Marble galls

Now, before the leaves open, is a good time to spot the marble galls that often occur on young oaks.  They are caused by the larvae of a tiny gall wasp.  As the grub nibbles at the bud where the egg was laid the oak tree produces this gall in reaction - green at first and then turning brown.  

The new adult female emerges from the gall in September or October and finds a place to shelter over winter. You can see a hole where she emerged.  Then in spring she will be ready to lay her eggs in new oak buds.