Oak trees are prone to infestation from a variety of pests, many of whom are identified by the damage that they inflict on the tree. These pests include a number of tiny wasps that create small balls, referred to as galls, on oak trees. The exact identification of the wasp is usually established by the size and color of the gall. Certain wasp species cause the growth of green galls on oaks.
The oak apple gall and the roly poly gall are among the green galls seen on oaks. The pests are common on all varieties of oaks in all oak growing regions. Though a number of other pests such as fungi, bacteria or mites may also cause the growth of galls on trees, wasps are the most frequently cause of this disorder, cites the University of Lincoln-Nebraska Extension.
There are about 1,400 different types of gall-creating wasps of which 80 percent infest only oak trees. The wasps are very small in size, measuring ¼ inch or less in length. The insects come in shades of red, yellow or black depending on species. The legless larvae and pupae are white. Every gall wasp is identified by the distinct characteristics of the resulting galls.
Galls are small, round balls that may be up to 2 inches in diameter or may be too small even to be noticed. The galls are actually abnormal plant growth resulting from wasp feeding that creates a disturbance in the normal growth process of plant tissues. As a reaction to pest feeding, the plant produces excessive amounts of growth regulating chemicals that in turn disrupts the normal cell growth. The galls contain the eggs and larvae of the wasp. The emerging larvae feed only the gall's tissues and not on tree areas.
Galls are primarily a cosmetic disorder of the affected tree and pose little threat to tree health. Foliage galls sometimes cause premature leaf drop or distortion by affecting the normal process of photosynthesis. There are no control options to reduce or eliminate galls once they have occurred. Using preventative insecticides may control populations of the adult wasps before they have a chance to lay eggs. Unless preventative pesticides are targeted precisely at wasp flight time, however, they will be of little use. Keep tree areas clear of debris and fallen foliage to remove overwintering pests in branches, twigs and leaves.