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(515) 294-9915

5/17/04

Mark Shour, Entomology, (515) 294-5963, mshour@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Yard and Garden Column for the Week Beginning May 21, 2004

The Inside Story of Common Gall on Oaks

By Mark Shour
Entomologist
Iowa State University Extension

Bur oak, Quercus macrocarpa, and swamp white oak, Quercus bicolor, are being planted as landscape trees in new developments in Iowa. Both species are native to the state and can be used for wet, low pH planting sites (swamp white oak) or across a range of site conditions (bur oak). Their slow growth and muted seasonal colors have not raised concerns for landowners, but conspicuous growths on the twigs and branches of some specimen trees have been the subject of several phone calls or emails to ISU Extension Entomology during the past two to three years.

So what is concerning these landowners? A type of gall called the oak rough bullet gall or oak bullet gall. At first glance, the structures appear to be acorns, but the shape and number of the structures indicate something is not right.

As a refresher, galls are abnormal growths or swellings of plant tissue caused by the attack of a living organism. Different gallmakers produce their own distinctive galls. In the case of the oak rough bullet gall, the gallmaker is a tiny wasp called a cynipid. Its scientific name is Disholcaspis quercusmamma (Walsh). The wasp is 2 to 3 millimetesrs (mm) long, winged and is brown or black in color (see photo).

In the fall, the wasps chew out of the galls, fly to the terminal buds on their oak host tree, and then lay one or two eggs in each dormant bud. Following several weeks of cold weather, larval development begins in the spring as the buds are expanding. When the white, legless larvae feed, the oak tree is stimulated to grow around the gallmaker, completely enclosing it. A single larva develops in and derives nourishment from the gall tissue. Fortunately only one generation develops per year in Iowa.

Oak rough bullet galls first appear as small green eruptions (bumps) on the twig. The color is changed to a red and then dark brown as the galls grow in size. Completely formed galls are rounded with a point at the apex and are 8 to 15 mm long. Clusters of galls can be found along a host twig or branch (see photo). Reports of the entire new growth of a host covered with galls have been made.

Because the oak rough bullet galls persist on a host tree for as many as five years after the gallmaker has left the gall, many homeowners are concerned about the damage done to their tree. It is thought that although the galls do not kill a twig or branch, they can cause reduction in the growth rate of a host.

There appears to be some variability in the susceptibility of a given tree to the gallmaker. In a Colorado study, only 7 percent of surveyed bur oaks were heavily galled (75 or more galls for every four terminals), while the rest of these street trees either were free of galls or had minor levels of galling (25 or less galls for every four terminals).

These galls also secrete a sticky substance (honeydew) that attracts wasps, bees, and ants. An unusual symbiotic relationship then ensues: these insects feed on the sticky substance and protect the gallmaker from its natural enemies (parasitic wasps).

In the Colorado study, the tiny wasp Sycophila dubia parasitized unprotected D. quercusmamma gallmakers. The parasitized galls were not as well developed, thus their length was 9 mm and shorter; unparasitized galls were fully developed with a length of 10 to 15 mm.

From this study, a control strategy developed for young host trees: remove and destroy the larger galls (10 mm and larger) in September, but leave the smaller galls (9 mm or less) on the host twigs. The smaller galls will have natural enemies in them and these insects will then emerge the following spring to parasitize any new galls formed on this or nearby hosts. No chemical control tools are recommended for management of this pest.

To learn more about galls on trees and shrubs in Iowa, go to your ISU Extension office and request IC 417 or download the publication from ISU Extension Distribution at: http://www.extension.iastate.edu/Publications/IC417.pdf.

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Editors: Two color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 172K and the bottom picture's fullsize photo is 284K.

Caption: OakRoughBulletGallMaker.jpg: Adult gallmaker at oviposition in dormant bud. Photo courtesy of Whitney Cranshaw, Colorado State University, www.insectimages.org.

Caption: OakRoughBulletGallLARGE.jpg: Fully formed oak rough bullet galls on bur oak twig. Photo courtesy of Whitney Cranshaw, Colorado State University, www.insectimages.org.

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