Anti-Vermin Barrier

the caster creates a 10X10X10 invisible force field that repels nonmagical insects, rodents, spiders, snakes, worms, and similar vermin of CR.0. The spell has no effect on giant-sized versions of these creatures. The barrier affects summoned creatures, such as those called by a summon insects spell. Any vermin within the area of effect when the spell is cast are not affected; however, when these creatures exit the area, they cannot return. 

At a higher spell lv. the CR. Rating increases by 1. Max at lv 9 is CR.5