WHAT ARE OAK GALLS?
Oak apple or oak gall is the common name for a large, round, vaguely apple-like gall commonly found on many species of oak. Oak apples range in size from 2–5 cm in diameter and are caused by chemicals injected by the larva of certain kinds of gall wasp in the family Cynipidae.
Different species of gall wasp develop inside distinctive galls affecting various structures on the tree. Oak gall wasps have complex life cycles, with alternating generations that are either sexual with males and females, or asexual with females only. The two generations often produce different types of gall on different parts of the tree, and in some species the two generations alternate between native and non-native species of oak.
The appearance of galls on an oak tree can cause alarm and some, such as the spangle and silk button galls might be mistaken for pests such as scale insects. Gall wasps however, cause no long term ill effects to oak trees. Gall wasps that attack the acorns can substantially reduce the acorn crop in some years, which may have consequences for pigeons, jays, squirrels and other rodents that eat acorns during the winter. The future of oak trees is not threatened by galls wasps as there are years when acorn gall wasps are scarce and plenty of acorns are produced.
Some commonly encountered oak gall wasps include –
- Oak apple gall wasp
- Oak marble gall wasp
- Oak artichoke gall wasp
- Common spangle gall wasp
- Smooth spangle galls
- Silk button gall wasp
- Oak cherry gall wasp
- Acorn or Knopper gall wasp
There are about 50 species on common oak and about 25 species on other plants. In recent years several additional oak-associated species have become established in Britain from elsewhere in Europe.