Not very organic, but then I am a traditional grower. The only thing that I have found that works under your conditions are the gopher gassers. Of course you have to find thier runs which can be quite difficult under plastic. Castor beans placed in their runs or tunnels will slow them down. Note there are several species of voles, some work almst completely under ground while other will have surface runs. traps will work on those like meadow voles which travel a lot on the surface.
http://garden.org/ideas/view/f...
