Summer 1999, Volume 8, Number 3
A JUMPING OAK GALL INVADES SOUTHERN INDIANA FORESTS
by Phil Marshall, IDNR Division of Forestry
The phones in district forester's, the Hoosier National Forest, extension agents and consulting forester's offices rang in late May and early June with the questions, "What is causing my white oak tree to turn brown in color? Is it going to die?"
The phone calls and questions continued through the summer because of an outbreak of a Jumping Oak Gall. One or more species of a Cynipid wasp in the genus Neuroterus are involved in the current outbreak that was first reported from Perry County.
The discoloration and defoliation - caused when the galls form on the under side of the leaf - has occurred on white oak, primarily, but has been collected from bur oak.
Aerial surveys conducted in south central Indiana found the brown discoloration present
throughout the heart of Indiana's forest resource (See Map). Forested areas in Perry
County, especially in the Hoosier National Forest, have extensive to severe discoloration
from the Jumping Oak Gall. The south facing slopes of the forests in this area are heavily
dominated by white oak, and large tracts of the forest are brown in color. Preliminary
estimates from the aerial survey found 1,000,000 acres of forest infested with this gall.
Moving north from Perry County into Orange, Crawford, Martin and Lawrence Counties, the discoloration becomes scattered but large areas of discoloration still occur in the forest. The upper limits of noticeable damage are lower Green County and northern Lawrence and Jackson Counties. The eastern boundary is the Knobs of Floyd, Clark, Scott and Washington Counties. To the west, discoloration decreases west of Jasper and Huntingburg in Dubois County. In addition to these counties, the gall has been found in Johnson, Hendricks and Wayne Counties.
The jumping oak gall is caused by a small wasp that has two generations per year. The first generation emerging in the spring is all females that lay eggs on new foliage. The eggs hatch into male and female wasps, which mate and start the next generation. It is the gall formed in the second generation that creates the discoloration. The galls from the second generation fall to the ground and 'jump' to help lodge them in the duff layer where they overwinter and emerge as females the next spring. The larvae inside the gall moves its body sharply hitting the side of the gall causing it to jump like the Mexican Jumping Bean.
The discoloration comes from the 'spot' that the gall creates on the upper leaf surface. This spot has a brown center, purple margin and a yellow hallo around the purple margin, and is 1/8 inch in diameter approximately. On the lower surface of the leaf below the spot, a tiny gall the size of pin head or smaller develops in a depression made by the wasp. If enough galls form, the leaf can turn brown in color. Some galls may form on the leaf petiole, also causing the leaf to turn brown.
What will happen to the white oaks attacked by the gall is everyone's concern. Although this is the first occurrence of an extensive outbreak by this gall in Indiana, other states have reported some damage from jumping oak gall. This primarily occurs to weak trees that are further weakened by the gall and may die.
The future is uncertain because of limited knowledge on the damage from this gall, but it is expected to return again in 2000. After that, parasites and predators are expected to move into the population and begin to reduce the gall's population. For now, valuable yard trees with heavy discoloration should be watered during the dry periods. Spring fertilization will help the tree tolerate the damage. Properly timed insecticide treatment for the first generation will help to protect valuable yard trees. Before considering an insecticide treatment consult with an arborist and/or forest health specialist.
In the forest, there is no management treatment recommended. Unless the tree is dying from other stresses, tree mortality is not expected and harvesting trees because of discoloration from the jumping oak gall is not needed. At this time, the forest will have to let nature take its' course. Do not harvest your white oaks just because you have the gall.
Consult with a professional forester before you harvest trees infested with the gall. Indiana is not alone in this outbreak of the gall. Jumping oak gall is reported in Missouri and Kentucky this year. It occurred in Missouri last year and other states in prior years, such as Wisconsin and Minnesota. It is also reported from Canada where a western species occurs throughout the west and into British Columbia feeding on Garry and other oaks.
If you have more questions, please contact Phil Marshall, Forest Health Specialist located at the Vallonia State Nursery - 812-358-3621, email - pmarshal@hsonline.net, or Bob Waltz, State Entomologist located at the Government Center in Indianapolis 317-232-4120.