Ants and Plants: Desert Willow Extrafloral Nectaries

What do you think is going on here?


These are Forelius ants visiting the flower buds of a common landscape tree in the Southwest, the desert willow, Chilopsis linearis.


Desert willows are not really willows at all, but belong the plant family Bignoniaceae, making them relatives of catalpa trees.

The trees have large, tubular flowers that attract hummingbirds and bees. Some varieties have dark magenta flowers,


whereas others have delicate, light pink flowers.


Forelius are heat-loving desert ants. Many of the Forelius in this area are Forelius mcccooki (key to US species). They nest in the ground, but commonly forage on plants where they are known to gather sweet fluids from nectaries.

forelius on desert willow0001

Which leads us back to the question:  what are these Forelius workers doing on the desert willow flower buds?


Both catalpa and desert willow are known to have extrafloral nectaries on the leaves. (Rico-Gray and Oliveira in 2007 defined extrafloral nectaries as sugar-producing glands found on the leaves, stems or stipules of plants.)

extra-floral-nectaries-desert-willow_0160Here are some buds from a desert willow tree that lacked ants. See the green spots?


Do you think the light green structures (circled) are possibly what Rico-Gray and Oliveira define as circumfloral nectaries, that is nectaries around flower structures that are not attracting pollinators?


Interestingly, a number of the flower buds on the tree without ants showed damage. What do you think caused this damage?

Looking into the literature, Ness (2003) found Forelius pruinosus workers attacked Ceratomia catalpae caterpillars on catalpa trees after visiting extrafloral nectaries. Ness also showed that leaf damage increased the sugar flow of nectaries within 36 hours. This supports the classic idea that plants attract ants to help fend off herbivores.

On desert willow, however, things might be even more complicated. Carey, Visscher, and Heraty (2012) found that an Eucharitid parasitoid of ants, Orasema simulatrix, laid its eggs in the extrafloral nectaries of desert willows, where the planidia had access to big-headed ant workers feeding there. The article has some fabulous photographs of extrafloral nectaries, by the way.

So, do you think the Forelius were visiting circumfloral nectaries? Have you seen any other ants visiting similar plants?

What do you think of Rico-Gray and Oliveira’s separation of exrafloral nectaries from circumfloral nectaries? Is there a clear need to make a distinction? Would circumfloral nectaries have more likelihood to contribute to successful seed production than extrafloral nectaries?


Carey B., K. Visscher, and J. Heraty. (2012) Nectary use for gaining access to an ant host by the parasitoid Orasema simulatrix (Hymenoptera, Eucharitidae). Journal of Hymenoptera Research. 27: 47-65. (Retrieved online)

Ness, J. H. (2003) Contrasting exotic Solenopsis invicta and native Forelius pruinosus ants as mutualists with Catalpa bignonioides, a native plant. Ecological Entomology. 28 (2): 247–251. (Retrieved online as .pdf)

The Ecology and Evolution of Ant-Plant Interactions (Interspecific Interactions) by Victor Rico-Gray and Paulo S. Oliveira, particularly pages 115 – 123.

Paperback: 320 pages
Publisher: University Of Chicago Press; 1 edition (July 15, 2007)
ISBN-10: 0226713482
ISBN-13: 978-0226713489

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Wordless Wednesday: Little Things

It has been too long, so let’s jump back in.



I would have probably not noticed these ants if an observant three-year-old girl hadn’t pointed them out.


Sidewalk ants (Forelius) are fairly small, after all. Plus, only a few ants were active on such a chilly day.


These ants are benefiting from the discarded lollipop stick left by another child.

Thank you, Rosie.

Ants of Tucson, Arizona: Forelius

The challenge: to see how many species of ants I could find in Tucson, Arizona in two hours.

Species 2. Forelius

Most likely they are Forelius mcccooki, the species I have identified in Phoenix, but because this was a public place I didn’t take samples.

Chris Schmidt indicates that Forelius are probably the ants you are most likely to stumble upon in Tucson (Backyard Ants of Tucson).

Forelius are always easy to spot because they are active even during the hottest parts of the day and they forage in long trails along sidewalks or up trees.

I had to laugh at what I found next.

Do you remember the person who fed colored water to ants to make them turn the colors of the rainbow? (Alex Wild shows a photograph by Maxim Piessen that uses the same technique.)

Now nature imitates art.

The color was provided by…

a prickly pear cactus fruit with help from this gal.

Do Forelius ants occur where you live? What color are they? 🙂

Forelius Ants on the Move

Sometimes it is hard to conceive how much ants move their colonies around. For example, this week a fairly large Forelius colony showed up in our yard.

Forelius are incredibly fast.


They do slow down when they find something interesting, like this dead honey bee.


What is on the honey bee’s wing? It had just rained so there’s water condensation.


I guess it could be refraction, but it looks almost like paint.


We also found the little ants will slow down for honey. Talk about exploding ants 🙂

I hope these stay for awhile.

Any ants on the move in your area?