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SCHMIDT: What are these strange growths on trees?

By Staff | May 29, 2020

A number gardeners and homeowners have been noticing strange growths on the leaves of their trees and plants so this week’s article will address these strange and sometimes alarming growths. These odd appearing growths are called galls and are usually seen on leaves or twigs of trees. Galls are just abnormal growths or swellings of plant tissue caused by the feeding or egg laying activity of tiny insects such as cynipid wasps (non-stinging with ant-like appearance), aphids, mites, or fly-like insects. These tumor-like growths are a defensive response of the plant to the physical or chemical irritation caused by the feeding or egg laying activity of the gall forming insect. This is because some of these insects have substances in their saliva that are irritating to plant tissue which causes the abnormal tissue growth.

Galls come in a wide variety of colors, shapes, and sizes. There is also variety in the surface texture of galls ranging from smooth, spiny, or fuzzy. They are essentially a protective capsule providing food and protective environment for various developmental stages of the gall forming insect larvae.

Different gall-making insects produce their own distinctive galls. This time of year, the most common gall specimen that I receive calls about is the Maple Bladder Gall. The Maple Bladder Gall appears as tiny pimple-like growths on the upper side of maple tree leaves. The growths begin on leaves as green later changing to a pinkish red color and eventually brown to black- almost appearing like pepper on the leaf.

Will They Harm my Tree?

Most galls do no harm to the host plant. In most cases, the only major consequence of galls is the reduced visual appeal of landscape trees. However, in cases of heavy gall formation where more than 30% of the leaf area of the entire tree is affected control may be necessary. Additionally when less than 30 percent of the circumference of a section of a twig is free of galls dieback can occur and control efforts would be advisable.


Population of gall-forming insects fluctuates from year to they have been especially prevalent. By the time you notice the galls, the insect that caused them is long gone and the gall protects the larva within it. Control of gall of gall-forming insects is also difficult due to timing of insecticide application and potential for making other insect problems worse. Control is best done in spring before bud break or leaf out to reduce the number of adult gall-forming insects and thus the number of galls formed. That means it is too late to treat for the pests that caused these abnormal growths they are long gone now.

Synthetic insecticides such as malathion, carbaryl (Sevin), or horticultural oils are recommended for control of gall-forming insects prior to gall formation. Keep in mind that if insecticides are used they may harm beneficial insects such as bees that act as pollinators and those that naturally prey on gall- forming insects. When natural predators are decreased this can lead to an increase in the less desired pests. Mites and insects can also develop resistance to repeated pesticide applications so rotating classes of control chemicals is recommended. Horticultural oils provide less risk of harm to beneficial insects but application timing is critical to prevent damage to trees.

Since the populations of gall-forming mites and insects fluctuate greatly from year to year, patience and maintaining overall tree and shrub health is often the best prescription.

During this time of COVID-19 social distancing, pictures of plant problems or concerns can be emailed to yolanda.schmidt@ndsu.edu.

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