Galls, Diseases, and Pests - Filbertworm Moth
Cydia latiferreana
The Filbertworm Moth does not cause a gall to form in an acorn. Instead, it lays its eggs so that the larva (caterpillar) can eat the acorn as it grows. So the Filbertworm Moth is a pest. There were two live larvae inside this acorn.
This is an acorn from a Valley Oak (Quercus lobata).