Ask the Master Gardeners: Red spots on oak leaves could be mites, gall wasps

By: Elaine Applebaum, Placer County Master Gardener
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Question: The leaves of my oak tree have red spots on them. Is this a problem? Answer: The spots are most likely caused by erineum mites or gall wasps. Both cause only cosmetic damage and will not harm your oak tree. The red spots you see are a reaction by the oak leaf to chemicals secreted by the mite or wasp, similar to how your skin swells when bitten by a mosquito. Erineum mites cause blister-like swellings on the upper surface of the leaf. On the bottom of the leaf under each swelling is a depression filled with mats of red or rusty brown hairs called erinea. The mites, which cannot be seen without magnification, feed and reproduce inside the blister, protected from the elements and predators. Galls are distorted swellings of various plant tissues, from twigs and branches to flowers and leaves. They are caused by the chemical secretions of certain moths, midge flies or, as is most likely in this case, gall wasps. Galls grow in a fascinating variety of sizes, shapes and forms, from large “oak apples” to bright pink spiny urchin galls to the red spots you describe. Almost all of our native oaks play host to one or more of the more than 100 species of gall wasps. These are tiny harmless insects, not the large stinging wasps most people think of. The adult is usually under 4 mm long. The female deposits an egg in the plant tissue. As the larvae hatch and begin feeding, the gall grows around them. Since erineum mites and gall wasps do not seriously damage oaks, no management is necessary. To have your red spots positively identified, you may bring some of the leaves in a plastic bag to the Master Gardener office at 11477 E Ave. (Building 306) at the DeWitt Center in Auburn. Sources: A Field Guide to Insects and Pathogens of California Oaks, Tedmund J. Swiecki and Elizabeth A. Bernhardt, U.S. Department of Agriculture. 2006. Pests of Landscape Trees and Shrubs, Steve H. Dreistadt, Jack Kelly Clark and Mary Louise Flint, University of California Agriculture and Natural Resources pub. 3359. 2004.