Balls of gall
"Often, strange structures appear on an oak tree and these attractive round balls seen swinging amongst the catkins or attached to the buds are known as Galls and house different types of wasp larva."
There are various types of gall and they are caused by a gall wasp that lays its eggs inside the acorn, causing it to mutate. These wasps do not usually cause damage to the tree and are nothing to worry about. The most common are the Currant galls, the red Cherry galls and the Marble galls. If you were to cut one of those galls open, you would find the wasp larva curled up inside.
Balls of gall
"Often, strange structures appear on an oak tree and these attractive round balls seen swinging amongst the catkins or attached to the buds are known as Galls and house different types of wasp larva."
There are various types of gall and they are caused by a gall wasp that lays its eggs inside the acorn, causing it to mutate. These wasps do not usually cause damage to the tree and are nothing to worry about. The most common are the Currant galls, the red Cherry galls and the Marble galls. If you were to cut one of those galls open, you would find the wasp larva curled up inside.