Forecasting the damages by voles on plant crops depends on a good forecast of vole population density. Biological work in Vendee from 1959 til 1968 has furnished the fundamentals of a population model. Further work on the relationship between breeding and climate enabled us to set up a practical forecasting model. This is applied by the Plant Protection Service, and the damage prevention system consists of a test trapping in winter, a population forecasting in early March, and, if necessary, poisoning in March-April. Generalization of the system is in progress.