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In a recent trip to western New York, I noticed a trend. Whenever I found an ant colony (flipping rocks looking for bait for a family member who likes to fish)...

cornfield-ants

an ant colony like these cornfield ants...

cornfield-nearby-thief-ant

I found thief ants.

thief-and-crematogaster

Everywhere.

lasius-claviger-group-1

Take these Lasius.

 

lasius-claviger-group-2

Can you see the thief ants?

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Let me give you clues. The thief ant is in the top right of first photo, near cluster of three larvae in second photo, and going into the tunnel just left of center in the last photograph.

Thief ants are named for their tendency to live with or near other ant colonies and then steal food from their "hosts."

They can also be found living in separate colonies.

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Some of the thief ant colonies I found living by themselves were quite large.

 

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Looks like quite a few new thief ants are on the way.

According to School of Ants, thief ants are distributed throughout the United States (although they don't show any records for New York State on the map). I had never noticed thief ants when I looked for ant colonies in that location in the past. I know my vision hasn't gotten any better, so that isn't it. It seems like thief ants have gotten a lot more numerous there.

Have you noticed more colonies of thief ants where you study ants? Do you think this a trend or a random happenstance?

 

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Solenopsis molesta:  they are tiny, they are secretive, they are thief ants.

Barely over a millimeter long, you really need a microscope to photograph them. This worker thief ant is running on a piece of cake frosting.

Now that's better.  (Photograph by © AntWeb.org / CC-BY-SA-3.0 retrieved from Wikipedia.)

The workers are often golden yellow-brown in color and may be confused with pharoah's ants at first glance.

The characteristics of the thief ant worker that you notice under a microscope are the small size of the eyes and the unusual shape of the antennae. Solenopsis (Diplorhoptrum) molesta workers have antennae that are 10 segments long with a relatively large 2-segmented club at the tip. The segments between the scape and the club, the funiculus, are wider than they are long and show darker bands of color, making them distinct.

Some taxonomists have separated the thief ants into a separate genus or subgenus, Diplorhoptrum. Renthal, et al. have found a ring of antennal glands in the thief ants that are also found in other Solenopsis, which they say might be further evidence thief ants are indeed members of the genus Solenopsis.

Biology

Thief ants are named for their habit of living near other ant colonies, with tunnels that actually infiltrate their neighbors. The thief ants use the tunnels to go steal brood. They are not looking for free labor like some brood raiders, but use the stolen brood for food. They also feed on seeds and carrion of various sorts.

Solenopsis molesta ants are native to North America. William Morton Wheeler listed them amongst the first native ants to be found in homes.

Vinson and Rao discovered that plots with high levels of Solenopsis molesta colonies prevented new Solenopsis invicta colonies from becoming established. Solenopsis molesta is one of the few ant species that can coexist with Argentine ants, as well. They may be small, but evidently these little gals are tough!

Do you know of anyone working on thief ants? Do they occur where you live?

For more photographs, See:

Series of photographs of Solenopsis molesta by Benoit Guenard
Alex Wild also has captured the tiny thief ants. This one shows the antenna particularly well. Male thief ant with workers from Tucson, AZ.

References:

Robert Renthal, Daniel Velasquez, David Olmos, and S. Bradleigh Vinson (2008), Occurrence of Antennal Glands in Ants. Microsc. Res. Tech. 71(11): 787–791.

S. B. Vinson and A. Rao. 2004. Inability of Incipient Solenopsis invicta (Hymenoptera: Formicidae) Colonies to Establish in a Plot with a High Density of Solenopsis (Diplorhoptrum) Colonies Environmental Entomology 33(6):1626-1631.

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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.

Identification:

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.

Foraging:
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?

References:

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.