There are around 70 different gall wasps of oak in Britain. 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 are harmless although they can cause alarm and some, such as the spangle and silk button galls might be mistaken for other insects such as scale insects. Gall wasps cause no long term ill effects to oak trees. Gall wasps that alter acorn growth can substantially reduce the acorn crop in some years, however, the future of oak trees is not threatened 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 (Biorhiza pallida) causes flattened rounded galls up to 40mm in diameter to develop on twigs in spring. The galls have a spongy texture and are brownish white, tinged with pink. Males and females emerge in mid summer and eggs are laid on oak roots. The next (asexual) generation produces marble-like galls on the roots, from which females emerge in late winter to lay eggs in buds on the twigs.
Oak marble gall wasp (Andricus kollari) causes hard woody spherical galls up to 25mm in diameter on the stems. Marble galls are initially green but later become brown and can persist for several years. The alternate generation causes rather small and insignificant galls in buds of Turkey oak, Quercus cerris.
Oak artichoke gall wasp (Andricus fecundator) lays eggs in buds at the shoot tips, which become greatly enlarged during the summer. The next generation in spring develops small hairy pale green or brown galls on the male catkins.
Common spangle gall wasp (Neuroterus quercusbaccarum) causes yellowish gingery brown disc-like galls 3-4mm in diameter on the underside of oak leaves in late summer-early autumn. The galls drop to the ground in autumn and females emerge in spring to lay eggs on the male catkins. The next generation causes spherical fleshy galls 5-6mm in diameter on the catkins. These galls are yellowish green or reddish in colour and are known as currant galls.
Smooth spangle galls (Neuroterus albipes). The galls are saucer-shaped, yellowish-green or pinkish-red discs without any hairs and up to 4mm in diameter. They form mainly on the underside of the leaf with each gall containing a single larva. Pupation takes place during the winter while the galls are on the ground and females emerge in the spring. They lay eggs which give rise to small, oval, green galls which are attached to the leaf margins or the catkins. Males and females emerge in May-June.
Silk button gall wasp (Neuroterus numismalis) creates 3mm diameter golden brown discs with a pronounced central depression on the underside of oak leaves in late summer-early autumn. The next generation in spring forms small oval galls on the male catkins and leaf margins.
Oak cherry gall wasp (Cynips quercusfolii) forms spherical pithy galls up to 20mm in diameter on the underside of oak leaves in late summer-autumn. The galls are yellowish green or red and often remain attached to fallen leaves. The spring generation forms inconspicuous galls in oak buds.
Acorn or Knopper gall wasp (Andricus quercuscalicis) became established in the UK during the 1970s and is now widespread. Eggs are laid during early summer in the developing acorns of Quercus robur. Instead of the normal cup and nut, the acorn is converted into a ridged woody structure in which the gall wasp larva develops. The gall is initially yellowish green and sticky but later comes greyish brown. The next generation forms inconspicuous galls on the male catkins of Turkey oak, Quercus cerris.