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Ants Tending Oak Galls

Ants are commonly known to tend Homopterans, such as aphids, scales, etc.

gall-ants-2

But what is this ugly thing?

Obviously it is attractive to ants. (I know, the photos aren’t the greatest).

gall-ants-3

Here’s another kind.

ants-oak-gall

Wow, that one is popular.

gall-ants-4

Any ideas?

The plants these structures are on are oaks (Quercus). I believe they are Emory Oaks, but please let me know if they aren’t.

oak

The reddish-brown structures are galls caused by cynipid wasps. There are a number of species that cause galls on oak trees, in particular.

The galls of some species secrete a sweet substance that is attractive to ants.

This one is a bit puzzling because the presence of ants are thought to protect against predators and parasites, but what could attack a gall wasp hidden inside a gall? Wouldn’t that plant tissue disguise and protect it?

It turns out that there are parasites that can kill gall wasps. In fact, scientists have shown via exclusion tests (by preventing ants from reaching galls), that survivorship of certain gall wasps is significantly increased when tending ants are present.

Security, and the gall wasps don’t even have to supply the payment. Now that’s sweet.

T. F. SEIBERT (1993). A nectar-secreting gall wasp and ant mutualism: selection and counter-selection shaping gall wasp phenology, fecundity and persistence. Ecological Entomology. 18(3): 247-253.

13 comments to Ants Tending Oak Galls

  • Very interesting. Do you know what causes the gall to secrete the sweet solution? Is it a function connected with the occupant species, or the species of the host tree?

    Great blog by the way!

  • Roberta

    I think when it comes to oak galls and ants, there are currently more questions than answers. Because there are numerous cynipid galls that are not tended, secretion would seem to be related to the gall species.

    Like your blog too. :-)

  • [...] Gibson, at Wild About Ants, describes some gall-forming wasps that apparently trick aphid gall-tending ants into protecting their homes without reward. Wasps are [...]

  • You’ve given me a lot to think about with this post. A cedar elm sapling in the front yard has a number of aphid-ish lumps on it that are definitely attractive to ants. I’ll watch them closely.

  • Roberta

    I’d love to hear what you find out.

  • Mark

    Glenn Keator’s book, The Life of an Oak, has a good introduction to gall wasps on several species of western oaks.

  • Roberta

    Thank you so much for letting me know about that. It looks wonderful. I’ll be sure to pick that up.

  • Eli

    Interesting! I found your site after noticing ants all over some new galls on a bur oak seedling in my front yard. Your photos are better than mine, but a couple of them are at http://www.flickr.com/photos/esagor/4752683184/ and http://www.flickr.com/photos/esagor/4752684390/.

    Thanks for the post, including the link to the study. Very interesting indeed!

  • Roberta

    Thanks for sharing your photographs. Always like to see what those ants are up to.

  • Ron Russo

    The wasp larvae inside the galls convert starch in the plant’s tissues (gall nutritive zone around the larvae) into to a sugary substance which when somewhat overstimulated is excreted to the outside surface. This substance then attracts yellow jackets, bees and ants, which due to their pugnacious behavior keep parasitic insects away, for at least a while, I suspect, during a vulnerable period of the larva’s life. There are several species of cynipid galls in which this “attractive defense” behavior is common, especially among white oaks such as valley, scrub, and blue oak. You might be interested in A Field Guide to Plant Galls of California and Other Western States, UC Press, 2007

  • Roberta

    Thank you for the great information.

  • Eileen

    The bunch of round structures that the ants are tending are called bullet galls. These cynipid (gall wasp) galls secrete a sugary exudate late in the summer, while the cynipid larva inside each gall is actively feeding, that attracts a lot of stinging insects, at least in Florida. The species that I’ve worked with on live oak is Disholcaspis quercusvirens. This is likely another Disholcaspis species.

  • Roberta

    Thanks for the info, Eileen.

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