Simulation of lethal control and fertility control in a demographic model for Brandt's vole Microtus brandtii.
Brandt's vole Microtus brandtii is considered a pest in China and is actively managed through the use of toxic baits. A time-dependent density-dependent Leslie matrix model was developed to investigate the effectiveness of fertility control as an alternative. Three annual cycles of Brandt's vole were observed between 1995 and 1998 at a site in the grasslands of Inner Mongolia, China. For each of these cycles, cohort-specific monthly survival rates were derived for the spring, summer and autumn months. Over the breeding season, monthly reproductive rates were obtained as numbers of young females per adult female. Survival of voles over winter, a 5-6-month period, depended on age at the onset of winter and the density in autumn. The model was parameterized with measured and continuously varying monthly data for vital rates. It was then used to predict the impact that fertility control, delivered with non-toxic baits, and lethal control would have had on the vole population if control had been imposed in the autumn of 1994 or spring of 1995. The timing and frequency of bait delivery were varied, and we considered the population response to bait that renders females sterile and bait that renders females only temporarily infertile. The model suggested that the effectiveness of lethal control in spring could be matched by fertility control if it was applied twice in the spring of a year in which high densities were reached, or in the previous autumn. The latter requires that (i) infertility is long-lasting in treated animals and (ii) either high-density years be reliably forecast a year ahead or fertility control be applied every autumn. Brandt's voles provide an exception to the general rule that fertility control is never more effective than lethal control applied at the same time with the same bait uptake. We found that when control was applied in autumn, fertility control was better than lethal control in reducing the numbers of voles over the following spring, summer and autumn. This is because normal density-dependent mortality over winter has a greater impact following fertility control than lethal control.