Solenopsis xyloni Stores Spurge Seeds

Last week I flipped over a rock about six inches square and found this:


Okay, it doesn’t look like much until you zoom in closer.


The grayish-mound is a solid mat of oval, wrinkly seeds. Apparently they had been gathered and stored by the Southern fire ants (Solenopsis xyloni) you see running around. The ants were in full defensive mode.

The seeds were both on the ground and on the underside of the rock, so there was quite a mass of them.




Seeing the seeds reminded me of an earlier timeĀ (above photograph) I had found a similar cache of seeds under a rock . At the time I didn’t know what plant they were from, but now I have figured it out.

These seeds are from a type of ground spurge, Chamaesyce prostrata. Another common name is sandmat. (See post about Southern fire ants and sandmat).

The University of Arizona has an illustration of the plant in their older weed manual under the name Groundfig Spurge, Euphorbia prostrata. See the seed labeled “d” in the illustration?

A quick search of the internet revealed the UC IPM website states “Weed seeds, particularly spurge, may attract the ants away from the bait…” This statement is referring to Southern fire ants in almond groves.

Seems like there might be something worth investigating going on here.

Have you ever seen Southern fire ants with seed caches? What kinds of seeds did you find in them?

International Rock Flipping Day: The Ants

Did you know that today is International Rock Flipping Day?

The idea is to go outside, flip over a few rocks, and record what you see. The resulting posts will be published at Wanderin’ Weeta.

After looking under a couple of rocks, I posted about most of the creatures I discovered (including a very cool case-bearing larvae) at Growing With Science. Of course flipping rocks is a fabulous way to find ants (and “experience” ants in other ways, too), so let’s take a look at what ants were hiding under rocks today.

The area I chose has mowed grass with a brick edging around it, as well as some rocks piled up in a drainage ditch. It isn’t uncommon to see Forelius running along the edging, so it was no surprise to find a few under the rocks as well.

Southern fire ants were in full force, too.

I was surprised how much more red these show that the ones in my yard a football field-length away.

I managed to get stung while taking this photograph. (Flipping rocks does has its hazards.)

Of course, Dolichoderinae don’t sting.

But they are more than willing to bite.

At least it was sitting still, so it is in focus šŸ™‚

Did you participate in International Rock Flipping Day? What did you find?

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 orĀ  “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.Ā  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). Ā Journal of the New York Entomological Society. 99 :141-198.

Southern Fire Ant

In the previous posts about the silverfish and the rove beetles, I mentioned that I found the insects in native fire ant nests. The native fire ant that is extremely common in Arizona is the southern fire ant, Solenopsis xyloni.

Like other fire ants, the southern fire ant has a clubbed antenna and two nodes in its petiole. As with many southwestern ants, it seems to be highly variable in color. Our local species are dark brown, although the major workers have some lighter red-brown, usually on the head.


Photo by Michael BranstetterĀ /Ā Ā© /Ā CC-BY-SA-3.0

As you can see from this photograph, Solenopsis xyloni does have a sting.

Fire ant workers exhibit a range of sizes.


They make distinct and heavily followed trails to certain foods.


These are consuming old cat food.


I noticed the soldiers often vibrate their gasters up and down while feeding.

I’m sure you will see more about this common species in future posts.