I think it is Tenthredopsis sp., am I right? It looks like T. scutellaris, but I don't have key for this genus and I'm not sure. 15.06.2008, northern Poland in forest, size ~11 mm.
For the moment, we do not have any expert for Symphyta here. I hope, we will get someone in the next year, when the seasons starts for new. And I cannot recommend someone now, unfortunately.