Issue 2, April 25, 2016

Woody Oak Galls

There are three common stem and twig galls on oaks that become woody. Gouty and horned oak galls cause tree dieback and can kill heavily infested trees. Oak bullet gall can cause some dieback, but is not likely to cause serious tree harm. All three are caused by gall-making wasps that attack the tree at this time of year during bud elongation.

Oak bullet gall, Discholcaspis quercusglobulus, looks like brown, woody marbles on the branches of white and bur oak. Although clusters of galls occur, they do not coalesce like those of gouty and horned oak galls. They initially are greenish, turning yellowish and then brown. Oak bullet galls do not typically cause dieback of the twig past the gall area and are most common in the northern third of Illinois.

Oak bullet gall.

Gouty and horned oak 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. Horned oak gall is common in the southern part of the state from about Mattoon and Litchfield south. The gouty oak gall wasp, Callirhytis quercuspunctata, attacks shingle, scarlet, red, pin, and black oak. Gouty oak gall is most common in central Illinois from Springfield to Peoria.

Gouty oak galls on pin oak.

Adult wasps of horned oak gall emerge in spring 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, mate, and the females lay eggs in young oak twigs. Young twig galls appear on the twig as small, greenish to yellowish to brown marbles, similar in appearance to oak bullet galls. They 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 in the spring through which the adult wasps emerge.

Horned oak galls on pin oak.

Gouty oak gall apparently has a similar life cycle to that of horned oak gall, but the gall has no horns and adult wasps emerge through 1/16-inch holes in the side of the gall.

Oak bullet gall can be ignored as it is unlikely to cause serious damage. They can be rubbed off where they are an aesthetic problem.

On small trees, prune off gouty and horned oak galls and destroy them, particularly in the northern third of Illinois. Their killing of branches can cause the tree to become misshapen. Pruning is typically all that is needed of gouty oak gall as those too high to easily reach are unlikely to cause serious tree injury.

Horned oak gall pruning may not be practical in southern Illinois, where the galls are common. Particularly in locations with large numbers of infested oaks nearby, removal of galls may have little effect.

Insecticide applications do not provide predictable control of horned oak gall. Bifenthrin (Onyx) sprayed at leaf expansion has provided control for some but not others. Some arborists and landscapers have seen good results with application of imidacloprid (Merit, others). If applying imidacloprid to the soil, remove dead organic matter such as leaf litter, mulch, or turf before drenching or inject the insecticide beneath the dead organic matter. Imidacloprid adsorbs to dead organic matter, reducing its availability for root uptake.

Although many trees have been killed in southern Illinois, we are generally powerless to prevent it. Keep the trees healthy with irrigation during droughts and fertilization where warranted. Even so, apparently healthy trees may succumb to the gall infestation. Biological control will eventually be achieved by parasitic wasps. These should naturally increase and control the gall wasps. To my knowledge, they are not available commercially. (Phil Nixon)

Phil Nixon

Return to table of contents