Grubs

I found a large white grub within a rotting (sweet gum) root in my garden ( Huntington, New York) It is approx 4" long x 1/2" wide, white, jaws?, has ribbed bands around its body except for 1/2" of tail. No indication of legs. I have kept it in a large container filled with soil and lots of the wood in the hopes it will pupate. What is it the lava of? Thank you. B. Pugsley