Answer: Based on your description I am not able to tell you specifically which insect is eating your grapevine leaves, however it is about time for Japanese beetles to be emerging and they will eat the leaves.
Japanese beetles are difficult to control, in part because they fly long distances. You can try to repel them using a spray containing neem, or you can handpick them (they are sluggish early and late in the day), or you can spray with carbaryl(the active ingredient in Sevin) but this is a contact insecticide with no residual action. You can also use beneficial nematodes or milky spore disease to kill them at the grub stage although this must be done over a wide area to be effective. Be sure to read and carefully follow all of the label directions on any product you use. The traps are not recommended because they seem to attract more than they catch. Most gardeners find they have to tolerate a certain amount of damage, and some years are more severe than others.
Since I am only guessing that Japanese beetles are the culprit, I would suggest you consult with your local Cornell county extension to obtain a specific identification of the pest. Based on knowing that, you can decide what to do. If a chemical control is needed, they will have the most up to date information on what to use and how/when is best to apply it for maximum results. Also, if there are other problems with the fruit, they can recommend what to do for that.
Good luck with your grapes!
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