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Growths on Oak Leaves and Twigs

Interesting lumps and bumps on the leaves of oak trees can be stressful for gardeners, but fortunately not the tree.

These abnormal growths, known as galls, are actually plant tissue that forms in response to insects feeding. The insect is housed within the gall protecting it from predators and pesticide sprays.

The wool sower leaf gall resembles a cotton ball. It’s actually a group of galls that formed at the same spot on the leaf. Pull this fuzzy gall apart and you’ll find the seed-like structures that contain the developing wasps.

The Jumping oak gall found on valley and California oaks contains one small wasp. Once mature, the gall falls off the tree and onto the ground. As the insect moves around inside the gall it can make the gall jump right off the ground.

Control is not effective once the gall forms and fortunately is not needed.

A bit more information: Twig galls, like the gouty or horned oak galls, take several years to develop on the twigs. They feed on the gall tissue, but once they reach maturity they do not feed on the plant. Trees can suffer some leaf loss and twig dieback when populations are extreme. Proper watering and mulching will help keep the trees healthy and better able to tolerate the feeding damage of twig galls.

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