One of the reasons voles are so dangerous is that they do most of their damage in the winter, when gardeners aren't paying attention. They prefer to eat in areas where there is plenty of cover, so that a fox or other predator does not discover them and can sneak up on them. Their small tunnels provide some protection, but the snow in winter provides far better protection.
Voles can consume plant roots in peace since they are hidden under soil and a coating of snow. Even when pests aren't eating on plants, they can cause damage by digging near them.1 As a result of all of this winter activity, gardeners who are looking forward to the first flowers of spring are met with dead plants.