Confidence: I'm as sure as I can be.
Notes: Black Dolerus sp. (s.g. Poodolerus) may be tricky to identify to species from a photograph, however good. Larvae of the black Dolerus species mostly feed on grasses, a couple on sedges. A quite likely candidate, given the slender build and longish antennae of your sawfly, would be Dolerus aeneus.