Schmidt: Strange growths on trees
Many gardeners and homeowners have been noticing strange growths on their trees and plants once again this year. 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. Two of the more common gall specimens that have come into my office this growing season are the Maple Bladder Gall and Chokecherry Gall Midge. 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. Chokecherry Gall Midge appears as enlarged fruit about the size of a pea that are hollow with tiny bright orange larvae inside.
Will they 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 percent 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. 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. 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 ag.ndsu.edu/pubs/plantsci/trees/f11292.pdf “Insect and Disease Management Guide for Woody Plants” and 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 ext. 5 or by email at firstname.lastname@example.org.