Adult 7.5 to 8 mm. One of four Nematus species in Britain and Ireland characterised by the enlarged basitarsal segment. This species is shining in the centre of the upper part of the mesopleura such that the punctures are not obscured (N. brischkei has fine microsculpture here). The femora are mainly yellow with any infuscation confined to the base and extreme apex. Males are rare.

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 Alders are present.

Adult flight period: Bivoltine. May to June and July to September. 

Nematus alniastri larvae feed on Alders.

Quite widespread, particularly in the southern half of Britain.

Rarely recorded in Leicestershire and Rutland.

https://www.sawflies.org.uk/craesus-alniastri/