Voles, in particular, like to gnaw the roots of young fruit trees and can cause severe damage. The tunnels are usually flat and just below the surface of the ground. The overlying soil is slightly broken upwards.
A good way to combat voles is by setting a vole trap. Bait the trap with carrot, apple or celery and place this inside the tunnel.
A particularly convenient method of combating voles is to use granulated bait. Ask at your garden centre for a suitable product.