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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 Andricus corruptrix agamic, A. kollari agamic, A. lignicola agamic, A. lucidus agamic, A. quercuscalicis agamic and Biorhiza pallida sexual. Its flight period is from february through to october.
The female has a head and body length of 1.3-3.4mm with an average length of 2.5mm
The head is a black/brown vertex fading more or less to a brown, though some are darker than others, some have only brown around the mouth but never yellow. The eyes are large and dark. The antennae are mid brown with 14 segments, slender but not tapered, and a dark scape.
The thorax is all black and with pale hairs. The notaulices are full and deep. The tegulae are mid brown which lead to the wings which are clear with neutral brown veins, inconspicuous hairs and a closed radial cell. The legs are brown, gradually becoming paler towards the tarsi and with paler joints.
The gaster (abdomen) is a single main segment which is dark brown and glossy and clearly punctate There is a large ploughshare underneath.
The male measures 1.3-3.5mm with an average of 2.2mm.
The head is often bright yellow with a black vertex and contrasting large dark eyes. Some however are like the female with a black vertex which blends into brown and maybe only showing around the mouth. The antennae are normally all translucent gold but sometimes darker, with a darker scape.
The thorax is all black with pale hairs. The notaulices are deep and full. The tegulae are mid brown and the wings are clear with at the basel halves, pale brown veins, and a closed radial cell. The legs of yellow headed insects have the first two pairs of legs translucent gold and the rear pair are brown with paler tarsi. The leg colour tends to darken with the colour of the face.
The gaster has a single main segment which is clearly punctate and dark brown a translucent rear edge.
More detailed descriptions and identification keys are available from Robin Williams at the British Plant Gall Society.
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