Hello Gary,

Thank you for reaching out to Ask Extension with your quesitons about Voles.

Voles can be managed by modifying their habitat to deter them, by exclusion, by trapping, and also with repellents, rodenticides, and poison baits. 

Here is more information about Voles, and these various methods of control:

https://extension.colostate.edu/topic-areas/natural-resources/managing-voles-in-colorado-6-507/

As you can read in the publication above, castor oil is considered a repellent: 

"Another product that is available that is labeled for voles is Ricinis communis oil (Castor Oil). Though its effectiveness is not known, it is labeled as safe around children and pets."

I hope this helps,

Dawn