Myrl,

from the post, i didn't gather that they were an infestation as much as
a nuisance. castor oil down the holes will in fact repel them. if the
problem is much bigger, a poison called Rozol, which comes in paraffin
blocks, seems to be the best. Push two inch pieces of the rozol back
into tunnels where it will stay dry - they won't eat it if it gets wet.

Voles are a Microtus species, there is an eastern variety and a
western one. The western variety feeds above the ground while the
eastern feeds on the roots and stems just under
the surface. You are correct when you say they can infest an area.
Voles reach sexual maturity in about 40 days, they have 4-5 litters of
3-6 babies per year, and they feed year round. Do the math and you can
see how a population can skyrocket.