This sawfly has a black head and thorax.  The abdomen is also mainly black but with a red area towards the tip. The antennae have a white band at the tip accounting for about 30% of the total antennae length. The larva of T. colon is brown with a distinctive diamond pattern. T. livida has a similar larvae that also feed on Great Willowherb amongst other plants, but the larva does not have the brown mark at the top of the head, just in front of the first thoracic segment. Also the diamond pattern is rather more distinct on colon with the diagonal cross lines being more obvious.

Often found on Great Willowherb although not exclusively so.

The larva can be seen around about September.

To follow

Widespread but infrequent in southern Britain and very scarce elsewhere.

Rare in Leicestershire and Rutland - the Sapcote record of 26th September 2013 is only the second county record, the first being in 1987.

https://www.sawflies.org.uk/tenthredo-colon/