Q. I have funny things growing on the leaves of my oak tree. Some of them look like little sea anemones and others like little stars. Is this a problem?
A. You are describing some of the wonderful variety of galls that can be found on the oak trees in California. Galls come in a large variety of shapes sizes and colors, and are growths produced by the oak tree in response to the egg-laying of several kinds of insects and mites. The Gall most readily recognized are the large round growths that occur on Valley Oaks. Green and smooth as an apple when young,, these galls age to a tan then blacken with mold if they remain on the tree. If you cut them open with a sharp knife you may see the tiny larvae living inside, or perhaps the predators that have learned to take advantage of a captive meal. Our native oaks support over 100 different kinds of distinct galls, on their leaves, stems, flowers, buds branches, roots and acorns.
When the tree is ‘stung’ by the insect’s or mite’s ovipositor at just the right moment in the life of the plant, it responds by pushing the nearby cells into overdrive. One of the insects responsible for gallmaking is a tiny cynipid wasp, although certain flies, moths, beetles and mites are also gallmakers.. The rapidly growing plant cells create a nursery and food source for the emerging larvae who are protected from some predators by the gall surface. The larvae live and eat in the gall until they emerge from pupae , chewing their way out as adults. On some galls you can see the emergent holes at the end of the summer.
Galls are not harmful to the host plants, although some may cause leaf tips to turn brown or small twigs to die. It is not realistic to try to stop the galls from forming, so it is best to learn to enjoy the incredible diversity of shapes and forms that decorate our trees.