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Overloaded Oaks

The king of forest trees is suffering—and dying—from a record number of horned oak galls.

The mighty oak, the symbol of strength in nature, can endure plenty of hardship. But a relatively tiny troublemaker called a gall wasp is overloading some Illinois oak species with its eggs and larvae, inducing unprecedented quantities of walnut-size galls to appear on afflicted branches.

In some cases, heavily infested oaks are dying.

"I used to tell homeowners oak galls won’t kill their trees," explained Department of Natural Resources Region 5 Forester Gary Stratton. "But not anymore." Stratton said he’s witnessed a massive proliferation of the lumpy, woody galls on species of red and black oak in the Rend Lake area, as well as other parts of southern Illinois. The once-harmless growths (commonly called horned oak galls) are becoming so common on shingle oak and pin oak, among others, Stratton said he is concerned about the future of Illinois oak forests. "It’s definitely gotten worse within the past 5 years," he added.

Many of us have noticed various galls on trees and other plants (those cyst-like growths are the plant’s response to the intrusion of a foreign object). While leaves and plants usually continue living despite the encapsulated larvae, the woody galls created by the gall wasp during its 2-year life cycle clog the vascular structure of the branch, cutting off water and nutrients. In small quantities, a few galls aren’t much trouble for a hearty oak. But massive losses of branch tips are proving too great a burden for numerous oaks, including prized timber species.

"The cherry bark oak is an important timber species in southern Illinois," Stratton said. "Down the road, if this trend continues, I’m concerned it could have an impact on the oak timber industry."

Although at least one restricted-use pesticide (Harpoon) is reportedly used against gall wasps affecting landscaping trees, forest trees appear to have some protection in their numbers—so far.
"The most severely affected galled trees seem to be the lone trees," said University of Illinois entomologist James Appleby. Additionally, while insect populations historically fluctuate, sometimes 'spiking' from one season to the next, there is no record of such a spike with the gall wasp.

"There has been a general increase in the occurrence of this gall, particularly from Effingham southward, during the last 25 years," Appleby said.

More information about plant galls is contained in the booklet "Some Plant Galls of Illinois," available from the Illinois State Museum in Springfield.

By: Joe McFarland
Oak galls appearing on winter trees
Oak galls on tree
Close-up of oak gall