Before the rain began on Saturday I took a walk in Schenley Park to check on the birds.
In addition to a flock of thousands of robins and starlings near Anderson Playground, I found American goldfinches foraging high in a stand of red oak trees. They seemed to be picking things off the backs of the leaves. At ground level I heard the sound of raindrops ticking on the dry leaf litter -- but it wasn't raining. The goldfinches were dropping the shells.
I collected a leaf and took its picture. Here you see the brown bumps the goldfinches were cracking open. They look like tiny acorns.
In fact, they're galls. When I searched the web to identify them, I learned from the University of Minnesota's Department of Entomology that there are more than 700 species of gall-forming insects in the US and Canada and 80% of them use oaks (read about it here).
Galls form when tiny insects lay their eggs on live leaves (not these dried brown leaves). The eggs emit chemicals that stimulate the leaves to grow covers around the eggs. This protects the larvae until they're ready to emerge -- unless a goldfinch finds them.
Were these galls made by cynipid wasps that are very common on oaks?
No. On 12 Dec 2012 Charley Eiseman at BugTracks corrected my original theory. He wrote: "I believe these are actually among the few oak galls that are not caused by cynipid wasps – they look to me like the work of Polystepha globosa, a midge (Cecidomyiidae)." This link has more information about the midge.
Thanks to the goldfinches I learned something new.
(photos by Kate St. John)
p.s. Here's some information about cynipid wasp galls.
Cynipids are very tiny wasps, harmless to humans, that lay their eggs on oak leaves. Each species of cynipid wasp uses a different site on the oak (root, twig, leaf) and specializes in particular species of oaks.
The most amazing cynipid wasp is the one that becomes the jumping oak leaf gall. The female lays eggs on white oak leaf buds in the spring. When the larvae reach an active stage in early summer they jostle inside the galls and the galls fall off the leaves. The larvae are so active that the galls jump on the sidewalk like Mexican jumping beans. This summer they created a stir in Illinois.
Too bad these aren't jumping oak leaf galls.