What is a bee grub?
Many bees and similar insects go through a larval stage. At that point, they resemble grubs. It would be unlikely for you to encounter them, unless you disturb a nest. If that occurs, I suspect you will have more pressing and immediate concerns than insect identification. Here is a link to some good info on bee life stages-
http://articles.extension.org/pages/21751/bee-brood-basic-bee-biology-for-beekeepers