Plants affected
Oak trees, especially the native oaks, Quercus robur and Quercus petraea.
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Symptoms
Bizarre structures develop on the foliage, buds, acorns and other parts of an oak tree, mainly in late summer but some develop in the spring. These include:
- Artichoke galls (see pic 1 above): swollen buds up to 15mm long and 10mm in diameter in late summer to autumn.
- Marble galls (see pic 2 above): green or brown spherical woody galls of up to 25mm diameter develop from buds and can be found throughout the year.
- Knopper galls (see pic 3 above): acorns become ridged and distorted, at first yellowish and sticky, later becoming brown and woody.
- Currant galls: spherical yellowish or reddish galls, about 6mm in diameter, that resemble bunches of redcurrants in the spring on the catkins.
- Oak apples: up to 40mm across, rounded, whitish-pink or brown and corky, the most familiar example of a bud gall seen in early spring as the tree comes into leaf.
- Common spangle galls: these are yellowish brown discs up to 5mm across. They are often mistaken for scale insects; they occur, sometimes densely, on the undersides of the leaves in late summer.
Cause
More than 30 species of gall wasps occur in Britain on the common oak, Quercus robur. Each species lays its eggs in a particular structure on the tree - the roots, bark, buds, leaves, catkins or acorns. Chemicals secreted by the gall wasp grub alter the normal cell division process and cause the plant to produce gall tissues that enclose the grubs.
The grubs feed on a nutritious layer of cells within the galls, where they eventually pupate and turn into adult wasps. These insects have complex life cycles involving alternating sexual and asexual generations that often cause different forms of gall on different parts of the tree.
Control
None is necessary. Gall wasps do not cause any serious damage even when abundant. In some years many acorns will be converted into knopper galls, but in other years galling is at a low level. Over a period of time sufficient viable acorns will be produced to ensure the future of oak trees.