This year seems to be an up year for gouty and horned oak galls. We have been receiving a large number of calls about them. These woody galls circle small branches and can girdle them, killing the branch out past the gall. The tiny wasp that causes horned oak gall, Callirhytis cornigera, attacks pin, scrub, black, blackjack, and water oak. The gouty oak gall wasp, Callirhytis quercuspunctata, attacks scarlet, red, pin, and black oak.
Adult wasps of horned oak gall emerge in May and June to lay eggs on the major veins of oak leaves. The resulting larvae cause oblong, blisterlike galls to develop in the veins. Adult wasps emerge from these galls in July, they mate, and the females lay eggs in young oak twigs. Young twig galls appear on the twig as small, brown marbles, which eventually coalesce into roundish, brown galls up to 2 inches in diameter. It takes two or more years for these large galls to form. The galls are covered with 1/8-inch-long horns through which the adult wasps emerge. Gouty oak gall apparently has a similar life cycle, but the gall has no horns and adult wasps emerge through 1/16-inch holes in the side of the gall.
These wasps are native insects whose populations rise and fall in relationship to the weather and natural enemies. Many native insects follow a cycle of 3 years of high numbers followed by 7 or 8 years of low numbers. These insects are more common in southern Illinois and very common in Kentucky and Tennessee. Perhaps the warm winters 2 and 3 years ago helped their numbers increase.
On small trees, prune off the galls and destroy them, particularly in the northern two-thirds of Illi-nois. Their killing of branches can cause the tree to become misshapen. Pruning may not be practical in southern Illinois, where the galls are more common. Particularly in locations with large numbers of in-fested oaks nearby, removal of galls may have little effect, with more damage being done by the pruning than the galls. Nurseries and others with large numbers of small oaks may wish to spray the trees with dimethoate (Cygon) in the spring when leaves are expanding. This treatment may reduce the number of twig galls, but remember that it won’t be noticeable for 2 years.
In general, if the tree is too tall for you to reach the galls and prune them, the problem can be ignored. Mature trees can contain thousands of galls without major branch loss. Heavily attacked trees appear to be as healthy and long-lived as those that are not attacked.