When conditions are favorable, voles are perhaps the most prolific of all rodents. There are cases of meadow voles producing up to 17 litters in one year for an amazing 83 offspring. Assuming that her offspring also mated and reproduced, several thousand voles could result from one female in a years time. You can see why this could be a big problem in our lavender field.
Voles are herbivores. The stems and leaves of grasses comprise the majority of their diet, but they will also consume other green vegetation and fruits. Voles do not hibernate and are active throughout the year. During severe winters and snow cover, when green vegetation is scarce, voles often girdle tree trunks and roots killing or damaging trees and shrubs.
I have been to a number of talks on controlling this little pests. One suggestion it to plant with chicken grit which can be found at your local farm store. Because voles don't like to dig in the grit they will stay away. This is great if you are planting tulips or any other tubular or bulbs. I don't think it will help the lavender.