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UC Cooperative Extension Ventura County
669 County Square Dr. Suite 100
Ventura CA 93003
Phone: 805.645.1451
Fax: 805.645.1474

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Oak Gall


The small balls you see on the ground under oaks are oak galls, and they are produced by the activity of a tiny wasp, called Dryocosmus sp. The wasp spends the winter in the gall on the ground. In spring, male and female wasps emerge, mate, and eggs are laid in catkins (male flower), where galls develop holding the first generation. Wasps again emerge from the catkin galls, and this time (late spring) they lay eggs in the leaves of the oak. The egg-laying and the activity of the wasp larva cause the live oak galls, which are now on your patio, to form. In fall, the galls or leaves fall, and the yearly cycle is completed. No control is recommended. However, if you wish to try to control this wasp, you will have to spray your tree in late spring to prevent the second generation female wasp from laying her eggs in the leaves. The dosage should be according to label directions. By the way, there are over 100 insects that use California oaks for a food source. Many of these are gall formers. There are books available on the insect galls of California oaks and other plants.