Synergus variabilis (Mayr 1872)

This wasp is a member of the family Cynipidae but is not a gall causing wasp, like many others. In fact it is incapable of forming a gall of its own so acts as an inquiline. This is basically a wasp that lays its egg into an existing gall at an early stage to share the same food store as the gall causer. Although the causer is not directly targeted, the fight for food resource can sometimes cause the death of the gall causer, especially if the inquiline is sharing the same cavity as the causer.

Synergus variabilis is an inquiline of Aphelonyx cerricola agamic. Its flight period is from march through to september.

The female has a head and body length of 1.3-2.2mm with an average length of 1.7mm
The head is a dark chestnut colour, which fades paler around the mouth. It has a black vertex and the striae radiate from the mouth to the eyes which are large and dark, contrasting with the head. The ocelli are mid brown and translucent. The antennae are testaceous yellow with 14 segments.
The thorax is black and hairy, except for the pronotum (without pronotal collar) which is a contrasting chestnut colour. The notaulices are variable from a steeply impressed short triangle to an only faintly indicated longer line. The tegulae are translucent pale brown which lead to the wings which are clear with light yellow veins and a closed radial cell. The legs are testaceous yellow with the rear, and sometimes the mid, coxae being dark brown and dark claws.
The gaster (abdomen) is a single main segment which is smooth and hairless but not high gloss. It is very dark chesnut tinged black with puncturations on the hypopygium but no punctuations on the main segment.. There is a short, broad and visible vetral spine.

The male measures 1-2.1mm with an average of 1.6mm.
The head is dark testaceous yellow fading to paler around the mouth. Larger specimens may have chestnut traces. The vertex is again black and the stiae radiate from the mouth to the eyes. The eyes, like the females are large and dark and contrast with the head, and the ocelli are a translucent mid brown. The antennae are all yellow and have 15 segments, of which the third has an excavated inner margin and is up to twice the length of the following segment.
The thorax is black and hairy, and the pronotum is without a pronotal color. The notaulices are variable from a long, feint indentation on the cuticle to a more sharpley defined short triangle. The tegulae are light brown to yellow and the wings are clear with light yellow veins and a closed radial cell.
The gaster has a single main segment with no punturations and a very dark chestnut tinged black in colour.

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

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