Ant-attracting Galls

A few months ago I mentioned that Hurricane Irene had brought down some interesting galls from the treetops, and I’m just now getting to that point in my photo sorting.  There was one spot in the woods where the freshly blown-down red oak leaves had lots of little round galls on the undersides, loosely attached to the lateral veins:

They weren’t just simple spheres; each one had a distinctive little “hat” on top:

As I sat down to inspect the ones on the leaves, I found that the ground was littered with others that had detached, with their “hats” in various stages of deterioration:

Then I saw a spine-waisted ant (Aphaenogaster sp.) come up to one of them and start gnawing on what was left of the appendage, and briefly try to drag the whole gall away.

I was suddenly reminded of elaiosomes, the nutritious little appendages that many plants’ seeds have.  These aid in seed dispersal, much in the way that having tasty berries facilitates seed dispersal in other plants, except that instead of the seeds being swallowed and passed through the gut of a bird or mammal, they are carried off by ants who will feed the elaiosomes to their larvae, then discard the seeds.  Would being carried off to an ant nest do the developing wasp larva any good?  It would probably be safe from parasitoid wasps in an ant nest.  It could also be that the gall isn’t meant to be carried off, and that this structure serves a similar function to the honeydew secretions of some other cynipid galls, attracting ants that discourage parasitoids from approaching while they are feeding. But if gall wasps can reinvent extrafloral nectaries, it seems reasonable that they could reinvent elaiosomes too.

Acrobat ants (Crematogaster cerasi) visiting an unidentified honeydew-secreting cynipid gall on scrub oak (Quercus ilicifolia).

An advantage of these elaiosome-type structures over the honeydew-secreting strategy would be that they are still attractive to ants after the galls fall off the tree, whereas (I presume) honeydew production requires that the gall be attached to the tree.  I have not been able to figure out what species causes these galls.  There is a common gall with a similar-looking structure caused by Dryocosmus deciduus, also on red (and black) oak, but those galls are smaller, skinnier, and come in clusters, erupting out of the leaf midrib:

A cluster of Dryocosmus deciduus galls erupting from the midrib of a red oak leaf, alongside one of the mystery galls and a leafhopper (Cicadellidae: Scaphoideus).

As the name suggests, Dryocosmus deciduus galls are also deciduous, dropping off the leaf before the leaf falls from the tree.  I don’t know if anyone has previously suggested that the  little nubbins on these galls function like elaiosomes.  The only thing I turned up in a Google search was this paper about a chalcid wasp that forms galls in elaiosomes.  I guess I’ll just have to keep an eye on these galls next fall and see what the ants do with them.