Voles are rodents that can achieve very high densities and become the main food source for many predators. However, they can get to pest levels, reducing considerably the grass and crop production. To give you an idea, it’s been already several times that someone in the lab come to ask me: what can I do with the voles in my orchard? So imagine if a farmer that depends on a small property have this problem… things can get more serious.
Maybe you want to have a look to this nice article recently published. You can see that voles have a great potential for expanding their distribution to not previously colonized areas, when food and landscape conditions improve.
Halliez, G., Renault, F., Vannard, E., Farny, G., Lavorel, S., & Giraudoux, P. (2015). Historical agricultural changes and the expansion of a water vole population in an Alpine valley. Agriculture, Ecosystems & Environment, 212, 198–206. http://doi.org/10.1016/j.agee.2015.07.006
In these situations voles achieve outbreak levels at the very first moments of their arrival. A farmer that did not deal with the problem in the past, may not know what to do in these situations.
Fortunately he has organizations like the FREDON (http://www.fredonfc.com/) to give them a hand. The range of options is not limited to use bromadiolone. In fact, in France bromadiolone is not permitted at high vole densities because of the high risk to intoxicate non-target fauna.