Vineyard insect

Ken,

Research the "Grape Cane Girdler" . You've described its modus operandi to a tee. They cut two rings, a couple of inches apart, and lay eggs inside the piece between, which eventually breaks off & falls to the soil. They are said not to do any significant damage, as the cane readily recovers, but I have seen where they cut off a cluster or two. I don't spray for them, but I do break off the egg-carrying pieces and remove them from the vineyard. Note that I have only 50 vines, not 50 acres!

Excellent. Thanks. First hit was the Cornell site with info that was right on the money. I went back out to the vines (I have 50 exactly, also) and even found two of the insects breeding. Little buggers, they are. I brought in the broken off stems and, using my loupe, could also see the little pockets where the eggs are laid, though I couldn't distinguish them. I also found this guy on the underside of a leaf:

I also found

Ken,

That resembles a tomato hornworm, the kind that chomps on your tomato plants & turns into a Sphinx Moth. A little different in appearance, though, but probably similar in habit & control. When I see leaf damage or frasse, I look for the culprit & remove him. BT spray is a good long term solution for these kinds of worms.