I've used a product called Triact 70, homeowners use it as neem, but it has a strong "garlic" odor (garlic is listed under scent on the MSDS sheet). Anyways, I sprayed about 50 roses with it and for almost a whole week with no rain, there were no beetles on them, not even in the flowers. They tore up 5 or 6 lindens across the aisle about 15 feet away. Based on this I would say give it a try.

The problems I have had is the rain, or any other overhead irrigation, plus the smell which on hot days was overpowering.

Garlic sprays are known to deter earwigs, and maybe could aid in detering the Japanese Beetles. I've not found anything, yet, that says it will but given that this spray would be less of a problem (other than your yard smelling like a Pizza Shop) than most anything else that people spray around to control "bugs".

We used homemade garlic spray on hibiscus one year. It did the job to repel them. But it did stink and had to be reapplied fairly often. We had to be sure to spray all new buds and newly opened flowers as well as the leaves.

Stompede, that 'aroma' of neem oil is one of the drawbacks, isn't it?

rhizo, neem is terrible smelling at first. I have not tried this, but a guy I know elsewhere in the trade says neem is less effective as the smell fades, saying in summertime heat neem may only be effective for 2 or 3 days for beetles.