Stem borer on Parthenocissus

This is likely to be a species of Phytobia. John van der Linden's notes on finding these larvae: "There's no external evidence as far as I can tell. The larvae I have found have all been in relatively young, slender, flexible segments of the vine that are green inside but also have distinct bark (gray and peelable). Most of these segments are maybe one or two years old, I think. I haven't found any larvae in the freshly growing tender shoots (the totally herbaceous parts that are all green, outside and inside). However, I haven't found larvae in the central pithy parts of thick tough old vines either. In fact, when I look for them I have been ignoring any vine segments thicker than a pencil."