This particular gall – the wool sower gall, is produced by a tiny wasp called Callirhytis seminator. There are many different types of galls but essentially they are all growths on various parts of plants, such as leaves, twigs, stems, roots, etc. They can be caused by flies, mites, moths, and gall wasps, but sometimes they are a response to injury or disease: Trees are affected by viruses and bacteria as well. Galls are produced when an insect larva releases a chemical that forces the host plant to form a gall. The larva will use this gall as food and shelter.
The insects and the mites are quite particular about not only the plant they seek out, but the location on the plant. The wool sower gall is placed on the twigs of the oak tree. I’ve seen other types of galls on the white oaks around my house, and many are attached to leaves. Oaks are important trees that provide shelter and food to over 500 species. Many of these are gall producing organisms so it should be no surprise that the genus Quercus is host to hundreds of types of galls.
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