Q&A – What is this? I found it growing on a young oak tree.

Question:
I found this growing on a small young oak tree in my front yard. What is it?

Answer: 
This is a gall caused by a cynipid wasp. The wasp cuts a slit in the bark of the tree to lay eggs. The tree reacts to the chemicals the wasp leaves behind by trying to isolate the damage which forms the gall. The gall also protects the wasp eggs. It is not harmful to the tree long term and you can remove it if you would like. Cynipid wasps are considered beneficial insects.

Related Videos:
Oak Galls

Related Resources:
Understanding Oak Galls
Galls on Oak