Galls: What are they?
Have you seen strange-shaped growths on your trees?
Growths like these are called galls. Galls are unusual, non-life-threatening growths on tree leaves or branches often formed in response to insect activity. They are unsightly but not cause for concern.
How are galls formed?
Galls are abnormal swellings of plant tissue, usually on leaves and twigs, that are a tree's response to insect, mite, bacteria, fungi, or nematode activity. The tree forms this tissue to encapsulate and protect itself from pest attack, yet the pests benefit from the formation of the gall. This type of symbiosis (relationship) is called commensalism.
Galls may appear as balls, knobs, lumps, or warts, each being characteristic of the organism present on the tree. They draw attention due to their range of colors: red, green, yellow, or black.
Galls may disfigure twigs and foliage, but they do not seriously affect the health of trees and shrubs. Chemical control is usually not recommended. The pests that cause the tree to form these woody growths are usually kept in control by their natural enemies. These pests must attack the target tree at a particular time during the year to be successful, otherwise they may not be able to stimulate tissue growth on the plant, which protects their eggs and larvae.
Common Galls
Wool Sower Gall - toasted marshmallow-like galls formed around wasp larvae that appear on white oaks in early summer. One gall is actually a group of smaller, hairy galls joined at a common spot on a twig. You can pull these apart to see seed-like structures that contain the developing wasps.
Eastern Spruce Gall - Galls on the bases of twigs of Norway and white spruce and occasionally black and red spruce are also caused by an adelgid. Prune out green galls in the summer to control.
Ash Flower Gall - this gall is formed around a small mite that leads to irregular distortion of male flowers. The galls are initially green, then dry and turn brown.
Honey Locust Pod Gall - this gall is caused by a small fly. Infested leaves have globular or pod-like distortions that contain one to several small maggots. Infested leaves often drop prematurely, and repeated damage can kill small branches. New shoots develop at the base of dead twigs. As a result, the natural shape of the tree may be lost.
Elm Cockscomb Gall - these distinct galls, caused by an aphid, are about 1 inch long. The irregular edge of the gall and its red color at maturity account for the common name. The galls dry, harden and turn brown as they age.
Hackberry Gall - caused by a small aphid-like insect with sucking mouthparts called a jumping plant louse. Feeding by the nymphs that hatch from these eggs causes abnormal plant growth that forms a pouch.
Maple Bladder Gall - This gall is caused by an eriophyid mite and is generally found on silver and red maple. These galls rarely require treatment.
Willow Pinecone Gall - The gall's appearance looks like a pinecone. Slicing the galls open lengthwise at this time of the year will reveal a single, plump, orange colored midge fly maggot nestled within an elongated chamber at the center of the gall structure. The chamber is protected by multiple layers of cone-like scales.
Horned Oak Gall - is a golf ball-sized growth in response to a small wasp. The 'horns' contain the wasp larvae that trigger the gall formation.
Roly Poly Galls - are similar in size and appearance to hollow green grapes. Inside, in a small, loose "seed like" structure, is the larva of a tiny wasp.
Jumping Oak Galls – caused by a small wasp, these round seed-like galls fall off leaves when mature. One wasp lives inside each gall, the wasp's activity may make the gall "jump" several inches off the ground.
How to treat galls
Most galls do not affect plant health, therefore treatment is usually not needed. Additionally, horticultural applications are generally not effective because the precise timing of treatment is critical. For maximum efficacy, apply the horticultural oil before gall formation begins, but when insects and mites are active. Once galls start to form it is too late for treatment, as the gall protects the insects. For insects that spend the winter on the host plant, apply the horticultural oil before insect activity begins in the spring.
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