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The gall wasp has 2 generations per year. One being sexual and the other agamic (all female, and needs no male to reproduce). The sexual generation causes Blister galls on oak leaves. Whereas the agamic generation causes Silk button galls on the underside of oak leaves. The agamic generation was previously named N. vesicator before it was realized to be two generations of the same species.
The female wasp is winged and about 1.8-2.4mm in length, averaging at 2.1mm.
Its head is a shiny dark brown, lightly reticulated, with large dark neutral brown eyes. The antennae are mid yellow brown and comprise of 14 segments and a fat and hairy pedicel.
The thorax is again a shiny brown nearly hairless and with no notaulices. The wings are clear with pale brown hairs and dark veins. The legs are hairy, mid brown, tapering to paler tarsi and pale joints.
The short and upright Gaster or abdomen is mid brown and smooth, not obviously segmented, but with fine microsculpture. A band of hairs at the back and a large ploughshare beneath.
The male wasp is winged and slightly shorter than the female, and ranges from 1.4-2.2mm in length, averaging at 1.9mm.
The head is dark neutral brown, lightly sculptured with large dark eyes. The antennae comprise of 15 segments and are pale straw coloured with a fat brown scape. The thorax has mid neutral brown side and is darker above. No notaulices and a pale tegulae. The wings are clear with heavy brown hairs and brown veins, with an open radial cell. The legs are pale testaceous coloured with a brown back coxae.
The blocky, segmented and unsculptures gaster is mid brown in colour, going darker underneath, with a thin, long petiole.
the is one inquiline species found in N. numismalis blister galls which is; Synergus albipes.
Common hyperparasites of N. numismalis blister galls include; Aulogymnus arsames, Aprostocetus aethiops, Eurytoma brunniventris, Mesopolobus fasciiventris, M. fuscipes, M. sericeus, M. tibialis and Torymus flavipes.
The agamic generation wasp is winged and measures in at approxiamtly 2.5mm in length.
The head is small and pointed, black and granulated, with medium sized pale brown eyes and pale straw ocelli. The antennae are dark brown with a large rounded pedicel, and consists of 15 segments.
The black thorax has pale hairs and short notaulices at the back. The tegulae are pale. The long clear wings have dark brown viens and hairs. The dark coxae and femora gradually lightening over the rest of the legs with pale joints.
The trapezoid, segmented, bulky (from the side) gaster is dark neutral brown and shiney, but paler underneath and along the ploughshare. The back end is hairy.
The one inquiline species found in N. numismalis silk button galls is; Synergus albipes.
Common hyperparasites of N. numismalis silk button galls include; Aulogymnus gallarum, Mesopolobus fasciiventris, M. tibialis, Pediobius lysis and Torymus flavipes.
More detailed descriptions and identification keys are available from Robin Williams at the British Plant Gall Society.
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