You have the larva of the scarab beetle, which does include the Japanese Beetle, and these are too mature for some of the least environmentally abusive controls to help.
Feeding them to birds is one good method of control right now, a more environmentally destructive way would be one of the broad spectrum synthetic poisons.

Here is a link that might be useful: About Scarab Beetle grubs

there must have been quite a population spike in the last year...

dont you have any recollection of the adult hordes????

i would look to attack this problem in the adult phase ....

rather than this stage ... to avoid the egg layers in the first place ...

are they in the remainder of the area??? ... besides this little 2 x 3 foot patch????

you dont mention what the patch is used for ... i would not drench a veg patch ...

if further excavation ... indicates its a yard/garden wide issue ... i would look for a soil treatment ... whether it is liquid or granule .... in no way can i advise what options you have in japan ....

all that said.. plagues come and go .... and odds are .... this is extreme ... and may never happen inside the next ten years ... if you had a county extension office .. as here in the states..

i would recommend you talk to some LOCAL experts ... even a good nursery ... but that said... i would talk to 2 or 3 .... to try to figure out.. who just wants my money .... get their info.. and come back and let us help figure out.. who is scamming you.. and who is honest .... if you wish ...

ken

Thanks for the replies!

This is the first bed I dug up actually, I'll have to wait until this coming weekend to tackle the other beds. Will report back on that.

This particular bed was only growing bush beans and okra. I really didn't notice too many of the adults, although I am sure they were probably there. I picked and squashed the ones I did see. I did pack in the bush beans pretty tight though, so they could have been hiding in there.

My other 2x3 grew basil and hot peppers, and my 2x6's grew pumpkin and hot peppers respectively. Never noticed any leave damage in these, so I am assuming that those were safe.

You mentioned tackling the problem at the adult stage, is there anything you recommend? I have read a bit about neem oil sprays, would that be a good deterent?

You might be able to ID the grubs yourself. You have to examine the distribution of the hairs at the rear end.

This doc has general info about JB:
http://www.entomology.umn.edu/cues/Web/242JapaneseBeetle.pdf

This one shows how to ID (JB is at the far left, 2nd row):
http://iaturf.blogspot.com/2010_08_01_archive.html

Here is a link that might be useful: to differentiate JB grubs from others

If you would like to see more images, do an image search using "ID grubs by raster" (but no quote marks).

As you suspect, many other grubs will also be shown. So verify you have the correct image.

Good luck.

Here is a link that might be useful: ID grubs by raster

filmost, I am surprised that there has been no bird foraging!
I have always seen some activity with far lighter infestations than the picture indicates.
This piques my interest because it might indicate an environment that is inhospitable to bird life.
Of course this is all conjecture on my part.