FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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OAK GALLS By Kathy Korbholz Have you ever seen a “tumor-like” growth on an oak or a willow? These growths, called galls, are caused by the interaction of a specific insect and a specific plant. Each kind of gall is the plant’s predictable defense against the intrusion of its tissue by a specific irritant.
Gall formation starts as the female wasp deposits her eggs into plant tissue, which causes a minor local swelling around the eggs. Once larvae emerge, their chemical secretions stimulate abnormal cell division. Threadlike fibers grow around the little creature and they continue growing until they are enormous compared with the larva. Galls provide both protection and food for the developing offspring. Soon, the larvae stop eating and change to pupae. When the pupal stage is completed, an adult insect emerges by boring a small hole in the shell-like wall of its home. This plant-insect relationship ends until the adult is ready to lay its own eggs; then it returns to the host plant. Oaks host more galls than any other native tree or shrub in the western United States. More than 200 species of cynipid wasps are associated with California native Oaks. It is not unusual for a single blue oak tree to be “decorated” with the galls of 20-30 species of wasps. Galls can be in a variety of shapes, including pinheads, tiny cups, flatter saucers, stars, sea urchins, clubs, and royal crowns. They are often brightly colored. One of the most noticeable galls, known as the “oak apple,” grows on the valley oak, Quercus lobata. It looks like a tan ball ranging in size from golf ball to baseball. Galls most commonly form on leaves, but can be found on stems, roots, seed pods, flowers, and acorns. Another very visible gall is the bright red-orange with yellow dots, marble-sized twig gall found on the scrub oak, Quercus dumosa. Editor's note added 12/2/2006: Drew Shell has pointed out that Q. dumosa has been reclassified as Q. berberidifolia. |
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This reaction
is so unique, scientists use galls to help differentiate between similar plant
species. Unlike animal tumors, galls do not continuing growing out of control
and rarely harm the host plant. One notable exception is the grape phylloxera
which causes serious damage to European varieties of grapes.