Some are little more than a blister on the surface of a leaf while others are large, warty growths on leaves, stems, or roots. Most plant galls are caused by mites and wasps. Each insect or mite produces its own distinctive gall formation. Often, the insect or mite can be identified by the gall shape. There are over 2,000 American plant galls caused by insects and mites.
More galls occur on oak than on any other host plant.
At least 750 different galls have been identified on oak alone. Nearly all trees, shrubs and other plants have at least a few galls. Generally speaking, most galls do not seriously harm the host plant. Some, in fact, are quite ornate and attractive. Unfortunately, most galls seriously detract from the beauty of the host.