A. This is the grape leaf skeletonizer, a dark blue black moth that lays its eggs on the underside of grape leaves. They usually begin laying eggs around April and that’s when control measures need to begin.
|Grape leaf skeletonizer adult. A blue black moth.|
The egg hatches and out comes a larva, fitting your description. This worm or larva begins to skeletonize, or nearly defoliate the grape leaves leaving behind the veins of the leaf. Hence the name, grape leaf skeletonizer.
|Grape leaf skeletonizer larva or worm eating the surface of a grape leaf. This is not my picture but I can't find who sent it to me so my apologies to whomever it was.|
Control is pretty simple and effective with organic pesticides containing Bt or Spinosad. Sprays should be applied in April prior to egg-laying or just after.
If you follow my blog via my newsletter or my tweets on twitter, I announce when to make these applications like these ahead of time. Otherwise, just mark your next years’ calendar and make this application some time in the first two weeks of April, depending on the weather.
Make sure you spray the undersides of the leaves, not just the tops. The spray will work now as long as the larvae, or the worms, present. It doesn't work on the adult moths. Your grape vine will put on new leaves to replace the damaged one so just be patient.