Gall wasps insert their eggs into plant tissue.
When larvae emerge from the eggs, they produce chemicals that induce the host plant to grow abnormally, producing galls. The larvae develop inside the galls. Galls induced by these wasps may appear as spherical stem galls, starlike leaf galls or horny twig galls. Oak trees in particular seem to host a large number of gall wasp species. If you place suspect galls in a glass jar, you may capture the adult wasps when they emerge.