6 to 7.5 mm. A stocky orange and black species with dark wings (except for yellow on the apical half of the stigma). Females have the head, thorax and first two tergites black and the remainder of the abdomen orange. In males the orange is more extensive but at least the clypeus, tegula and pronotum are black. The larva has a greyish green body with a paler underside, orange head and white spots distributed evenly all over.

Unless identified by a recognised expert, a photo is required. If the photo doesn't show the key ID features then in the comments box describe the size and identifying characters you have observed.

Areas where Alder, the larval foodplant, is present.

Flight period: May to July.

The larvae feed on Alder.

Widespread and fairly frequent in Britain.

Status in Leicestershire and Rutland not known.

https://www.sawflies.org.uk/nematinus-fuscipennis/