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A. solitarius has a life cycle with encompasses a sexual generation as well as an agamic (all female with no need to mate) generation which cause different galls in different locations of the common oak trees Quercus robur and Q. petreae.
The sexual generation was previously named A. occultus before it was realized to be two generations of the same species. The gall, known as the Bisected gall, is common and can be found in the base of stunted catkins. It measure about 3x2mm, is oval in shape and has a ridge on each side of the gall, meeting at the tip, splitting the gall, hence the name, and coloured hairs of red brown or yellow. The gall is found in April, with the gall reaching maturity and the wasp emerging in the same month, usually through an exit hole which is commonly in the side of the tip.
The female measures 1.9-3.2mm averaging 2.8mm
Its head is a slightly glossy, very dark brown/black and granular with mid sized, very dark brown eyes. The antennae consist of 13 segments and are yellow, darkening to the tip. The sensillae and hairs are pale.
The thorax is very dark brown with a semi glossy mesocutum. The wings are clear with pale brown veins and hairs. The legs are translucent deep yellow and lightly hairy.
The gaster (abdomen) is a deep, bulky, non segmented, glossy, deep translucent chestnut and a few hairs are present at the front of the segment. The ovipositor sheath is long and thin with a tuft of hairs at the tip.
The male measures 3.3mm
The head is black and granular with mid sized very dark brown/black eyes. The antennae consist of 15 segments and the flagellum are dark yellow, darkening to the tip.
The thorax is semi glossy black. The wings are clear with pale yellow veins and hairs. The legs are deep yellow with darker coxae and hairy.
The gaster is pointed and not segmented or sculptured, high glossy chestnut in colour.
There is no recorded inquilines or hyperparasites associated with this gall.
The Hairy spindle gall is caused by the agamic generation. It is common and found on the terminal and axillery buds of oak trees from june to september and measures about 9x3mm. The gall is in appearance pear shaped with a pointed tip. It is normally smooth and hairless, although may have red or golden hairs when young and contains a single cell. The gall reaches maturity at the end of august and the adult wasp emerges august and september.
The agamic wasp measures 3mm in length.
The head is yellow brown and granulate. The eyes are mid sized yellow brown and the ocelli are dirty yellow. The antennae are dark yellow brown and there are 14 segments.
The thorax is dark and chestnut, lightly granulate, with a chestnut mesoscutum, with a largely dark top and a yellow brown mesopleuron. The tegulae are chestnut and the wings are clear and the legs are dirty yellow brown.
The gaster (abdomen) is yellow brown and paler underneath. It is smooth and segmented.
Two inquilines, Synergus gallaepomiformis and S. nervosus, and four hyperparasites of this gall include; Eupelmus urozonus, Eurytoma brunniventris, Mesopolobus amaenus and M. sericeus.
More detailed descriptions and identification keys are available from Robin Williams at the British Plant Gall Society.
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