Tenthredopsis sordida is some 10 to 12 mm long. It has quite a lot of yellow markings and the one in the picture is a male. In males the legs and antennae are darker than in females. Females have yellow legs and almost completely yellow antennae. In both sexes the edge of the forewing is black, interrupted by a very clear yellow dot, which actually is part of the pterostigma. The adult sawflies are seen from mid April to the beginning of July, sometimes a little later. They hunt for other insects, but are seen feeding on nectar regularly.
We couldn't find much useful information on this species. It is suggested the larvae live on -among others- hornbeam. Like all other sawfly larvae they are strict vegetarians.