Sometimes you see something, and you're just not sure if an insect did it or not.
This is one of those things.
Discolored/dead/dying stems of willow, Salix sp., with a raised scar circling the stem in an irregular way right where the discoloration begins. My bug radar is going off...but I suppose the girdling could be unrelated to insects, too. That would be so much less exciting.

Similar girdles, same plant species, same place, same day:
(A)
(B)
(C)
(D)

In (A) and (B) note how the girdle scar appears to be a tunnel encircling the stem just underneath the outer bark.

Normally I would not post pictures of so many different individuals. In this case the sign is subtle enough that it seemed it would be best to show several instances of it, in order to help folks understand what it looked like.

There are fungal diseases that cause twig dieback in willows, and it's certainly possible that's what's going on here. However, the presence of the girdle-like scar surrounding the twig at the point where it changes color makes me think the sign shown here may be insect-associated.