We have voles. We suspected this after the snow melted, and saw tracks running through the yard/grass. We researched them on-line, and figured it out. In the spring, we bought a trap. We put a sticky trap-paper in the bottom, and figured out their "runway," and set it in their path. We have caught 5. We planted 4, 4x4 garden squares, and planted various vegetables. First, during the week it was really hot, something nibbled off our pea plants. Over the last week, I have noticed nibbles out of our tomatoes. Today, literally in 1 day, I noticed that the tops/greens of our carrots have been lopped off. I noticed a hole in the ground on the outside of the boxes, and a matching hole right on the other side, inside the garden box. I took a stick to close up the hole, and 2 little voles came running out. I moved the vole/vermin trap near the exterior hole, hopefully in their pathway. We live next to a pond. I hate to use poison because when the corpse decays, the chemical will contaminate the ground and potentially our lake/large pond. Please advise how to get rid of voles!!! Thank you.
If you don't want to use poison bait, then all you can do is use mouse traps. Here is our publication on voles: http://www.extension.umn.edu/garden/yard-garden/wildlife/voles-in-the-landscape/