A sawfly bites for 1-4 points of damage, and on the following round it causes 1-4 points of damage automatically due to blood drainage. Afterward, it attempts to bite again.
Fair enough. Let's go with that.
Attach and blood drain like a stirge?
All suggestions are fine.
CR 1, I think. It's substantially better than a stirge, but not all that much.
Maybe advancement: 3-6 HD (Medium) or -.