it is an excellent situation for the voles, having the food right into
their cover. If you can't get a bunch of snakes, who will go under the
tarp to get them, you should get human urine by the gallon. It is an
excellent repellent, though it works best to prevent colonization of a
virgin place. You should also dig where a dead plant is, find the
tunnel with your fingers, drop a piece of rat poison, and close.