are you positive it is japanese beatle and not just a closely related beatle. The true test for a Japanese beatle is it will raise its back legs in the air if you approach it. If it does not do this then you have another type.
If that is the case then it is a quarantine pest in the UK and you should report it to your local plant health office who can positively identify it for you.
It is possible that it is a **** chafer beetle, or may beetle. There have been a lot of these around lately, they buzz around very loudly in the evening, and their grubs appear in the soil in the garden. Google image '**** chafer' and you will get a fairly comprehensive selection of pictures - they are quite harmless.