Some cats are easily capable of killing 5-10 rodents a day. However:

1) Fewer pet cats than you think are actually any good at mousing. Successfully catching and killing rodents has a strong learned component which many pet cats lack.

2) Cats are not just predators. They're also small enough to be prey, and they evolved with the nervousness and safety consciousness of prey animals in addition to their predatory instincts.

3) Most cats do not have blanket trust in human beings the way many dogs do but need to form trust with individual people over time, and they are territory-oriented and need to be in a place they consider their own territory to feel safe.

Therefore, you can't just plunk a cat down in unfamiliar territory belonging to a person it doesn't trust for the night and expect it to go to work cheerfully decimating your local pest population. The cat will almost certainly freak out and immediately run away and end up dead or just hide all night terrified and doing nothing. Even if it sticks around and goes exploring, there's no guarantee the cat will be any good at catching rodents. It could just as easily be a dud at hunting, or it could prefer to decimate other animals you don't want dead.

In the old days, if you needed mass rodent extermination, you'd call in a pack of trained dogs, specifically terriers, which were originally bred specifically to be rodent-slaughtering machines. But today, your best bet, as Dave says, is an exterminator. More reliable and probably cheaper and definitely easier to find than someone with a pack of trained, working terriers. Certainly easier to watch working than a pack of dogs joyfully tearing rodents apart alive.

kit...whatever you decide on...do not be nice about it. These varmits mean business...and they want your tomatoes!

My neighbor has 3 outdoor cats, which are hunters judging by the dead animals I find around, but we still have to trap for gophers here and there. They have put a dent in the gopher population, but cats alone won't end your problem.

I have learned a lot from all of these comments. I am persuaded not to get a cat. I have also been hearing about the vole/chimpmunk/rabbit explosion from so many people lately that it seems it's an outlier year (hopefully- and not a harbinger of the new climate). The most important thing I think I can do is to fill in and turn over EVERY hole EVERY time i see one, poking holes in the direction the tunnels point. And I will put out a dozen snap traps at a time.

Place the traps next to the vole "runs" (paths they wear into the grass/weeds). Put something over the traps, like a box with a weight on top or a cheap planter with a weight on top (weight to keep them from blowing over), and cut 'mouseholes' only just large enough for voles to get in down at the bottom where the box meets the ground. That way, you can bait the traps with highly desirable food like peanut butter without worrying about killing birds in your traps.

You can also buy granular castor oil based deterrent (usually marketed for moles but will also work on many other creatures). You water this stuff into the ground, and it irritates the critters living in the soil with its bad odor and other qualities and encourages them to move elsewhere. YMMV, as with all DIY strategies. Some people report great success, others report disappointment.

I looked for castor oil and only found it in unscented versions! Where can i find the real deal?

Or, seriously--if the problem is so bad, call an exterminator.

RE: population explosion, note that last year was a "mast year" for acorn oaks. This could be why there are more chipmunks and rodents this year and why they might be bigger pests in the garden than usual, as your garden could be a more desirable and necessary source of food for them as the greater-than-usual population feels the food pressure this fall without the acorn jackpot that produced it.

I'm seeing effects in that the local fruit trees (apple trees that are mostly neglected, etc.) were totally stripped bare of fruit very early in the season, presumably by a very hungry and unusually numerous squirrel population, and now my tomatoes are occasionally raided by local rodents, which is unusual since there's usually so much for them to eat around me.

kitasei: I looked for castor oil and only found it in unscented versions! Where can i find the real deal?

Are you trying to find and use castor oil marketed for human use? It will not only be ineffective, but it will be wildly expensive to get as much of it as you need. As I recommended, buy the animal repellent that is castor oil based and designed to penetrate into the ground when you water it in. There are a number of brands on the market.

I had no idea, gorbelly. Thanks.