Elm leaf gall - Ask Extension
This question concerns an aggressive leaf gall on elm trees on a wooded property. it is only present on the affected tree at the moment but appears to...
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Elm leaf gall #869951
Asked May 24, 2024, 2:12 PM EDT
This question concerns an aggressive leaf gall on elm trees on a wooded property. it is only present on the affected tree at the moment but appears to be beginning development on a neighboring tree. Is this benign or potentially harmful? If it is harmful, how can it be treated, and can anything be done to prevent spread? Refer to first picture for image of gall and second picture for its incipient development on neighboring tree.
Harford County Maryland
Expert Response
Hi David,
The gall in your picture is most likely caused by aphids (Tetraneura ulmi) overwintering on elm bark. They are not harmful to the tree, but they are filled with aphids which will be released in June/July and spread to other plants and trees. The adult aphids return to the trees in September to lay eggs and start the process all over.
Usually by the time the sack galls are discovered on elms, it is too late for a pesticide application to "undo" the galls. Should a specimen elm important in the landscape be heavily infested year after year, a systemic insecticide could be applied during the winter to prevent gall formation next spring. If a tree is small enough to spray, an application of a systemic insecticide applied when the galls are new should control the aphids inside. One such systemic insecticide is imidacloprid available for landscape use in big box stores, garden centers, and nurseries (The active ingredient of every pesticide is listed on the front label although in very small type near the bottom).
If you decide to try and treat the galls, but feel you cannot do it yourself you can contact a tree company with a certified arborist on staff. An arborist can also tell you if removing the tree is your best option.
Thanks for contacting us,
Judy
The gall in your picture is most likely caused by aphids (Tetraneura ulmi) overwintering on elm bark. They are not harmful to the tree, but they are filled with aphids which will be released in June/July and spread to other plants and trees. The adult aphids return to the trees in September to lay eggs and start the process all over.
Usually by the time the sack galls are discovered on elms, it is too late for a pesticide application to "undo" the galls. Should a specimen elm important in the landscape be heavily infested year after year, a systemic insecticide could be applied during the winter to prevent gall formation next spring. If a tree is small enough to spray, an application of a systemic insecticide applied when the galls are new should control the aphids inside. One such systemic insecticide is imidacloprid available for landscape use in big box stores, garden centers, and nurseries (The active ingredient of every pesticide is listed on the front label although in very small type near the bottom).
If you decide to try and treat the galls, but feel you cannot do it yourself you can contact a tree company with a certified arborist on staff. An arborist can also tell you if removing the tree is your best option.
Thanks for contacting us,
Judy
Thank you for your help Judy! It’s much appreciated.
