At a glance, a person – or a young scrub jay – might think that the sphere pictured above was an acorn. It is a gall on a myrtle oak (Quercus myrtifolia). For a wonderful photo of a juvenile scrub jay – a brown head – ‘mouthing’ a gall, visit Bob Montanaro’s amazing blog.
Galls come in a variety of shapes and sizes. Galls can occur on different parts of plants: Leaves, leaf buds, stems, roots, flowers and fruits. Galls can be caused by fungi, bacteria, nematodes, and insects. Insects are the primary cause.
Many kinds of insects cause galls: Midges, aphids, jumping plant lice, mites, and wasps. Gall – host plant relationships are quite specific. Most gall-makers cause galls only on a particular species (or genus) on a specific part of a plant. More than 50 species of tiny gall wasps have been identified on scrub oaks at Archbold Biological Research Station.
The myrtle oak forms the gall pictured above in response to chemicals produced by the gall wasp as it inserts its eggs into the plant tissues and by the larvae that develop inside the protective structure of the gall. Note the obvious exit hole left by the emerging adult!