Answer: You have described perfectly the grapeleaf skeltonizer, a common pest of grapevines in our area. They get the name because they can decimate the plant, leaving behind just a "skeletal" outline of the leaf's veins. They tend to cover a leaf in large groups. The adult form is a tiny blue-black moth. You can handpick any eggs, larvae or adults you see and squish them. Wear gloves and a dust mask because the larvae are covered with stinging hairs that may become airborne. Bacillus thuringiensis (BT) is a biological control for them. (Any nursery should have it.) It's applied to the leaves and after eating them, the caterpillars die in a couple of days. Follow package instructions for application and be sure to apply on the underside of the leaves where they are. The adult moths are harder to control because they flit about sporadically. Good luck!
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