Mystery guest

I went to the greenhouse this morning to perform the usual morning check and maintenance and found I've been visited by a mystery guest.  This "guest" (or guests?) had demolished two plants overnight.  A beautiful little red and gold primula that had been in full bloom a few days ago was nibbled down to the ground, and a daylilly start that I had repotted a few days ago (it had a beautiful big, healthy root ball) had been dug around, leaving a large empty hole surrounding what was left of the roots, with the soil and other root remnants had been kicked out of the drainage holes in the bottom of the pot and piled around it.  WEIRD!  From my reading since then, I'm assuming my visitor or visitors are voles, since they have to be small enough to enter through the drainage holes and move all that dirt.   ARGH!  Sounds like there aren't a lot of viable options for getting rid of them without a lot of work, since I don't want to poison them and risk poisoning whatever predators might eat them.  Until a month ago, my cat lived in and patrolled the greenhouse, and during that time I never saw any damage, although the previous winter (before the cat moved in), a number of my weaker shrub starts suffered from what I thought was mice-damage (chewing of bark and branches).  When we put in the greenhouse several seasons ago, we dug down about 6 inches throughout, covered with weed barrier, and then covered the weed barrier with

3-4" of pea-gravel, and since then have placed concrete pavers on the aisles to both act as heat sinks and make walking in the greenhouse easier.  Still, it's just a hoop greenhouse, and from what I've read, sounds like that wouldn't have provided much of a challenge for the voles - assuming that's what is doing the damage.  Mouse traps baited with apple will go down tonight (voles supposedly like apple), and we'll see what happens.