Size 5 to 7 mm.  A black and orange species. The thorax, head and wing stigma are black whilst the abdomen is mainly orange but with tergite 1 black. The saw sheath is also black. There may be further black marks along the top of the abdomen and on the apical tergite but these are variable. The legs are orange but darker at the base of the tibia and on the tarsi. Only females are known in Britain and Ireland.

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.

Damp areas where Soft-rush occurs.

Flight period: May to July.

Larvae feed inside the stems of Soft-rush.

Widespread but uncommon in Britain.

Rarely recorded in Leicestershire and Rutland.

https://www.sawflies.org.uk/eutomostethus-luteiventris/