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.
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.)
Here 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)
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