Wasps infect the trees and use them to overwinter their larva. Every once in awhile we will find a gall that is green with pink dots. It almost looks like an alien pod of some sort. They are spongy and filled with all kinds of interesting parts. I'll try to save one next spring and take a picture of the contents.
When we crush the brown ones, they are empty. The larva has escaped and is out in the world ready to repeat the cycle.