Strange Growths on Trees
Many gardeners and homeowners have been noticing strange growths on their trees and plants this year. These odd appearing growths are called galls and are usually seen on leaves or twigs of trees. Galls are simply 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. Some of these insects contain substances in their saliva that are particularly irritating to plant tissue, provoking the development of 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. Two of the more common gall specimens that have come into my office this growing season are the Maple Bladder Gall and the Oak Marble Gall. The Maple Bladder Gall appears as tiny pimple-like growths on the upper side of the leaf of maple trees. 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. The Oak Marble Gall appears as marble sized clusters of green colored individual galls ranging in size from 25 millimeters-1 inch somewhat resembling immature acorns at first glance. Upon opening this gall, the observer will typically find a spongy mass of material and at its center a single larva. As the galls mature they will harden and become brown.
Harm my Tree?
Most galls do no harm to the host plant. 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 year and this year 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. 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. 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 and should be applied before and after bud break.
According to Aaron Bergdahl, Forest Health Specialist with the North Dakota Forest Service “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.”
For more information, visit www.ag.ndsu.edu/pubs/plantsci/trees/f11292.pdf “Insect and Disease Management Guide for Woody Plants” and www.ag.ndsu.edu/pubs/plantsci/trees/e296w.htm “Common Insect Pests of Trees and Shrubs in North Dakota”. You may also contact Yolanda at the NDSU Extension Service Pierce County office by calling 776-6234 or by email at firstname.lastname@example.org.
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