Above: Adult Gall wasps. Left: Cherry gall wasp Cynips quercusfolii. Centre: Oak apple gall wasp Biorhiza pallida - the wings would normally be over the back. Right: Oak apple gall wasp hatching from the gall.
Species in Britain and Ireland
There are 86 species of cynipid wasps in Britain and Ireland. Most are associated with oak trees where various species produce galls on the roots, buds, foliage, catkins and acorns.
One of the first oak galls to be seen in spring is the oak apple, caused by Biorhiza pallida. The pithy, bun-shaped galls, up to 35mm in diameter, often appear before the foliage has expanded. Later in spring, spherical currant galls, 5mm in diameter, caused by the sexual generation of Neuroterus quercusbaccarum develop on the catkins. During the summer, artichoke galls, which are greatly enlarged buds, caused by Andricus foecundatrix, develop on the stems.
In late summer-autumn several types of gall occur on the foliage. Common spangle galls are slightly domed, yellowish brown discs up to 4mm in diameter that can be thickly clustered on the underside of the foliage. This gall is caused by the asexual generation of Neuroterus quercusbaccarum. Other common leaf galls are silk button galls, caused by Neuroterus numismalis, and cherry galls caused by Cynips quercusfolii. The latter are pithy, spherical galls up to 20mm in diameter. Silk button galls are up to 3mm diameter, golden brown and shaped like ring doughnuts.
Several gall wasp species convert developing acorns into galls. Andricus quercuscalicis is the most common and causes knopper galls. These are yellowish green, later pale brown, ridged structures that partly or wholly replace the usual cup and nut arrangement of an acorn. Andricus grossulariae converts the acorns into a mass of blunt spines. These two galls wasps are European species that have become established in Britain in recent years.
Visible throughout the year are marble galls, caused by Andricus kollari, especially on small sapling oaks. The hard woody spherical galls are up to 20mm in diameter and are on the stems. They are initially yellowish green but later turn brown.
Roses, especially wild roses in hedgerows and sucker growth on cultivated roses, are affected by several gall wasps. Diplolepis rosae causes growths known as bedeguar galls or Robin’s pincushion on the stems. These galls have a hard centre but in late summer are covered in moss-like yellowish green or red modified leaves. Large bedeguar galls can be about 50mm in diameter. Diplolepis nervosa causes spherical green or reddish galls on the foliage. These pea galls are up to 9mm in diameter and can be smooth or with several projecting spines.
Gall wasps insert their eggs into the part of a plant where the gall will develop. The plant grows the gall in response to chemicals that are secreted by the larval stage. The inside of the gall has a layer of nutritive cells on which the larva feeds. Most galls caused by gall wasps contain a single larva but some, such as oak apples and bedeguar galls on rose, will contain numerous larvae.
Some of the larger galls, such as marble galls and oak apples, may also contain larvae of other cynipid wasps. These are effectively ‘lodgers’ that live and feed in the gall but do not cause it to develop. Such insects are known as inquilines.
Many oak gall wasps have complex life cycles with alternating sexual and asexual generations that cause different types of gall on different parts of the plant, and sometimes on different species of oak tree. The sexual generation has both male and female adults, the asexual generation has females only through parthenogenesis
. The oak gall wasps in the table below develop on native oaks, Quercus robur
, or, where stated, have a generation on Turkey oak, Q. cerris