From Family Plot.
This is the effect of river birch aphids which also are known as the witch hazel gall aphids. UT Extension Agent Chris Cooper says the feeding of the aphids causes the bumpy ridges on the leaves. The tree will tolerate the damage and he would do nothing to treat it. University of Memphis Director of Landscape Joellen Dimond says the aphids are feeding on the young leaves. She recommends collecting and throwing away the leaves when they fall. This will reduce the aphid problem next year. She also recommends letting beneficial insects control the aphids.
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