Anyone who takes the time to look closely at several branches of oak will soon find one or another peculiar anomaly among the leaves and twigs. One can easily find structures resembling Ping-Pong balls, hard knots, fluffy tufts, horns—either single or clustered, or irregular thickenings, to mention just a few possibilities. These abnormal growths are galls, structures caused by the presence of small insect larvae living inside the tissue of the plant. Galls can be found on a wide variety of plants. They are common, for example, on the stems of goldenrods, and the leaves of maples, but oaks are host to a bewildering diversity of these little parasites. As many as 400 species of gall-forming flies, wasps, and mites have been documented to occur on white oak alone; and some 700 species of gall-forming wasps have been recorded for North America. Many, but certainly not all, parasitize oaks. Some oak gall-formers are restricted to white oak (Quercus alba), the 2011 VNPS Wildflower of the Year, others may occur on other species within the white oak group, and still others parasitize the red/black oak group.
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Hayden, W. John. "Oak Galls: A Strange Biology Indeed!" Bulletin of the Virginia Native Plant Society 30, no. 3 (Summer 2011): 4-5.