Landscape - Oak Galls

Yesterday, a sample of an oak twig with galls was brought in for identification. Galls can be caused by pathogens and insects. Oaks are particularly susceptible to insect galls. The following is information on common galls caused by insects in oaks in Delaware.

Oak trees have a number of galls. Gouty oak gall is caused by a tiny wasp (order Hymenoptera). Most frequent on red oak the gall occurs on many other native species. Galls form on twigs and coalesce to form a long mass on the branch. Control is not usually necessary.

Another oak gall seen mainly on white oak is wool sower gall; the insect pest involved is a tiny wasp (order Hymenoptera). The insect stimulates the twig to produce a rough, shaggy, reddish globular gall 1 to 1 1/2 inches in diameter. Gall becomes covered with creamy-white growth of fibers like wool, turning light brown with age. Control is not usually necessary.

Another oak gall, mainly pin oaks is horned oak gall; the insect pest involved is a tiny wasp (order Hymenoptera). Infested twigs of pin oak usually have several galls forming an irregular woody mass of 2 or more inches. Tgall is covered with numerous conical, hollow horn-like projections. Control is usually not necessary.

On white nut and scrub oak is oak bullet gall; the insect pest involved is a tiny fly (order Diptera). It occurs on small branches with galls bullet-shaped and 1/3 to 2/3 inch in diameter. They usually occur in groups of two or three. Galls are yellowish in color, tinged with red. Control flies that emerge from galls in October with a general-purpose insecticide.

On numerous oaks are oak-apples; the insect pest involved is a tiny wasp (order Hymenoptera). Spherical galls are found on leaves and twigs. They have a strong outer wall, 3/4 to 2 inches in diameter and are usually fastened to a vein or leaf petiole. Control is not usually necessary. On bur oak and swamp white oak are found noxious oak gall; the insect pest involved is a fly maggot (order Diptera). Galls form on the leaves and later on the stems. Galls vary greatly in shape but are more or less tuber-like. They deform the branches. Cut and burn all twig galls before the first of March. Insecticide spray may do more harm than good by killing beneficials.

Extracted from "Gall Insects" by Dewey M. Caron, Extension Entomologist and Derby Walker, Extension Agent (retired), University of Delaware Cooperative Extension. Go to http://ag.udel.edu/extension/horticulture/pdf/ent/ent-03.pdf for the full fact sheet.