We are seeing an interesting number of galls already this spring. Galls have always fascinated me, mainly because of the many curious shapes and colors on such a wide range of plants. Most galls we see in trees are caused by very small insects, often tiny wasps. Galls are triggered to form when a gall-making insect lays eggs in the leaf tissue. The plant’s tissue begins to grow around the egg(s) forming the gall. Once triggered by egg-laying, the growth cannot be stopped. The developing larva inside the gall feeds on the tissue. Each gall maker produces a distinctly shaped gall on its specific host. Therefore, they are usually named based on the host plant and shape of the gall.
Several people have asked about large, unusual growths on their red oak
trees. These galls, called oak apple galls, are very conspicuous, about the size of a golf ball. The oak apple gall, which is mainly formed on red oaks, is kind of hollow, soft and full of spongy tissue, and in the center is the young developing insect inside a harder core.
Most galls are mere curiosities in the plant world, and for the most part do not interfere with or hurt the host. As mentioned above, once formed, gall development cannot be stopped, and there is no control. Nature, however, provides its own control through various kinds of parasites that prey and feed upon the larvae inside the gall. This is why you rarely see heavy gall infestations on a tree year after year. Occasionally galls can get so numerous that the leaves may drop, and a couple of types of galls are formed on stem tissue rather than on leaves. These can cause significant twig damage and dieback if in they occur in large numbers.