Ant of the Week: Southern Fire Ant

The first thing you notice about workers of the Southern fire ant, Solenopsis xyloni, is what fierce foragers they are.

You never see just one foraging worker. Instead, there’s almost always a teeming mass.

Even when they are collecting sweets at extrafloral nectaries, Southern fire ants show up in greater numbers than most other species.

Southern fire ants are thought to be originally from throughout the southern and western United States. They have been displaced in many areas by the imported fire ant, Solenopsis invicta, but still occur widely in the dry areas of Arizona and California.


Solenopsis ants are relatively easy to tell from other ants because of the antennae have ten segments with a two-segmented swollen area or  “club” at the end.

Solenopsis xyloni workers vary considerably in size and color, even within colonies. The larger workers tend to have lighter-colored heads and trunks than the smaller workers. In the area around Phoenix, Arizona, the Southern fire ants seem darker than those found elsewhere.

(These workers are feeding on their favorite meal of dried cat food.)

Where Solenopsis xyloni and S. invicta overlap it is difficult to distinguish the two species. Jacobson et. al. (2006) have developed a pcr technique and guidelines for identification (see references).

To make things even more confusing, it seems that Solenopsis xyloni hybridizes with Solenopsis geminata where the two overlap.  Obviously this group is “interesting” from a taxonomic standpoint.

The foraging workers are often seen carrying bits of hard food or arthropod parts back to the nest. They also gather some seeds.

Wet food, like this watermelon, go straight to the crop.

Often the foraging trails around their nests are underground or partially covered, so you might not notice them until you dig into the soil or pull up a weed. Then they come boiling up seemingly out of nowhere.

When foragers cross a man-made structure, such as a walking trail, sidewalk, or tile floor, they form a dense foraging trail of numerous workers traveling in both directions.

You have to admire the ability of Southern fire ants to find, process and transport food very rapidly. Plus they seem to eat just about anything they encounter. It is no wonder the colonies can grow to a relatively large size.

Do you have fire ants where you live? Have you ever watched them gather food?


Solenopsis xyloni by Dale Ward

The Navajo Ant Project has a brief review of taxonomy

Ant Web shows some of the color variation within the species

Jacobson AL, Thompson DC, Murray L, Hanson SF. (2006). Establishing guidelines to improve identification of fire ants Solenopsis xyloni and Solenopsis invicta. J Econ Entomol. 99(2): 313-22.

Trager, J. C. (1991). A revision of the fire ants, Solenopsis geminata group (Hymenoptera: Formicidae, Myrmicinae).  Journal of the New York Entomological Society. 99 :141-198.

6 Replies to “Ant of the Week: Southern Fire Ant”

  1. Fire ants are weird to photograph. They are very shiny, more so than other ants, and they tend to move all their parts. The only days I get anything even halfway decent is when it is pretty cold out.

  2. Hi,
    I am a life long ant lover from the greater Pheonix area and am currently on a hobby to collect a colony of Arizona harvester ants.
    I have found what I think is messor pergandei but I’m not quite sure. They are jet black and polished and workers vary between three different sizes in the colony reaching up to a half inch in length. Although they can eat other arthropods, their diet mainly consists of a seed combination between chaparral and flowering weeds. They are hardly aggressive. These ants look similar to your pictures of the black southern fire ant which is what gave me doubt. If you could clarify this for me with a discription of some distinguishing features I would be very greatful.

  3. Hi again,
    Please disregard my last comment for on closer examination I have confirmed that they are indeed the native Arizona fire ant.
    I have encountered yet another type of harvester native to the Senora desert that I believe to be pogonomyrmex rugosus. My question about this type is how deep and wide their nests can be and what is the average colonial population size? I only hope I don’t have to be burdened with carving out a nine foot deep canyon just to reach the queen and larva/pupa.
    I have aspirations for capitalizing on the wonders these social marvels of nature tend to deliver and lack the patients it takes to wait for the prenuptial flights in spring. Your input is very valuable to me and I look forward to your response.

  4. Robert,

    I’m not that familiar with Pogonomyrmex rugosus, but I did find that some work has been done. According to this reference, (Walter G. Whitford and Rudolfo DiMarco. 1995. Variability in soils and vegetation associated with harvester ant (Pogonomyrmex rugosus) nests on a Chihuahuan Desert watershed. Biology and Fertility of Soils
    Volume 20, Number 3, 169-173.):

    Pogonomyrmex rugosus nests have adundant interconnected burrows in the first 20 cm underground and then ends in a single main tunnel, with an average diameter of the nest disk of 1 m. Walter Whitford has done quite a bit of work with these ants.

    In addition, William P. MacKay has a paper in Psyche 88:25-74 (you can get a free .pdf at the link – look towards the top for “Full text”) that gives extensive data on excavating and colony size in three different Pogos. He indicates P. rugosus numbers around 3000 workers, but gives one reference of 22,000+. He does discuss his technique for excavation. Also, as he mentions, the position and composition of the ants will change with season. As cold as it is right now, the ants will be deep underground. You might be better off waiting for those flying queens.

    Anyone else here have any advice for Robert?

  5. Hi Roberta,

    Well you’ve done me a huge favor with that research. It’s nice to have a knowledgeable friend on such matters seeing as how it’s not every day you run into someone with interests that exceed sports stats or American Idol contestants. I have jumped the gun and already begun an excavation, but I will indeed look up that excavating data for proper modifications. My technique involves a 7 ft deep trench around a 3 sq ft periphery and, based on my limited research, seemed logical at first. However, I’m finding signs that their main tunnel deviates into the trench which further validates how important your reference may be. Thank you.
    I don’t mean to be so long winded, but are you also able to direct me towards the proper sites where I can research oxygen influences on insects. I don’t want to completely let the cat out of the bag, but I’m planning on setting up an environment in which I can force-feed oxygen to these ants on the hopes of increasing their size. There will soon be a market niche for this which can expand through many possible applications. For any bug-lover with imagination, this is very exciting.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.