Oct 29 2012


Published by at 7:00 am under Insects, Fish, Frogs,Schenley Park,Trees

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).

Though I can’t tell you which species, I think these are cynipid wasp galls.

{NOTE on 12/12/12:  Today Charley Eiseman at BugTracks 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).”     He knows much more than I do!  The news has derailed my paragraphs below about cynipid wasps, but they’re still interesting even though they don’t apply in this case.   This link has more information about the midge.}

The tiny wasps, harmless to humans, lay their eggs on oak leaves.  Their 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.

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 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.

Thanks to the goldfinches I learned something new today.

(photos by Kate St. John)

6 responses so far

6 Responses to “Galling”

  1. Sharonon 29 Oct 2012 at 7:34 am


    I became intrigued and found this video of the “jumping galls” … kinda cool …

  2. Kayon 29 Oct 2012 at 8:51 am

    I have seen these on oak leaves but never knew what they were. I have a question off the subject, why are there so many robins at this time of the year. I didn’t see many of them this summer, but Saturday was a revolving door at both of my birdbaths. A line up waiting to use the birdbath, much like cars waiting to get in the car wash. At least 8 or 9 on the patio at one time “hunting”. Is this like the starlings gathering and getting ready to migrate? You start each of my days off on the right foot with an interesting story. Thanks.

  3. Kate St. Johnon 29 Oct 2012 at 9:37 am

    Kay, at this time of year migrating robins spend about 2 months at Pittsburgh’s latitude before heading further south. Their numbers can build to 100,000 by the end of the year. Here are two blogs with more information:

    Lots O’Robins from November 2011

    100,000 Robins Near Carnegie from January 2008

  4. bhanceyon 29 Oct 2012 at 4:04 pm

    Thanks! I have always wondered about those bumps on leaves, and now I know.

  5. Charley Eisemanon 12 Dec 2012 at 2:26 pm

    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).

  6. Kate St. Johnon 12 Dec 2012 at 2:30 pm

    I can see from your blog that you’re an expert on galls and midges. I defer to your judgement and will alter my blog entry to point out your comment. Thanks for letting me know.

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