Imported Fire Ant
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The Main Ant Pages On Wayne's Word: Images Taken With Nikon & Sony Cameras
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Ant Page #3: Fire Ants In Mira Mesa (San Diego County)
   © W.P. Armstrong 4 December 2012     Back To Ant Page #1   
Imported South American Fire Ant (Solenopsis invicta)
Special Thanks To Dr. Tracy Ellis, Entomologist, San Diego County Dept. of Agriculture

Fire Ants (Formicidae: Subfamily Myrmicinae)

The South American fire ant (Solenopsis invicta) has been introduced into southern California. It is very similar in general appearance to our native southern fire ant (S. xyloni), except the head and thorax of the imported fire ant is a little darker. The dirt excavations are much larger and approach the size of gopher mounds! The sting of both species is about the same pain level, and leaves a raised reddish welt, especially in tender areas between the fingers. The sting has a value of 1.2 out of 4+ on the Schmidt Sting Pain Index, just under the sting of the bullhorn acacia ant (Pseudomyrmex ferruginea) of Central America. According to references on Wikipedia, the venom in the sting is mostly piperidine alkaloids including isosolenopsin. These are single nitrogenous rings found in black pepper (Piper nigrum), wild tobaccos (Nicotiana) and poison hemlock (Conium maculatum).

The excavated dirt mound of Solenopsis invicta resembles the loose diggings of a gopher mound in size. The mound in above photo was 5 inches in height. The slightest disturbance to the mound will bring out a swarm of stinging ants including thousands of angry major and minor workers. Excavations of the native southern fire ant (S. xyloni) are much smaller. Lower image shows close-up view of mound with a plastic container placed in the center.

Plastic container showing hundreds of swarming worker ants (mostly majors) that have dropped into bottom.

Close-up views of swarming major and minor workers in plastic container.

Raised welts between your fingers are unavoidable if you handle these aggressive ants.

Major and minor workers of the imported South American fire ant (Solenopsis invicta) from Mira Mesa in San Diego County, California. The head and thorax are a little darker than the native southern fire ant (S. xyloni). There are other technical differences beyond the scope of this article.

Winged Fire Ant Royalty Deep In The Mound Nest

A winged female (queen) guarded by swarming major workers.

A winged haploid male guarded by swarming major workers. His only purpose is to mate with a queen on her mating flight. Like other males in the insect world, his seemingly "easy life" is very brief and comes to a rather abrupt end. See following link if you don't believe me!

  See The Wayne's Word Article About Sexual Suicide  

Possibly The Native Fire Ant Along A Nearby Sidewalk?

Disclaimer: The following images and nests greatly resemble the closely related southern fire ant (Solenopsis xyloni). I found them both in Hourglass Park in the City of Mira Mesa. The following ants have a much smaller nesting mound and appear brighter (lighter) red. In addition, they appeared to clash with imported fire ants in a container by fighting and gaster wagging. I doubt if this behavior would occur if they were all from one supercolony. If I am wrong I will certainly stand corrected.

Small mounding (nest) along sidewalk (black arrow) with average screwdriver as size relationship. Right: Swarming (angry) major & minor workers in lid of plastic container.

Swarming (angry) major & minor workers in lid of plastic container. If these are indeed the native southern fire ant, their sting is just as potent as the imported South American species in my humble opinion.

Will The Invasive Imported Fire Ant Go To War With The Argentine Ant?

Aggressive Argentine ants have definitely eliminated other species of native ants in southern California, including the southern fire ant (Solenopsis xyloni). Will this be the destiny of imported fire ants (S. invicta) that also form massive, multi-queen supercolonies? Quite frankly, I doubt if the fire ants could overtake the well-established Argentine ant supercolony that extends from coastal southern California to San Francisco. I placed a bowl of southern and imported fire ant workers (all sterile) on my sidewalk on a cool December night. Within an hour they were attacked by literally thousands of Argentine ants pouring out of my foundation and from under stepping stones. This actually surprised me because I didn't see any patrolling Argentine ants or ant trails when a set the dish down. I actually felt sorry for the fire ants, but I must conclude from this brief unscientific experiment that they could not win an ant war with Argentine ants, at least not in my yard.

I have no great love for Argentine ants, although I do admire them. They are a serious pest that tends aphids and scale insects, kills off the native harvester ants that our coast horned lizard depends upon, and destroys baby birds in their nest. On a recent trip to Maui I discovered that Argentine ants in Haleakala National Park are threatening the native pollinators of the endemic, rare and endangered silver sword (Argyroxiphium sandwicense ssp. macrocephalum). On a positive side, I have observed them attack rottenwood termites (Hodotermitidae) or drywood termites (Kalotermitidae) in an old picnic table; however, I have no direct proof that they will attack subterranean termites (Rhinotermitidae). I do know that they are very fond of soft-bodied insect larvae and eggs. For this reason, and only this reason, I allow them to live under the foundation of my home and inside my walls; not that I could do anything about it anyway!. Regarding their fondness for insect larvae and eggs, I have observed them attack the nests of paper wasps, driving the much larger wasps away. I also watched them devour hundreds of blow fly eggs and larvae on the putrid blossoms of my starfish flowers (Stapelia gigantea).

Although a constant nuisance in warm weather, Argentine ants are relatively easy to outsmart and control in my house without poisoning myself with excessive use of toxic biocides. Please see the following article about Argentine ants.

  The Wayne's Word Article About Argentine Ants  
The Wayne's Word Article About Termites
Starfish Flowers That Smell Like Carrion

Most Argentine ant images on my web pages were taken on a light box with backlighting. This explains the lighter
(transparent), yellowish-brown legs and body. Without backlighting the ants are darker as in following 2 images.

Argentine ant trail on my backyard patio and on a hard-boiled egg yolk.

According to Mark W. Moffett (2011), battles among ants can be strikingly similar to human military operations. See his article in Scientific American (January, 2011 pp. 84-89) entitled "Ants and the Art of War." In some polymorphic species, the smaller, minor workers are sent to the front lines like expendable cannon fodder. Their job is to hold the legs of larger enemy ants until the larger "soldiers" arrive to decapitate the enemy with their powerful mandibles. Since the soldier caste represent a much larger investment of energy and materials for the colony, their position behind the front lines is less hazardous. Suicide bomber ants rupture their own bodies to spray a toxic yellow glue on the enemy ants, killing themselves and the enemy instantly. Moffett also mentions Dorymyrmex bicolor of Arizona that drops small stones onto the heads of enemies! Dorymyrmex is also known to plug the entrance holes of rival colonies with small stones.

Resistance is futile when fighting Argentine ants. In the above two images, an unfortunate fire ant major worker is hopelessly overwhelmed by three Argentine ant workers. The fire ant was still alive, but greatly weakened and near death.

The Winter Ant
(Prenolepis imparis)

Remarkable Native California Ant
With A Potent Chemical Defence
Against Invading Argentine Ants

Worker ants of Prenolepis imparis may be able to survive the attacks of Argentine ants. I first noticed these shiny black ants on 3 December 2012 near Palomar College, hence the common name of "winter ant." The thorax (mesosoma or alitrunk) is deeply constricted in center and resembles the shape of an hourglass when viewed from above. These ants are slightly larger than Argentine ants. According to Trevor R. Sorrells, et al., Department of Biology, Stanford University (2011), winter ants secrete a hydrocarbon mixture that is lethal to Argentine ants, thus providing an effective defense against this invasive species from South America. "Prenolepis workers are more likely to deploy their chemical defense in encounters near their nest than in encounters while foraging on trees. This indicates that P. imparis modifies its behavior according to the value of the resource it is defending. This may be because the production of the secretion is metabolically costly." Please refer to following link in peer-reviewed journal PLOS ONE Volume 6 (4) 2011.

The North American winter ant looks practically identical to a sister species in Europe (P. nitens) that resembles an ancestral species (P. henschei) that flourished in Baltic amber forests of northern Europe 44 million years ago.

William Morton Wheeler. 1930. "The Ant Prenolepis imparis Say."
Annals of the Entomological Society of America. Vol. 23 (1) 26 p.

Winter ant workers (Prenolepis imparis) feeding on a sunflower seed along steep trail to Owens Peak.

According to Univ. of Florida "Featured Creatures" (Winter Ant Prenolepis imparis) Entomology & Nematology UF/IFAS: Young adult workers emerge from pupal stage in autumn and spend the next 10 months inside the colony, while older workers forage outside the nest and return with liquid food. The foragers pass their collective food reserves via mouth-to-mouth regurgitation to the young workers in a process called trophallaxis. Young workers have softer, more flexible cuticles and their gasters can swell and distend to accommodate large amounts of liquid food. This is similar to repletes in the genus Mymecocystus (honeypots or honey ants) that store liquid carbohydrates. However, Tschinkel (1987) described them as “corpulents” when he discovered that the distention of their gasters was actually caused by the storage of lipids in hypertrophied fat bodies. Corpulents store more than twice their body weight in fats and nutrients. Unlike true honeypot ant repletes, the corpulent (obese) state seen in Prenolepis imparis is reversible; once a corpulent worker has exhausted its food stores it becomes a forager during the next active season. This indicates that workers may live for up to two years, serving as a corpulent for the first and a forager during the second. In fact, winter ants are sometimes called "false honey ants" because their food storage corpulents are similar to Mymecocystus repletes.

W.R. Tschinkel. 1987. "Seasonal Life History and Nest Architecture of a Winter-Active Ant
Prenolepis imparis." Insectes Sociaux: Volume 34, n ~ 3, pp. 143-164. 9 Masson, Paris.

From the Bakersfied Area Skeptic Society: A honeypot ant (Myrmecocystus) "replete" engorged with nectar. Like a living larder, It remains deep in the nest and supplies other workers with liquid food during times of drought. Running fast is no longer an option for this ant! This tasty, sweet morsel is highly prized by aborigines in several countries where honeypot ants live.

Winter ant workers (Prenolepis imparis) feeding on the stalk of a mushroom (Russula ?).

According to "Controlling Fire Ants Takes a Group Effort" published in the July 2009 issue of Agricultural Research Magazine: "In a battle with the Argentine ant, Linepithema humile, the fire ant won 80 percent of the time." Other on-line articles (without references) say that well-established Argentine ant supercolonies in the southwestern United States have the advantage. Argentine ants have spread to many other countries of the world by hitching rides in human cargo. According to Mark Moffett, the biggest of these supercolonies ranges from San Francisco to the Mexican border and may contain a trillion individuals. Each month millions of Argentine ants die along battlefronts that extend for miles around San Diego, where clashes occur with other ant colonies in wars that may have been going on since the species arrived in the state a century ago. The Lancaster Square Law of military combat, where small-sized competitors use their numerical advantage to overcome large-size competitors in simultaneous combat, certainly applies to Argentine ants.

Regarding invasions of South American fire ants into Argentine ant territory in southern California, only time will tell. Perhaps stringent control methods against imported fire ants will tip the balance in favor of Argentine ants. The bottom line here is that both invasive species are ecological disasters in our area.

Ant Colonies Compared With The "Borg" Of Star Trek

I have compared Argentine ants with the "Borg" of Star Trek, a fictional cybernetic race controlled by drones. This is probably not an accurate comparison because the Borg apparently have centralized control by the "Borg Queen." Ant colonies exhibit "swarm intelligence" with the collective behavior of a decentralized system. The individual units (workers & soldiers) are sterile females that operate without a power hierarchy or permanent leader. The queen's role is basically an egg machine to reproduce and perpetuate the colony. The individual worker ants are like cells of an organism, and they are able to communicate with each other by the release of pheromones. This social unity is evident in extreme warfare between different colonies, and explains how colonies recognize "self" from "alien" ants. Recognition between cells of an organism is also controlled by chemicals.

Cells are recognized as "self" by our immune system because of chemicals imbedded in the cell membranes. Some of these are special proteins (glycoproteins) with unique carbohydrate antennae. Just as foreign bacteria and viruses are recognized as "enemies" by our immune system, ants recognize rival colonies by different pheromones. The poison oak rash is actually a complex cell-mediated immune response where special white blood cells (T-lymphocyes) attack and destroy the poison oak allergen (urushiol) bound to cell membranes deep in the skin epidermis. These "killer" T-cells are programed to recognize urushiol with "urushiol receptor sites" on their membranes. See the following article for more information.

  The Wayne's Word Article About Poison Oak  

Simplified diagram of cell membrane structure. This model shows large protein (glycoprotein) molecules embedded in the membrane. Membrane proteins may serve as carriers in which molecules and ions pass through channels in the protein. Glycoproteins may also be associated with cell recognition in which patrolling T-cells and antibodies recognize the shape of membrane proteins as "self" or "foreign." These membrane proteins often contain unique carbohydrate chains (antennae) which are involved in the cell recognition process.

2 Ant Species Living In Argentine Ant Territory In Twin Oaks Valley, San Marcos

On a recent (May 2016) walk along a bridle path in San Marcos, CA I noticed thriving nests of 2 different ant species in close proximity to Argentine ant trails from nearby residences. Perhaps the bridle path was too dry for an Argentine ant invasion.Or maybe it is the $500 per day fine proposed by the local Vallecitos Water District for overwatering!


  1. Evans, A.V. 2007. Field Guide to Insects and Spiders of North America. Sterling Publishing Co., Inc. New York, NY.    

  2. Hogue, C.L. 1993. Insects of the Los Angeles Basin. Natural History Museum of Los Angeles County.

  3. Hölldobler, B. and E.O. Wilson. 1990. The Ants. Harvard University Press, Cambridge, Massachusetts.

  4. Moffett, M.W. 2011. "Ants and the Art of War." Scientific American (January, 2011 pp. 84-89).

  5. Sorrells, Trevor R. 2011. "Chemical Defense by the Native Winter Ant (Prenolepis imparis) Against the Invasive
    Argentine Ant (Linepithema humile." PLOS ONE 6 (4): e18717. doi: 10.1371/journal pone.0018717

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