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Galls

Oak gall © SBF 2014

    

     One of the great parts of late fall and early winter is that we are able to get a new perspective on our natural surroundings. The leaves have fallen, the light reaches places it hasn't in months and there is a stillness that exposes what has been there all along. A few weeks ago as we were raking our yard I noticed many oak galls. Oak ball galls, or oak apple galls, are spherical growths found on oak leaves. Galls grow in many shapes and sizes on a variety of plants. They are caused by insects, generally a type of wasp or fly. The round oak galls that I find in my yard are most likely caused by the oak apple gall wasp, or Amphibolips confluenta. This small wasp lays its eggs on the center vein of an oak leaf. The tree responds to a chemical reaction caused once the larva has hatched and begins munching on the leaf. The leaf grows a mutation around the egg.

Oak gall © SBF 2014

Some galls are troublesome to the plant, but this type of gall is rarely harmful to the tree. This is the first step in a fascinating life cycle. As the larva continues to eat and grow, the gall grows too. When the larva is full grown, it pupates and then eats its way out of the gall, emerging as a winged, full grown adult. The cycle begins again.

Partial Oak gall ©SBF 2014

     

     Another type of gall that you may have observed is the Goldenrod ball gall. If you have access to a field that has not been mowed, you can easily find these galls in the winter months. There are many great informal and classroom investigations that can be done with these galls. The goldenrod ball gall is another spherical gall that grows in the stems of the Goldenrod plant. This gall grows in a similar way to the oak gall, but is caused by the larva of the spotted-winged fly, Eurosta solidaginis. The fly lays its egg on the stem of the plant during the summer. 

Goldenrod ball gall (source)

When the egg hatches the larva burrows into the stem of the plant, hollowing out a living chamber for itself. The plant forms a gall around the chamber. While still a larva the creature chews an exit tunnel for itself. It must chew its exit tunnel while still in the larva stage as adult flies do not eat and therefore do not have the mouth parts necessary to escape their childhood home. The larva pupates, emerges as an adult and the process begins all over. I have always found galls interesting. To read more about types of goldenrod galls see this article that I wrote several years ago for the Squam Lakes Natural Science Center. You may also want to check out this post that Sue at Archimedes Notebook recently wrote about galls that she observed on her hickory leaves. It includes a link to some other types of wild looking galls.

    

     If you are interested in learning more about Goldenrod galls specifically, as they are fairly easy to spot this time of year, I would encourage you to check out this interesting study at the American Museum of Natural History's website. If you are a teacher or just an inquisitive naturalist, you might want to try this Goldenrod gall lesson from the Cornell Institute for Biology Teachers, or this activity from Cornell for ages 12 and up: "Exploring Plant Galls".

Happy gall hunting! 

Have you made an awesome science observation this week? Comment below!