Galls, Diseases, and Pests - Andricus albobalani Wasp
Andricus albobalani
These galls occur on the inside of an acorn shell. The acorns are usually stunted or misshapen, and they do not fall from the tree. More often there is only one gall in an acorn, but sometimes there are two.
This is a stunted acorn, and the gall is on the right side--it looks like a shell within the shell of the acorn.
The arrow is pointing at the gall.
Here you can see the entire shell of the acorn within the dried-out acorn cap.
This is another galled acorn. You can see how the acorn has not grown up out of the cap.
Looking down at the top of the acorn, you can see two exit holes.
The gall on the right is opened, and the one on the left is still enclosed.