Outbreaks of the water vole Arvicola terrestris cause severe damage in grasslands of upland regions of Europe. The sheer speed of this phenomenon is a challenge to effective pest control measures. While there has been some research into factors that promote outbreaks, especially landscape composition, little is yet known of the biological mechanisms underlying the speed of colonization of grasslands during the population growth stage. Like A. terrestris, the mole Talpa europaea digs vast tunnel networks that may be used by A. terrestris. The availability of extensive networks of this sort might greatly boost the colonization potential of A. terrestris and so explain the speed of onset of its outbreaks. 2. Areas occupied by networks of A. terrestris and T. europaea tunnels, and their respective locations, were evaluated and mapped with a global positioning system (GPS) during low-density, growth and the first weeks of abundance phases of cyclic fluctuations of A. terrestris. 3. During the growth phase and the first weeks of the abundance phase nearly 80% of new A. terrestris colonies were found in T. europaea tunnel networks, although these occupied just 20% of the area under study. 4. The regulating influence of wooded areas on the risk of outbreaks, which can be identified at larger spatial scales (region, sector), was imperceptible at local scales, probably because of the influence and relative extent of T. europaea tunnel networks. 5. There was a threshold level of occupancy of grassland by T. europaea, as indexed by surface indices, below which A. terrestris populations were regulated naturally and over long periods. 6.Synthesis and applications. These findings have implications for controlling outbreaks of A. terrestris. Temporally, chemical pest control of A. terrestris can be reduced by taking action during the A. terrestris low-density phases, preferably in the autumn. Spatially, control operations should be targeted at T. europaea and early A. terrestris networks. Mechanical destruction of tunnels (e.g. ploughing) and trapping should be considered as an alternative to chemical pest control.