Answer: Tiny wasps and flies commonly cause a wide range of swollen abnormalities, or galls, on plants. More than 800 different kinds of galls are found on oak trees. Each species of insect causes a particular type of deformity, and each gall has a distinct shape, size, sturcture, and color, depending upon the insect involved.
When the insect lays her eggs, plant tissue is stimulated into abnormal growths which surrounds and protects the eggs. The eggs hatch into larvae and the larvae feed within the gall. Then they turn into adults and bore their way out of the gall. Galls seldom cause serious injury, but you can treat by spraying with horticultural oil during the dormant season to kill overwintering insects.
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