Jumping oak galls, made by tiny non-stinging wasps, are causing white oak leaves to turn brown (Figure 1) and drop in some areas. Brown-to-black spots appearing on upper leaf surfaces are discolorations created by gall formation. Tiny button-shaped galls can be seen on lower leaf surfaces. The more galls there are on a leaf, the larger the brown areas (Figure 2).
Gall development begins when female wasps place eggs in expanding leaves during spring. Chemicals from these insects cause leaf tissue to develop around grub-like wasp larvae; this leaf tissue (gall) provides protection and food for larvae. When wasp larvae mature, galls drop to the ground, leaving a small scar where it was attached. Larval movements in galls cause them to jump like jumping beans. Pupae may end up in soil cracks or under leaf litter, where they remain until spring. Adult wasps emerge in spring to begin the cycle again.
Some jumping oak galls are present every year but are not noticed. The occasional outbreak that causes significant visual effects may last for 2 consecutive years before natural enemies reduce wasp numbers to low levels.
Established, vigorously growing trees usually can tolerate moderate leaf loss without significant harm; however, trees under stress may be adversely affected. White oaks in the landscape may benefit from fertilizer applied next spring. Trees should be watered if they become drought-stressed this summer. It also is important to avoid injury or other stresses to trees. There is nothing practical to do for forest and wood lot trees; most should be able to cope with infestation.
Insecticide applications are not recommended for gall wasp control in home landscapes. The main reason is that research has generally shown that insecticide treatments are more damaging to beneficial insects, which are natural enemies of the wasps, than they are in preventing gall formation. Even if insecticides were available, powerful sprayers are necessary to treat large trees thoroughly. The best strategy is to promote tree health and reduce stress.
By Abe Nielsen, Forest Health Specialist, Kentucky Division of Forestry and Lee Townsend, Extension Entomologist