Jill - NW Missouri - zone 5
I don't post very often, but wanted to jump in and tell you what works here in our neck of the woods. We have had pretty significant problems with voles in the past, and the very best treatment for us has been to keep vegetation to a minimum. We mow the surrounding hay fields late in the summer so the hay fields winter over with short growth - the hawks do the rest of the work for us. Any time voles live in an area with sufficient cover so their predators can't reach them - they multiply at alarming rates. If you are in a rural or semi-rural setting, keeping all the surrounding areas free of tall growth will help you more than you can imagine. The poisons may be needed short term, but for long term controls, short vegetation is the way to go.
Best of luck to all of you having these problems. They are really destructive little critters.
Painted Acres Farm