Solenopsis invicta Buren

Solenopsis invicta worker face
Solenopsis invicta, full face view of major worker (click image to enlarge).
Solenopsis invicta worker profile
Solenopsis invicta, profile view of major worker (click image to enlarge).
Solenopsis invicta queen head
Solenopsis invicta, full face view of queen (click image to enlarge).
Solenopsis invicta side view of queen
Solenopsis invicta, profile view of queen (click image to enlarge).
Solenopsis invicta profile view of male
Solenopsis invicta, profile view of male (click image to enlarge).
Solenopsis invicta profile view of worker
Solenopsis invicta, profile view of major worker (click image to enlarge).

Solenopsis invicta pen drawing by Joe MacGown

Solenopsis invicta, profile view of worker,
pen & ink drawing
(click image to enlarge).

Solenopsis invicta colored drawing by Joe MacGown

Solenopsis invicta, profile view of worker,
pen & ink drawing colored with Adobe Photoshop
(click image to enlarge).

The genus Solenopsis includes both the "fire ants", known for their aggressive nature and potent sting, and the minute "thief ants", many of which are lestobiotic subterranaen or arboreal species that are rarely collected. Many species may be polygynous.

Solenopsis invicta, the scourge of the South, commonly referred to as the red imported fire ant (RIFA), has been the bane of many of man in the deep south. These ants are more than a nuisance with their large mounds that dot the landscape, but instead are agressive antagonists in a pitched battle for dominion of the open landscapes. Their stings are quite potent individually, but they attack enmasse, and it is the lucky person who escapes with only one sting! The red imported fire ant is a major agricultural and urban pest throughout the southeastern states that also causes both medical and environmental harm resulting in a cost of many millions of dollars per year for southeastern states.

Generic level identification of Solenopsis is relatively straight forward, although sizes are greatly variable ranging from approximately 1.0 mm to over 4.0 mm. The genus can be basically characterized by the following: mandible with four teeth (usually), bicarinate clypeus with 0-5 teeth, median part of clypeus with a pair of longitudinal carinae medially or at lateral edges, 10-segmented antennae that terminates in a distinctive 2-segmented club, overall shiny appearance and general lack of or reduced sculpture (when present usually restricted to rugulae or striae on the head, alitrunk, petiole, and postpetiole), lack of propodeal spines or other protuberances on the alitrunk, well developed petiole and postpetiole, and a well developed sting. Workers are either polymorphic (especially in the fire ant group) or monomorphic (especially thief ants).

Hybridization is not uncommon among the larger fire ant group, which can make identification of some species difficult. Identification of thief ants is perhaps even more challenging due to their minute size, similar appearance of workers of one species to another, taxonomic problems, and lack of knowledge of all castes.

Solenopsis invicta can generally be recognized by their large mounds, polymorphic castes (varying sizes of workers), 10 segmented antennae ending in a 2 segmented club, lack of spines on propodeum, reddish color, median tooth on the anterior border of the clypeus, lack of humeral processes (as found in S. richteri), and lack of a large reddish orange colored spot of the first segment of the gaster (as seen S. richteri). However, because S. invicta hybridizes with S. richteri, it can be a challenge to differentiate them from the hybrid, which may have characters of both species. The most reliable method for identification of this group is a cuticular hydrocarbon test, which some labs are now equipped to do.

Biology and Economic Importance
The red imported fire ant, is thought to have been introduced into the U.S. through either Mobile, Alabama or Pensacola, Florida, from Brazil sometime between 1933 and 1945. The red impored fire ant together with Solenopsis richteri, the black imported fire ant, which was introduced sometime near 1918, have wreaked havoc on the economy of the South. As if these two species were not bad enough, they both can mate with one another producing a hybrid, which is as bad or worse than either the black or red fire ant. Two native fire ants, S. geminata and S. xyloni, have not been collected in either MS or AL in many years and it is thought that the two imported fire ants and their hybrid have out-competed them for resources and effectively driven them out from this area.

The native range of S. invicta is Brazil. In the United States, it has been reported from AL, AR, CA, FL, GA, LA, MS, NM, NC, OK, SC, TN, TX, and VA. Solenopsis invicta is found in the southern halves of both AL and MS and also found in the western portion of MS following the Mississippi River northward, whereas S. richteri tends to be found in the northeastern part of MS and northwestern portions of AL, with the hybrid found in a band between the two populations. This is not a static situation and S. invicta appears to be on a continuous path northward, bounded only by temperature restraints (which it appears to be overcoming). As S. invicta moves northward, so also do the populations of S. richteri and their hybrid.

Literature Cited
Global Invasive Species Database. 2009. Solenopsis invicta. Available online at: Accessed 25 February 2009.

Imported Fire Ant Links:
Extension Fire Ant Site-
Texas A & M Fire Ant Site-
The Imported Fire Ant and Its Control -
Imported Fire Ants in Tennessee -
Imported Fire Ant and Household Insects (IFAHI) research site-
LSU red imported fire ant research-
Control of the Red Imported Fire Ant -
List of Fire Ant Web Page Links -