Gouty Oak Gall
Galls are abnormal swellings of plant tissue, usually on leaves and twigs. Insects, mites, bacteria, fungi, or nematodes may cause them. Chemicals produced by the egg-laying and feeding activities cause most insect and mite galls. The chemicals cause the affected plant cells to swell. Aphids, midges, wasps, psyllids, beetles, and eriophyid mites can cause galls.
Galls may disfigure twigs and foliage, but they do not seriously affect the health of trees and shrubs. Chemical control is usually not recommended. If a plant appears unhealthy, search for additional causes such as cultural problems or diseases.
Maple bladder gall Cooley spruce gall
Hickory leaf stem gall Eastern spruce gall
Spiny witch hazel gall aphid
To prevent completion of the insect or mite life cycle prune out twig and stem galls while they are green. Small holes in the gall indicate that the inhabitants have escaped to repeat the cycle. To control leaf galls rake up and destroy infested leaves.
This aphid is a common pest on birch, particularly river birch. Injury from this aphid ranges from premature leaf drop to dead twigs and branches. It has a complicated life cycle in that it alternates between two hosts: birch (Betula) and witchhazel (Hamamelis spp.) The aphids become active in the spring when the leaf buds are opening. The growth and reproduction of the aphids is rapid, and the leaves soon develop characteristic “corrugations”. The corrugations on the undersides of the leaves fill with aphids and a white granular material. Winged aphids develop on the birch leaves then seek witchhazel on which to lay eggs and complete the life cycle. This activity takes place before the end of June.
Examine the undersides of leaves for beneficial insects such as ladybird (ladybugs) beetles and their larvae, predaceous midge larvae, Syrphid fly larvae, lacewing eggs and larvae, and parasite activity. Any combination of these predators and parasites can give sufficient control without having to spray with an insecticide. This is not a serious pest. If damage is heavy or the tree is young, spray tree with a horticulture oil. Coverage of the lower surfaces of the leaves is critical, as the aphids are fairly protected in the corrugated folds of the leaves.
Author: John A. Davidson (retired), Extension Entomologist, University of Maryland.
Revised: Mary Kay Malinoski, University of Maryland Extension Specialist, Home and Garden Information Center
Rev. 2020