Galls, Diseases, and Pests - Urchin Gall Wasp
Cynips quercusechinus
Urchin Galls get their name from their slight resemblance to sea urchins. They are a bright pink to purple, which can make coming across a tree full of them quite a treat. The galls stay attached to the leaves--at Rockville I've found them on Blue Oak (Quercus douglasii)--when they fall to the ground (see the last two photos). Around November, the adults emerge and lay eggs in oak leaf buds.