Yard and Garden
Yard and Garden

Insect Galls

Overview of galls

Galls are abnormal growths or deformities found on plant leaves, stems and flowers caused by the presence of a tiny insect or mite. Plant galls come in a wide variety of forms, textures, and colors; bumps, warts, spheres, spines, fuzzy, hairy, smooth, etc.

Signs and symptoms of galls

The tiny insects and mites that cause galls to form are called gallmakers. Galls occur on young, developing plant tissue because of an intricate interaction between the gallmakers and the plant. There are thousands of species of insects and mites that produce galls on hundreds of species of plants. In the presence of a gallmaker the plant abruptly changes the course of normal growth and modifies growing tissue into a special swelling that surrounds the tiny insects and mites. Each gallmaker attacks specific tissues on a specific plant producing a unique and distinctive gall.

Oak tree heavily infested with horned oak galls.
Oak tree heavily infested with horned oak galls.
Horned oak gall, the 'horns' contain the wasp larvae that trigger the gall formation.
Horned oak gall, the 'horns' contain the wasp larvae that trigger the gall formation.
Woolly oak leaf gall, one of many common galls on oaks, and common in northern Iowa this year.
Woolly oak leaf gall, one of many common galls on oaks, and common in northern Iowa this year.

Most insect galls do not seriously affect the vigor of healthy plants. Leaf galls are aesthetically displeasing and may cause some premature leaf drop but they do not directly kill the plant. Occasionally, a heavy or prolonged gall infestation on small branches or roots may weaken or kill portions of a tree or shrub.

Management of galls

Galls cannot be "cured" after they have formed. That is, spraying or treating does not make them go away. Preventive treatments applied before the galls form may be effective but are not usually practical. Control with insecticides is not recommended.

For more information on the 24 most common galls in Iowa, consult this publication: Insect Galls on Trees and Shrubs.

Do you live in Iowa and have an insect you would like identified?

The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them.  Please see our website for current forms, fees, and instructions on preserving and mailing insects.   

Contact information for each states diagnostic laboratory for U.S. residents.  If you live outside of Iowa please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.  

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 1, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.