foraygardengirl said: If I can just get this group out from under my tree for the winter, I can take steps to repel them starting in the spring.
Yes.
I'm thinking that winter is probably well underway in MN and that your chances of encouraging the voles to relocate of their own volition (
pun intended) at this seasonal point are nil.
I suppose if your ground is not frozen yet you could try habitat destruction at the base of your tree, but all the potentially effective eviction extremes I can think of --like digging out all of their dens and decapitating them with the shovel or flooding them out by turning it into a a swimming pool--could threaten the tree as much as the voles (or more)
Your prized tree is in danger of at least some amount of vole-induced damage, though, and to protect it you will probably have to resort to the gruesome options for exterminating the voles.
Trapping--if you can even get them to go in a trap to get caught--will only eliminate the few voles that you can trap. It would be great if all the pregnant mommy voles would venture into the traps but they don't generally cooperate like that.
So your best bet is probably something toxic to kill the voles and depending on your choice of poison and bait and their bait acceptance, you may have to prebait without the toxicant first and then offer multiple--successive feedings with the toxicant to get all of the voles.
I'm sorry Jeanie--good luck with that!