Friday, July 1, 2011

Distribution and Identification of Solenopsis

Opening Statement: This is a home made key and work in progress intended to help simplify identification of North American Fire and Thief ants, Solenopsis. This genus in North America is represented by 3 subgenera currently recognized as informal species groups, the fire ants (former subgenus Solenopsis), the globularia group (former subgenus Euophthalma), and the thief ants, or fugax group (former subgenus Diplorhoptrum). This genus is easily one of the more confusing ones I've done. The genus should really be renamed Gemini or something along those lines. The Fugax group (thief ants) needs some serious revisionary work done too! So many of the original studies describe species in ways that don't match up with pictures on Ant Web. More than once I've found a picture of a brown workers for species described as being solid yellow.

Also, while I love to make these keys so that a microscope isn't necessary, the majority of ants in this genus are 2mm long making them near imposable to identify to species level without one. The better magnification you can get the better!

Fire Ants Workers are polymorphic and armed with a powerful sting that will cause a welt to form on human skin. Often times the minors and media will look identical to those of other species. You Will Need to Look At a Major Worker to Identify the Species! A queen can be a close second but even these can blur the line a little. This group contains two complexes. They are the Saevissima and Geminata complexes. Geminata are native to North America while Saevissima are not. All Saevissima species have a pointed media clypeal tooth, while all Geminata species are either lacking this tooth or it is dull and rounded. 

Useful Anatomy: The media clypeal tooth is a rather small spine or "tooth" that occurs at the bottom of the clypeus. When viewing the head from the front the "teeth" are visible on the clypeus. All Solenopsis have at least two, even if they're reduced down to barely visible nubs. The media clypeal tooth, if present falls in between and often has a hair protruding out the tip or towards the pointed area.

Saevissima Complex:
Solenopsis saevissima is only found is South America. However two of it's members of it's group have been imported.

The Red Imported Fire Ant
Solenopsis invicta (2) SW, SC, SE
Nests underground, in grassland and savanna type areas where the moisture level is moderate to wet, and never dry. They're also adept in nesting on flood plains, where, when the wet season comes, all members of the colony link together and form a living raft. When living rafts float into one another, rather than fight the colony simply combines. This behavior would suggest the polygynous form of this species is dominant in flood plains. Mounds are not always made but when present are usually in a dome shape to absorb the sun's rays. In the heat of summer the colony may retreat down into the earth where it's still relatively moist. Workers are very polymorphic ranging from about 2.5mm to 5.5mm long. They have a media clypeal tooth too, which comes to a point. The most common color pattern is bicolored with the body either yellow-red to dark brown and the gaster ranging brown to blackish brown.

This ant was likely imported with cattle from Argentina or Paraguay. Populations there are similar in color, venom chemistry, and even allele frequencies (DNA stuff). When it comes to number of queens per colony this species has both a polygynous and monogynous form. Monogynous colonies produce larger queens who are more able to fly from the nest and start their own colonies, no other queens are added from this point on. Polygynous colonies have to many queens to feed as it is and typically produce smaller, weaker queens that mate and land close by the nest where they are typically reintegrated into the colony. The Polygynous form spreads by colony division. Colonies of both forms often spread around with sod and nursery stock. In arid areas it can survive around suburban homes where lawns and landscaping are being maintained through supplemental watering. Isolated populations in towns surrounded by dry arid land these populations are easily eliminated with pesticide. This species has been known to hybridize with Solenopsis richteri.

The Black Imported Fire Ant
Solenopsis richteri (2) SE
Nests underground, in grassland and savanna type areas where the moisture level is moderate to wet, and never dry. This species is very similar to S. invicta but the media clypeal tooth, is short but pointed. (The specimen on AntWeb is simply not positioned to see this.) The pronotum is also slightly more bulky than S. invicta in the shoulder area but this is only noticeable in the majors. Color does not vary as much as it does in S. invicta either. This species leans towards darker colorations only with the exception of a light coffee brown area on the gaster but that is not consistent. This species has been known to hybridize with S. invicta.

Solenopsis invicta x richteri (2) SE
As this is a hybrid between two species, expect to find traits from both mashed into one species. While S. invicta does produce polygynous colonies this form does not integrate into them.

Geminata Complex
Solenopsis geminata (2) SC, SE, Mexico, CR, PR
Nest underground in sandy soil, especially along rivers and beaches, but not restricted to these locations! Workers are polymorphic with more separate division between castes than S. invicta. They range from 3.5mm to 7mm long. Majors have an enlarged head more typical of granivores and sure enough they do mill seeds. Foraging is done both day and night. The media clypeal tooth is completely lacking in this species. The petiole is also wider compared to S. invicta. Typically body texture is shiny if not a little towards the dull side, with lots of small dimples dotting the body. There are also numerous long erect hairs all over the body. Color varies dramatically in this species! Colony uniforms range from the body being all red with rear part of the gaster black in color, to the body being a brownish black and only the mandibles and appendages lighter in color, typically reddish brown. The darker forms tend to be more tropical and prefer high humidity microclimates, (poor drainage, damp perhaps forested and near water,) while the lighter brown and red varieties can handle a wider range of conditions. While colonies tend to only be one color form a few colonies have been found that display the full range of traits and everything in between. In Texas they tend towards being shinier than average, perhaps due to hybridization with S. xyloni. Hybrid majors measure between that of S. giminata and S. xyloni in length.

Solenopsis xyloni (2) SW, SC (Texas), SE (but possibly extinct in this region) 
Nest underground in open sunny places. This species measures 2mm to 5mm in length. Compared to S. geminata the majors's head and petiola are smaller. The head shape of this species is similar across it's size range to S. invicta's. They're not as hairy either but this can be harder to notice. Color tends to be more uniform in a single colony than S. geminata too; with majors, minors, and queens all tending to share the same colors. Color once again is highly variable. The body can be orange-red with the rear of the gaster a dark brown. This form seems most common around Texas, Arizona, and Mexico where it nests in deserts. There's also the brownish black body with brownish red on mandibles and appendages.

Solenopsis aurea SW, SC
Nests underground at elevations ranging from below sea level to almost 2000m. This species supposidly has a narrower head but I wasn't able to find anything for a good comparison on the internet. Color can be reddish yellow with the rear of the gaster brown to dark brown. I believe this can range to a solid yellow tone too.  

Solenopsis amblychila (2) SW
Nests underground at elevations ranging between 1500m to 2500m. Distribution is believed to be divided between S. aurea where their range would overlap. Minor and media workers are indistinguishable from those of S. aurea. A major worker is absolutely needed for an identification. In face view some majors heads' will appear more cordate (heart shaped). The body is shinier and has less hair than S. xyloni and S. aurea. This ant has a greatly reduced media clypeal tooth where it's either nonexistent or a barely visible nub. Queens are said to have larger head margins than those of S. aurea.


Globularia group these are said to be the intermediate species that have some degree of polymorphism, however feeble it may be. Eyes are larger in comparison to the head than in members of the Fugax group. This is all well and good but the only member of this group that occurs in North America is also the one noted exception to these rules. Solenopsis globularia is still different enough though that it deserves it's own group.

Solenopsis globularia (2) SC, SE, Islands of the Bahamas, Caribbean sea, and Gulf of Mexico
Solenopsis globularia littoralis

This ant is considered native to the US but has a preference of nesting in rotted out wood in wet and often muddy conditions in swamps, along rivers, and beaches. I would theorize it's this nesting habit that has spread it about it's island and coastal range. It's easy to imagine a high tide taking a piece of wood out to sea containing one or more colonies and perhaps washing ashore somewhere else. Both are easily identified by having a huge post petiole in comparison to the gaster when viewed from above. Despite the species and the variety listed above, Dr. Ant tells me the two are indistinguishable and I trust his judgment. Solenopsis globularia should be the name given to any ant thought to be either, at least in the US. However, I've read the most common color pattern is that of S. globularia littoralis but it doesn't differ so much from that of S. globularia itself.


Thief Ant group all members of this group have a monomorphic worker force and the eyes are rather tiny in comparison to the head. The post petiole is little or not at all wider than the petiole. These ants are called thief ants because of their habit of combining nests with larger species. These are not parasitic ants however, in fact all but one species I've put in this group is fully capable of sustaining a colony without a host. It's more likely their small stature makes them overlooked by the larger ants and they are free to roam and feed on whatever unguarded brood and food stores they can have access to.

This group was formerly named subgenus Diplorhoptrum, but the validity of this as a monophyletic subgenus has been challenged by both Bolton and Trager. A lot of work still needs to be done on this group especially. Numerous species are described as one color form only to have images show up of another.

Solenopsis molesta (2) SW, SC, SE, NW, NC, NE
Nests underground with the entrance tunnel far away from the nests itself. There is no mound to speak of either, and in fact are barely noticeable besides nuptial flight season in the summer. As with most Solenopsis species, the reaction to rainfall is usually to head for higher ground. I've seen whole colonies evacuate into tunnels left over from earth worms, in wood chips, and on one occasion, into the hallow cavity of a log, but underground is the standard for this species. Workers are 2mm with Queens being 5mm long. They are covered in a fair amount of long erect hairs which is unevenly distributed and with less of it on the mesosoma. Given this ant's size though these are really hard to see without good magnification. Color ranges from pale yellow to orange-red to a chestnut-like brown tone, and always with darker patches on the body, especially the head and gaster area. Regardless of color, queens always have a darker head.

This ant is polygynous and queens will get along from the start. I recommend groups no bigger than 4 queens though, anything bigger and none of them seem to take care of the eggs. Nuptial flights take place in the afternoon and mating is as low as eye level. Something else I noticed, queens typically dig holes that are much larger than their needs. I came across a area in a parking lot where lots of sand and gravel had collected and there were dozens of these queens all digging nests. Despite the fact this ant is thin enough to just push their way through the soil they all dug tunnels large enough for a Lasius queen to fit in. Perhaps they're laying the ground work to encourage a queen to a host species? Despite this observation I know from experience queens of this species are fully capable of starting colonies all on their own and do so regularly. That said, combine nests in the wild have been observed with the following genera: Camponotus, Formica, Lasius, Stenamma, Myrmica, Pachycondyla, Odontomachus, Linepithema(?), Crematogaster, Ponera, and Pheidole. In captivity however this doesn't always work out. I read about a Camponotus colony that couldn't have cared less about them, unlike a Crematogaster colony that lead a campaign to irradiate every last S. molesta from their enclosure.

Solenopsis carolinensis (2) SE
Similar to S. molesta but I believe this ant is slightly smaller, closer to 1mm long. Dr. Ant says this species probably replaces S. molesta in the deep south.

Solenopsis validiuscula SW (Nevada)
This one I'm completely confused on. It's described as both being it's own species as well as the darker brown variety of S. molesta itself??? What? The darker form is supposed to be larger than S. molesta while the lighter form is 2mm long. This is described as a variety of S. molesta. Assuming the image on Ant Web is accurate this ant lacks the darker areas on S. molesta and the gaster and head are slightly larger. (And depending on who you believe this species might just be a synonym with S. truncorum.)

Solenopsis truncorum SW, SC, SE
Out west it's been found in mid-elevation mountain locations nesting under rocks. To the east it's been, "sifted out of material from tortoise burrows in the sand 'hills' of Florida," - Dr. Ant. So it nests underground in a variety of conditions. This species is described as being chestnut or reddish-brown in color, just as the darker form of S. molesta but with a paler mesosoma. Queens are a dark brown color.

Solenopsis texana (2) SC, SE, NE
This species replaces Solenopsis molesta in southern Texas. Workers measure 1mm to 1.2mm with queens about 3.5 to 3.8mm long. They're described as being paler than S. molesta but the color patterns match up. Both have an orange to brown color form. Both have queens with darker heads and gasters, though sometimes not equally so.

Solenopsis texana catalinae SW (California)
Taxonomic status of this ant is vary unclear. Besides being yellow in color, and apparently only found on an island off the coast of California, I'm not sure how this ant differs from S. texana.

Solenopsis nickersoni SE (Florida)
Though widely distributed in Florida this species is uncommon and believed to be completely subterranean. This is the only brown colored species in this group that occurs in Florida. The legs and antenna are always a pale yellow-tan color. Workers measure about 1.2mm long. Queens are reddish brown and described as being larger than those of S. abdita, S. picta, and S. tonsa but no measurements are given for any of those species in general. Alates don't appear to be attracted to lights.

Solenopsis abdita SE (Florida)
Nests underground and in rotten wood, with a preference for palm, typically in a forest edge habitat with moist conditions. Similar to S. carolinensis, but shorter, less shiny, with smaller, rounder, darker eyes. Color is pale yellow-orange shading towards reddish orange on the gaster. Nuptial flights occur in June. This species is polygynous, and reproductives will mate within the colony.

Solenopsis picta (2) SC, SE
Nests in trees. Legs, antenna, and mesosoma are a deep yellowish brown color with the head and gaster a dark brown to blackish tone. Because of this pattern and life style choice it's hard to mistake this ant for any other.

Solenopsis corticalis (2) SE (Florida), West Indies
Not as wide spread as S. picta and restricted to habitats containing Red Mangrove, Rhizophora mangle, which is a neat tree. Presumably they nest inside the tree somewhere but this isn't mentioned. Hairs are erect and rather sparse compared to other species in this group. Hairs also don't arise from punctures on the body. All castes are yellow to light brownish yellow in color.

Solenopsis pergandei (2) (3) SE (Florida)
Nests underground in the dry sandy soil areas of oak-pine savannas (savanna = partial sun or open woodland). Dr. Ant writes, "It is among the largest of the small Solenopsis in our fauna, has a broad and almost square head, is very pale, almost cream colored, and is quite punctate, giving it a much duller appearance than others. S. tonsa is also dull like this, but smaller and with a narrower head, and S. tennesseensis is smaller and narrower, still." Alex wild has a few excelent pictures of the bright orange queens to this species (3)

Solenopsis tonsa SE (Florida)
Similar to S. pergandei and S. tennesseensis but the short grayish hairs on thorax and head are dense and uniformly short falling into neat rows. They are especially bristle-like on the head and mesosoma. Hair elsewhere on the body isn't as dense or uniform looking. In face view workers have a bald streak going down the center of the head. Alates are not attracted to lights.

Solenopsis tennesseensis SW, SE
The post petiole is very much round looking when viewed from above. Hair is very sparse and typically short except for the tip of the gaster, near the mandibles on the head, and on the antenna.

Solenopsis subterranea (2) SC
Nests are completely subterranean. Head and mesosoma are a light yellow brown color with the gaster and legs darker. Differs from S. tennesseensis in post petiole isn't circular. Hairs cover the body and are typically short except for the tip of the gaster, near the mandibles on the head, and on the antenna.

Solenopsis puncticeps (2) SC (Texas)
The head is somewhat more rounded than wedge shaped. Post petiole is nicely rounded when viewed from above. I would love to add more but this species doesn't appear to have been collected very much or described against other species very well. They're believed to be completely subterranean but this could be untrue.

Solenopsis salina SW (California)
Nest underground under stones, in grasslands, typically but not always encrusted with saline (salt) deposits from hot spring water. Hence the name, S. salina. The entire body is a deep yellow color. Body hair is numberous and erect.

Solenopsis krockowi (2) SW (Arizona)
Workers range from 2 to 2.5mm long, and queens are around 7mm long. The head is somewhat more rounded in shape. Antenna scape are slender. Hairs are all over the body and erect. Queens are described as a rich chestnut brown color.

Solenopsis phoretica (2) SE (Florida)
This is a socially parasitic species, that seems to have only ever been collected once. This specimen was found perched on top a Pheidole dentata queen within the host colony. This is similar to other inquiline relationship. There were no workers belonging to this species found within the host colony, which is to assume this ant has a worker caste at all. Males and method of infiltration of host colonies is unknown. 

Solenopsis papuana (2) HA, Polynesia
Found in forests, but sources are unclear weather they prefer wood or soil. They've also been imported around tropical parts of the world largely from coconut and taro plants. Color ranges from yellow to brown. Antenna and legs are much hairier than the body. The few hairs that do occur on the body are long and erect.

Solenopsis pilosula Mexico
The head is unusually small and square for Solenopsis in general. It's described as a deep yellow color but online images show this ranges at least to an orange color. Workers are 2mm to 2.7mm.

Also see "Ants of the Southereastern United States."


Sources
Thompson, C. R. 1989. The thief ants, Solenopsis molesta group, of Florida (Hymenoptera: Formicidae). Florida Entomologist 72: 268-283.
Thompson, C. R. 1982. A new Solenopsis (Diplorhoptrum) species from Florida (Hym.: Formicidae). Journal of the Kansas Entomological Society 55: 485-488.

MacKay, W.P. and Vinson, S.B. 1989. Two New Ants of the Genus Solenopsis (Diplorphoptrum) from Eastern Texas (Hymenoptera: Formicidae). Systematic Entomology Laboratory, USDA, pp 175-178.

Creighton, W. S. 1930. The New World species of the genus Solenopsis (Hymenop. Formicidae). Proceedings of the American Academy of Arts & Sciences 66: 39-151

Wheeler, W. M. 1908. The ants of Texas, New Mexico and Arizona. (Part 1.). Bulletin of the American Museum of Natural History 24: 399-485.

Trager, J. C. 1991. A Revision of the Fire Ants Solenopsis Geminata Group (Hymenoptera: Formicidae: Myrmicinae). Journal of the New York Entomological Society, Volume 99(2)

Fun Reading
Are red imported fire ants facilitators of native seed dispersal?

A Preliminary Report on the Life Economy of Solenopsis Molesta Say by J. W. McColloch and WM. P. Hayes, Assistant Entomologists, Kansas State Agricultural Experiment Station.

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