Wow, that’s quite something! Though you seem to have been able to identify the insects on sight, your picture definitely shows the tell-tale “lacy” cutout pattern on the leaves that are the hallmark of the voracious “Japanese Beetle” (Popilia japonica). These little half-inch long bugs tend to be present in the Midwest and eastern United States and like to feast on roses, hops, and, as you well know, grapevines. Like just about all vineyard pests, there really is no way to eliminate them entirely but they can be “managed,” especially in the long-term. First of all, Japanese beetle traps seem to have limited functionality as the pheromone bait, in various studies, was shown to actually attract more beetles then they trapped. Since they are slow-moving and are especially inactive on cold mornings, small infestations can be managed by hand picking them off and discarding them into a bucket of soapy water. Larger areas affected by many of them can be treated with pyrethrin sprays, which are natural, biodegradable insecticides and repellants derived from plants. Insecticidal soap and kaolin sprays have also shown to have good effect as a barrier to feeding.
A really innovative and completely natural way to control Japanese beetles is to implement a longer-term biological control program utilizing one of the insect’s natural enemies, the “milky spore” bacteria. Also known as Paenibacillus popilliae, these bacteria are sold in powder form and are lethal to the grub phase of the beetle’s lifecycle. It’s available at garden supply stores, especially those catering to organic gardening, and can be applied to large areas, so this might be especially suited to your situation. It’s considered harmless to food crops and can be used around pools, in gardens and around pets and kids with no ill effects. The downside is that it can take three to five years to be completely successful as it can only act on the larvae and not on the adult beetles. It won’t be any good against an active infestation, but with time it will definitely reduce the population.
I’m not at all surprised the grapes tasted sour. Without their “photovoltaic cells” (aka leaves) to produce carbohydrates during photosynthesis, it’s not hard to guess why their natural sugar levels were a little bit lower as they approached ripening this year. I hope that you and your daughter have luck managing your Japanese beetle population. There’s no magic bullet but implementing a program now will reap benefits for your vineyard, and her entire yard and garden, in the long run.