Identification

Gall wasps are small insects, mostly black or reddish brown in colour. Most have two pairs of small transparent wings but some are wingless. Many of the gall wasps associated with oaks (Quercus spp.) have complex life cycles with alternating sexual (male and female) and asexual (all female) generations that cause different forms of gall on different parts of the host plant.

Galls are abnormal growths that are produced by the host plant in response to chemicals secreted by the developing gall wasp grubs. The galls are often more visible and recognisable than the adult gall wasps or the grubs feeding within the galls.

Size

Adult gall wasps are 2-4mm long.

Distribution

Gall wasps occur throughout Britain, where the majority of species produce galls on common oak, Quercus robur.

Status

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.

Habitat preference

Gardens, hedgerows, oak woodland.

Where to find them in the garden

Mainly on oak where galls form on the foliage, buds, catkins, acorns, roots and stems. Other gall wasps occur on wild roses and sucker growth on cultivated roses. These include the Robin’s pincushion or bedeguar gall, caused by Diplolepis rosae.

Role in the garden

The galls created by gall wasp larvae have little harmful effect on their host plants, so these insects are not garden pests.