Shade Tree Galls

Gouty Oak Gall 

How Galls are Formed?

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.

Are Galls Harmful? 

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.

Management

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.

• Maple Bladder Gall: This gall is caused by an eriophyid mite, and is generally found on silver and red maple. Spray trees with horticultural oil at the dormant rate before bud break in the spring. Once galls have formed on leaves, it is too late for treatment. These galls rarely require treatment.

• Hickory Leaf Stem Gall: This gall is caused by an aphid. Spray trees with horticultural oil at the dormant rate in late spring just as new growth begins. Sprays are not effective once the galls begin to develop.
• Spiny witch hazel gall aphid (Hamamelistes spinosus)

Publication: (PDF) HG 58 IPM Series: Birch

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.

Control: Examine the undersides of leaves for beneficial insects such as ladybird beetles and their larvae, predaceous midge larvae, Syrphid fly larvae, lacewing eggs and larvae, and parasite activity. Descriptions of these beneficial insects may be found in publication (PDF) HG 62 IPM: A Common Sense Approach to Managing Problems in Your Landscape. Any combination of these predators and parasites may give sufficient control without having to spray with an insecticide. If damage is heavy, spray tree with a registered systemic insecticide. Coverage of the lower surfaces of the leaves is critical, as the aphids are fairly protected in the corrugated folds of the leaves.

• Cooley Spruce Gall

Pineapple-like galls on the tips of blue spruce and Douglas fir are caused by an adelgid (aphid-like insect). Spray with a horticultural oil at the dormant rate before bud break in March, or use the summer rate in early April, or July-August. Prune out small infestations in summer.

• Eastern Spruce Gall

Pineapple-like galls on the bases of twigs of Norway and white spruce, and occasionally black and red spruce are also cause by an adelgid. Spray with a horticultural oil at the dormant rate just before bud break in March, or spray the summer rate in July to August just as the galls open. Prune out green galls in summer.

Additional Information

• (PDF) HG 31 - Insect and Mite Galls on Plants
• Also see - Eyespot Galls

Author: John A. Davidson, Extension Entomologist, University of Maryland.
Revised: Mary Kay Malinoski, University of Maryland Extension Specialist, Home and Garden Information Center