I use rugs to keep weeds off garden paths and under them are non stop vole tunnels. I have no interest in killing these creature, just don't want them eating dahlia, lilium and day lily bulbs or roots. I have removed the rugs. Thinking I will use a castor oil brew on lilium, but would need caster oil by the gallon to treat my whole garden.
Suggestions? http://wdfw.wa.gov/living/mole is an informative mole (or vole ?) site.

You don't want them dead?
The only alternative to a cat and/or traps would be to wrap Wire mesh around the bulbs or plant them all in above ground pots.

At my house, voles also eat iris, beans, squash, watermelons, even pokeweed!

Also, Irish potatoes, sweet potatoes, and any vegetable seeds that I attempt to direct plant.

Personally, I do want those evil things to die!
And.... Apparently they taste so nasty that when the cats catch them.... They leave them lying around.

We have a relatively crowded mixed perennial garden and there's been evidence of voles/meadow mice in it (e.g. tunnels under the snow). The only resulting assumed vole damage we've ever noticed, however, has been to the (non-poisonous) spring bulbs. Apparently, voles can mine bulbs out and consequently be more of a problem here (for bulb loss) than squirrels. I'm really not aware of any vole damage at all to any other of the wide variety of perennials we grow/have grown.

Since, a chipmunk moved in, I'm thinking that our vole population has gone down. Chipmunks are quite tough characters and I've already rescued one fledgling bird from it. They also hibernate during winter. I'm guessing that the single chipmunk (chipmunks are solitary) is eating fewer bulbs than families of voles.

I have read of evidence that vole tunnelling improves soil quality (releases nutrients along the roofs).

A friend of mine (David Tomlinson, Merlin's Hollow) protects his spring bulbs by putting warfarin in beer cans, after he's increased the size of the hole in the cans. He says that nothing else can get into the cans except for the voles he's targeting. He also says this never eliminates all the voles but does keep their numbers down. Personally, I've just continued to add more spring bulbs (especially the crocuses we love) each fall.