The best way to stop the Japanese Beetle is to catch them by hands. They are most easy to catch at mid day as they are most active and liminous. Wear a pair of gloves and have a tray ready when you go near them. Use the tray to fetch them if they hop off. To escape from predators they will hop off and disappear to nowhere. The idea of catching them is the break their re-production circle. Japanese bettles lay their eggs in the soil and they can take ages to hatch and are very resistance to weather and pesticides. The re-occuring Japanese Bettles you have might be the eggs laid last circle.
Try to catch and kill them for a week or 2 in a roll and you will completely remove their terrority.
Hope it helps.