What Is a Gall?

Get the facts on galls and why they form on plants.

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Abnormal growths that appear on the surface of leaves (or stems) are usually a sign that an insect is at work. There are hundreds of gall types, caused by mites, midges, tiny wasps, aphid-like insects and thrips and they vary in shape — from red and spindly to round and green to large and woody — depending on the type of insect and the plant affected.

A gall forms as a reaction between chemicals produced by the female insect and hormones produced by the plant. Sometimes the insect expresses those chemicals when she lays eggs on the plant part. The plant reacts by developing abnormal tissue.

To the insect, the gall offers protection — a kind of womb. The eggs hatch inside, and the young insects later emerge, sometimes after having fed on material inside the gall. With the elm pocket gall, a female mite injects saliva into the leaves, initiating the formation of elongated red galls, and other mites enter the galls from the underside of the leaf.

Galls in ornamental trees cause little damage except for aesthetics. No treatment is usually necessary. If treatment seems called for, a dormant oil spraying is usually effective, but this must be done before bud break.

The grape phylloxera is all too familiar to grape growers, however. The round galls formed on leaves are a benign problem, but galls that form on the roots can cause stunting and death of the plant.

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