What to cover underside of shed subfloor
Normally a shed does not have an insulated floor but since yours is going to be insulated i would not leave it open for critters to nest in or chew up and take away for a nests. 

I would cover the bottom of your joists with treated plywood or Metal lath.

The plywood keeps out bugs and squirrels etc, the lath keep out only things larger than the opening of the lath you choose. 

Building wrap (tyvek?) or polythene is no match for squirrels. Unless you put some lath around the structure as skirting, i would bury the bottom of it in a trench several inches deep to keep critters from burrowing under it. 

Mice a voles are different, they can chew through plywood, so you may need both PW &amp; lath with very small openings.