Pin oaks are great looking trees. They are fast growing. They tolerate poorly drained soils, even some flooding. They have a deep red fall color. But they have one fatal flaw - Horned Oak Gall and for the last 15 or more years this pest has lead to the sad decline and eventual removal of many pin oaks in the area.
The problem is a tiny cynipid wasp called the horned oak gall wasp. Although a native species, this wasp can sometimes reach outbreak densities on pin oak causing woody twig galls on more than 80% of a tree's branches, disfiguring the tree and causing crown dieback.
In research done at the University of Kentucky, three tactics were tried in an effort to manage this pest. The first was to use a foliar spray to kill the adult female wasps before they could lay eggs in the developing leaf buds. This method proved unsuccessful in reducing twig galls. The second method involved injecting concentrated solutions of insecticide into the tree to target developing larvae in the galls. This method did not reduce the number of developing stem galls. The third tactic utilized foliar applications of systemic insecticides targeting the larvae. This was also unsuccessful.
Controlling existing stem galls with insecticides is not very effective because insecticide uptake is very much reduced in woody tissue compared to leaf tissue. Essentially there is still no really effective means of controlling horned oak galls.
One of the reasons control of the horned oak gall wasp is so difficult has to do with its alternating generations biology. In early spring, wasps emerge from the woody stem galls and females lay eggs in the swelling leaf buds of the tree. The eggs hatch in a couple of weeks and feed and develop in the expanding leaves and change to adults only two to three months later. These new females lay eggs in the tree's young shoots in a tight spiral completely around the branch. The tree forms a swelling in this area and eventually the woody gall forms. The larvae within these stem galls may take two to three years to complete their development. This variance in life cycle with both leaf and stem infesting wasps in the same growing season is different than most other insect pests and contributes to the difficulty in finding an effective control.
During this time, as the gall is increasing in size, the movement of water and nutrients along this branch stop and dieback occurs to the branch.
When the outbreak of horned oak gall is severe, repeated galls continue to girdle the new growth leading to decline, and over a period of time, death to the tree.
As a matter of interest, occasionally opportunistic insects such as borers, ants, midges and other wasps may use the older galls as nesting sites or shelter. Dogwood borer is one that is frequently found in oak galls using them as a reservoir. In some heavily infested pin oak trees as much as 15% of the galls may contain dogwood borers.
Dogwood trees, therefore, growing in landscapes with galled pin oaks may be at a higher risk from borers. Finally, if you find twigs with the galls attached littering your landscape you can probably blame the squirrels. They have been known to sharpen their teeth on the galls and branches and drop them down from the tree.
Because of the likelihood of pin oaks in our area developing horned oak gall, planting pin oak or at least large groups of pin oaks is not a recommended practice. Some alternatives worth considering include Swamp White Oak, which is not susceptible and a tremendously adaptable oak; American Yellowwood, which has fragrant pendulous blooms; or Yarwood Planetree a great fast growing shade tree with attractive bark.
Kelly R. Jackson is the Christian County Extension Agent for horticulture. He can be reached at 270-886-6328 or visit Christian County Horticulture online at www.christiancountyextension.com.