Much to my delight, the Elderberry I got last autumn is going to flower, and has also attracted some attention from mother nature. Taxonomy on sawflies doesn't seem to be all that perfect so I'm left to conclude this little worm to be a
Macrophya species.
Sawflies are the odd cousins of the Hymenopter order (Ants, Bees, and Wasps) who have a caterpillar-like larval stage that even feeds on host plants, though some are parasites on beetle larva. The adults are wasp like in appearance and it's this unusual larval stage among a few other things that separates them from wasps.
It's just the two larva that I saw on the plant, so they shouldn't eat too much.
