What are those strange growths on tree leaves?

Leaf galls and wasps

The strange growths on the back of the leaves, such as those picture below, are known as leaf galls. Galls are formed when a wasp lays an egg. Along with the egg(s), the wasp secrets a growth regulating chemical which causes the leaf or stem to swell. The resulting swelling is called a “gall.” The wasp larvae feeds on the gall until it is grown. (It’s baby food!) When the adult wasp emerges, it will exit through the gall. The time that it takes for the adult wasp to emerge will depend on the species. Some wasps can take up to two years!

Can you prevent galls from forming?

Galls, and the growing wasp inside, are harmless to your tree. There is no need to prevent them or to attempt to remove them.

If you’re concerned about the presence of wasps, the female wasp does not stick around. She lays her eggs and leaves. The emerging adult wasps likewise do not remain on the tree. The tree is merely a temporary host.

There are many types of galls. And not all form on oak trees, but a majority of galls can be found on oaks.