Oak galls won't cause much permanent damage

Published Date

ROLLA, Mo.–Oak trees across a broad swath of Missouri are experiencing an unusual outbreak of a common parasite. Tree experts say the parasite is not fatal to healthy trees, even though the condition can look awful.

The Missouri Department of Conservation is receiving numerous reports from St. Louis to Branson of oak trees turning brown. The problem is especially severe in the area around Rolla, Lake of the Ozarks, Springfield and Table Rock Lake.

The main cause of the problem is a wasp of the family Cynipidae. These tiny wasps deposit their eggs on emerging leaves in the spring. As the eggs hatch and young wasp larvae begin feeding, leaves develop brown spots that grow into “galls.” Each of these button-like growths, which are about the size of a pinhead, provides food and shelter for one wasp larva. In severe cases, entire leaves turn brown over much of a tree.

“It looks worse than it really is,” said Conservation Department Resource Scientist Rob Lawrence. “Some people mistakenly think a tree has died when it turns brown. However, it is extremely rare for oaks to die from this insect. Only a few trees that are in serious decline before infestation are likely to succumb.”

Lawrence said trees that have been stressed in recent years by ice or wind damage are more vulnerable to declining health if they lose most of their leaves to gall damage. If most of a tree’s leaves turn brown or drop by early summer, it may produce a second flush of leaves. This added stress can lead to a long-term health decline.

Lawrence, a forest entomologist, urges tree owners not to cut down infested oaks. He said all that most trees need to recover is a little tender loving care, including supplemental watering during dry weather and possibly fertilizing next spring. Tree owners also can help minimize the severity of future outbreaks by burning or composting fallen leaves.

Avoiding any further injuries, such as wounds from lawn mowers or trimmers, is important for all trees, but especially those that already are stressed by previous storm damage. The use of pesticides is ineffective on the current gall damage.

Most jumping oak galls eventually drop to the ground. They get their name from the larval wasps’ habit of moving violently, causing the entire gall to jump. This causes them to settle into vegetation and soil crevices, where they are more protected.

Winter weather is one factor that helps determine how prevalent jumping oak galls are from year to year. Fewer jumping oak gall larvae survive in winters with repeated wide temperature swings, which encourage them to emerge early. Larvae survival is better in years when winter temperatures are more constant. Extended snow cover also helps the larvae survive by insulating them from cold.

More information about jumping oak galls is available at mdc.mo.gov/22967.

Although jumping oak galls are the main cause of oak trees turning brown, they are not the only cause of leaf discoloration in oak trees. Fungus infections also are causing some problems this year. Like the galls, this problem seldom is fatal to oaks. No actions are needed to help trees recover, other than those noted above.