I'm not sure that it is a caterpillar. Though it is VERY difficult to tell for certain from this picture, I believe that I see some pointed little true legs on right-hand side of this picture, making that the 'bow' or front. If so, I can count as many as 7 body segments after that, each of which would be supported by a proleg. True caterpillars don't have that many prolegs by a long shot.

So, unless my eyes are deceiving me, this is one of the many, many species of sawfly larvae that appear every year on plants of all kinds.

If you can find others and take improved images, an ID could be more certain.