Question

GreenBallonTree   I found two of these green balls on one of my oak trees.  The other one is a bit smaller.  I've never seen them before.

Answer
Thes are oak apples. Oak apple galls are sometimes mistaken for an actual crop of the tree, such as apples on on apple tree, but are actually deformed leaves. The gall is about 1 1/4- 2 inches or larger in diameter, globular, and smooth. The outside is green and darkens with age. The inside has a juicy, white, spongy substance with a small, hard center where the parasite is located. When the parasite leaves the gall, the gall dries and the insides become a mass of fibers.

The oak apple gall wasp is identified by its gall. These wasps are tiny and are usually seen.  A gall is an abnormal growth on a plant. The life of a gall wasp is very interesting. They do not hatch from the gall until they are a full grown adult. They quickly find another adult of the opposite sex and mate. After mating they both fall to the ground. The female burrows itself down into the ground next to the oak tree from which the gall grew. It finds the root of the tree underground and injects its eggs into the roots. The larvae hatch and feed on the roots for a year and then become pupae. The wingless females hatch underground, burrow out, and inject an egg into a leave's center vein. The larvae that hatch are round. They cause a chemical reaction that makes a gall form around the larvae. Inside the larvae eats and grows until it is an adult and then it will hatch out and the life cycle of the oak apple gall wasp will start again. These gall wasps cannot sting, so they don't bother us people and since they don't really bother trees either, galls are just something interesting to learn about.

Do not worry about these galls.