My new shrub that was planted last year is doing well, but it has odd-looking, cone-shaped bumps on some of the leaves. Is this something to worry about?
— Maria Gonzalez, Waukegan
Your description suggests your shrub has galls. Insect and mite galls are common on trees and shrubs throughout the Chicago area and generally are nothing to be concerned about. Leaf galls rarely cause serious harm because unaffected leaf tissue on the plant, and even on the same leaf, will continue to produce food for the plant. A very heavy infestation of stem or twig galls may cause more serious harm to some trees and shrubs but is an unusual occurrence and is not like what you are describing with your shrub.
You may or may not have a recurrence of these galls next year in your garden. Most galls are caused by native insects and mites and are attacked by parasitic wasps and other natural enemies that serve to control their populations naturally. As the gall insects’ and mites’ population increases, so will the natural enemies. Thus, infestations come and go over the years.
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Insecticide applications rarely provide any effective control of insects and mites that cause galls. The plant will wall off and compartmentalize the insect or mite as it feeds, which creates the galls you are seeing in your plants. Since the plant’s nutrient and water-conducting tissues have been “walled off” and greatly reduced from the gall, systemic insecticides that are transported inside the plant are not very effective in killing the gall-forming insects while they are feeding. Also, since the gall tissue is different from the plant, insecticides are generally not well absorbed into the gall so do not affect the insect or mite. This is a good example of the importance of identifying an insect causing a problem and then careful consideration before applying an insecticide that is either not necessary or will not work as applied.
Adult gall-making insects and mites are the only life-cycle stages outside the gall, so contact insecticides do not work well either. Most gall makers are wasps that are difficult to control even when you time the application correctly. Just continue with a normal maintenance program for your shrubs. With the current dry conditions, the shrub will benefit from supplemental water since it was planted last year and is still getting established. In summary, there is no need to be concerned about the galls or to take any special action at this time.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.