Rams-horn gall wasp Andricus aries

A recent arrival in Britain, this gall was first recorded in Maidenhead Thicket, Berkshire in 1997. The first colony of them was recorded at Hamstead Heath in 1999. It is now common in most of the South East and rapidly spreading nothwards. At the current time it is found as far north as Cheshire's northern border. Any sightings north of this would be appreciated by the British Plant Gall Society.

It is found on lateral and terminal buds of oak trees (Quercus robur and probably Q. petraea) from july to december. It has a globular base with 2 long, slender, tapering projections, which may sometimes be only points, or may fuse together as one. Its shape may be variable especially when attacked by inquilines, eg Synergus spp. which causes an enlargement of the bulbous area at the base. The gall is green at first but as it matures, goes brown, hard and woody. The galls are unilocular (containing a single larvae), but there may be upto 3 galls per bud, and can grow to 7mm across and 30mm in length (including horns).

As far as is known, there is no sexual generation, only agamic females.

The gall matures in August and the wasp emerges from august to october or april/may the following year. The wasps head is small, hairy, pale chestnut gold in colour with a reticulated vertex. The eyes are medium sized and black. The antennae have 13 segments, is orange brown in colour and covered in inconspicuous hairs.
The thorax is orange brown and hairy. the propodeum is dark brown. The legs are translucent orange with a lot of white hair. The wings are clear with pale veins and hairs and with an open radial cell.
The gaster (abdomen) is bulky, glossy orange brown and segmented. Bands of long pale hairs are present on the edges of the segments. There is a long, slender ventral spine with sparse hairs.

Inquilines of Andricus aries include; Ceroptres arator, Synergus albipes, S. gallaepomiformis, S. pallicornis, S. pallidipennis, S. reinhardi, and S. rotundiventris.

Hyperparsites of Andricus aries include; Megastigmas dorsalis, Ormyrus nitidulus, Sycophila biguttata and S. flavicollis.

More detailed descriptions and identification keys are available from Robin Williams at the British Plant Gall Society.

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