Damaged grape leaves?  Defoliation? Look closer and you may see the grape-leaf skeletonizer.  These destructive caterpillars are yellow, with black and purple stripes, they typically feed in clusters on the underside leaves of grapes, leaving nothing but the veins.  They reach a length of 15 mm when in larvae stage. The adult moth is bluish-black with a yellowish orange collar 12 mm long.  When you see this moth, be on the look out! 

The skeletonizer moth lays clusters of eggs on the undersides of the leaves.  When these eggs hatch the larvae feeds voraciously.  Leaves become lacy, thin, brown and eventually nothing left but the skeleton of the leaves veins.  Defoliation can cause fruit to sunburn and fruit loss and vine can become week due to the reduce carbohydrate reserve storage. 

Organic Grape Skeletonizer control can be taken by applying Bacillus thuringiensis (Bt) or Spinosad.  The earlier the better the control is. Either of these should be applied early morning or evening time when the sun isn't as intense.  Sometimes you may need a second application two or three weeks later to catch any late-hatching larvae you may have missed. If left untreated, the pupa can overwinter creating a larger problem the following year.