Why aren't you using traps? They are very effective, IMHO, I've had dozens go in a trap per day, and we only have very light J.B. populations in my part of Oakland County, unlike, say, the East Side around St. Clair Shores, the Grosse Pointes, etc, where I understand they can be very bad.

I would go buy a couple of traps for each vine, they should trap the majority of the offenders.

Thank you for replying, denninmi. I haven't used traps because I've read anecdotal evidence that traps are a mixed bag. Yes you are definitely right that they trap many beetles. They work great, but do they work too well that they bring even more beetles to that area than you have traps for?
I would prefer if possible to deter the beetles from coming into my yard rather than drawing them into it with traps.

Thanks again for your response. I will think about what you wrote.

The obvious solution is to spray your grapes with an insecticide that kills by ingestion.

I have used traps, sprays, covers and so on to control these pesky insects in Wisconsin.... we have severe infestations of them.... they defoliated my cherry tree in one day! Needless to say, since then I rely mostly on netting to prevent the contact with them. Other than that, I have also tried another method, though it is not fun. If you manage to collect enough beetles from your traps, and have an old blender, Grind those suckers up with lots of water, strain the emulsion, and use a pump sprayer with a spreader sticker added to the water. They do not like the smell of dead japanese beetles, and tend to stay away from the plants sprayed with this.... it has to be applied often if you have lots of rain ( a downfall, depending on the size of your vines) but given the relatively dry summer we have had at least, the re-application has only had to be once a week or so. Some long term solutions include adding milky spore to your lawn, this takes a while for the fungus to build up in the soil, but it kills the grubs before emerging as beetles, so I have had success with this. In all honesty, a combination of all of these will do the most good, as well has hand picking in the morning while they are inactive. I hope that this helps you out!