with 2 or 3 sensilla, each with 1 spinule. Integument of head with minute spinules. Occipital setal row normally with 6–8 bifid setae, base ;0.66X total length of seta, setae 0.08–0.10 mm long. First setal row on vertex with 2 bifid setae, base ;0.66X total length of seta, 0.05–0.07 mm long. Second setal row on vertex with 4–6 simple setae, ;0.10 mm long. Setae anterior to antenna level simple, 0.08–0.14 mm long. Clypeus with transverse row of 4 setae, inner setae shorter than outer setae, 0.06–0.08 mm long. Labrum small, short (width 2.5X length). Labrum with 4–6 minute sensilla and 2 setae on dorsal surface of each half and apex with 4–6 sensilla on each half. Each half of the epipharynx with 2–3 isolated and 2 contiguous sensilla. Straight medial portion of mandible with 2–5 teeth that decrease in size dorsally. Maxilla with apex conical, palpus peg-like with 5 sensilla, each bearing one spinule. Galea conical with 2 apical sensilla bearing spinules. Maxilla with sclerotized band between cardo and stipes. Labium with patches of spinules dorsal to each palpus, in 2–3 rows. Labial palpus slightly elevated with 5 sensilla, each bearing one spinule.

Body. Stout. Spiracles small, first spiracle larger than others. Body setae of 2 types. Simple setae (0.06–0.11 mm long) arranged in transverse row of 6–12 on ventral surface of each thoracic somite and on each of 3 anterior abdominal somites, some with short denticulate tips. Bifid setae (0.06–0.09 mm long) occur elsewhere, base ~0.5X length.

Length. About 3.1 mm.

Fourth instar worker larva (O-18).— Head. Large, subpyriform in anterior view. Cranium slightly broader than long. Antenna with 2 or 3 sensilla, each bearing spinule. Integument of head with minute spinules. Occipital setal row with 4–6 setae (0.06–0.08 mm long), median pair simple, other setae bifid with base 0.66–0.75X total length of seta. First setal row on vertex with 2 simple setae,;0.09 mm long. Second setal row on vertex with 2 simple setae, 0.10–0.12 mm long. Setae ventral to antenna level simple, 0.09–0.12 mm long. Clypeus with transverse row of 4 setae, inner setae shorter than outer setae, 0.05–0.10 mm long. Labrum small, short (width 1.8X length), slightly narrowed medially. Labrum with 5 minute sensilla and 2 setae on anterior surface of each half and ventral border with 6 sensilla on each half. Each half of posterior surface of labrum with 2–3 isolated sensilla. Straight medial portion of mandible with 2–5 teeth that decrease in size basally. Maxilla with apex conical, palpus peg-like with 5 sensilla, each bears one spinule. Galea conical with 2 apical sensilla. Labium with patch of spinules dorsal to each palpus, spinules coarse and isolated or in short rows of 2–3. Labial palpus slightly elevated with 5 sensilla, each bearing one spinule.

Body. Spiracles small, first spiracle larger than others. Body setae of 2 types. Simple setae (0.07–0.12 mm long) arranged in transverse rows of 6–9 on ventral surface of each thoracic somite and on each of 3 anterior abdominal somites, some with short denticulate tips. Bifid setae (0.08–0.12 mm long) occur elsewhere, base ≤ 0.33X length, branches more or less perpendicular to base, tips recurved.

Length. 2.7–2.9 mm.