Hi,
Against voles try the following : plant jonquils or some "Fritillaria imperialis" in the ground all around the trees. The Jonquil's bulb is poisonous , if the voles touch, they go / to the opposite don't plant tulips .
The big advantage is once the jonquil are in place they do the job for you and they are quit hardy, not to mention that they are beautiful.
You would just need to replace some of the jonquils until you get rid of the voles.
Did I already mention that it helps keep Madame happy to have a nice looking garden with flowers in april ?