Neem oil has been around for a looong while. It is reputed to do all sorts of helpful things, including repel Japanese Beetles. I'd suggest careful research, contact with (in the USA) local Cooperative Extension offices, and appropriate application, just for starters.

There have been some concerns about its effects on humans, other animals, water supplies, and beneficial insects and arachnids. To my knowledge, when correctly and appropriately applied, no problems with these issues have been verified.

I haven't worked with it yet because I can't afford it. It is available here in Maine at farm supply, hardware stores as well as various garden centers. Experts in IPM (Integrated Pest Management) have generally good things to say. For example, the University of Connecticut has this to say about Neem

When evaluating pesticide information on the internet, pay attention to the source. Commercial sites selling Neem products are not always the best sources of fully disclosed and explained information.