Are Japanese Beetles often times the grub worms that one finds in their grass soil in Spring?

I don't think so they are not the grub worms. I farmed for years and had grub worms then. That was in the 60's. The beetles start here about oct each year. And they are a big problem here.The beetles here have been here for 13 years. You can put out the grub worm granules but they are really hard to kill out. They were put here to help the crops. Brought her from japan. Hope this helps you some.