Google "voles hosta" and you'll find more remedies than you can shake a stick at, most of them imperfect. This posting sums up most of the tips:
You know you have voles if your entire hosta clump disappears into a subterranean hole, or returns in the spring as a fraction of it's last years size. Voles (herbivorous mole relatives) are easy to control if you follow all three steps. First vole bait (rat poison) must be applied every 10 feet through the infected area. If you can find the runs, then apply the material there. If a run isn't evident, then put the material on the ground (they find your hostas don't they). Secondly, cover all of the bait, as the voles only feed in the dark. We like to use clay flower pots (it makes visitors ask questions). Thirdly, repeat the procedure in 2 weeks. This doesn't mean three or four weeks...TWO. If you follow this procedure, you will get rid of your vole problem.
It sounds too good to be true, and I despise rat poison, but maybe that's what I'll have to do. You'd think the neighbor's cat, who's a good hunter, could help me out here, but apparently not. She spends a lot of time in the driveway staring at the junipers - if she'd just turn around she'd find some delicious voles to eat!!
The other suggestion is to pot your hostas and plant them pots and all, or add pea gravel to the bottom of the planting hole. Anyone have any other ideas??