Depends on the wasp I guess..

Most species of gall wasps attack specific tissues of the host plant, forming galls, a type of defense response, on roots, stems, twigs, leaves, buds, or flowers. Salivary secretions from the wasp larva stimulate the plant to produce the gall, so the insect actually modifies the plant's normal response to injury, resulting in food and shelter for the larva. Many leaf galls are red and white and resemble miniature stars, sea urchins, cones, cups, or saucers.

-EverythingAbout.net