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Levonorgestrel and quinestrol, commonly known as EP-1, has long been used in the control of wild rodents. Up to the present time, however, no method for simultaneous quantification of levonorgestrel and quinestrol in rat plasma has been reported. In the present study, a sensitive reverse-phase high-performance liquid chromatography with ultraviolet detection (RP-HPLC-UV) method for quantification of levonorgestrel and quinestrol in rat plasma has been developed. It uses a Kromasil ODS C(18) column and acetonitrile-0.1% formic acid (85 : 15, v/v) mobile phase at ambient temperature. The plasma sample was prepared by hexane-isoamyl alcohol extraction (90 : 10, v/v). The flow rate and detection wavelength were 1.0 mL/min and 230 nm. The correlation coefficients were greater than 0.9995 within 0.08-50 microg/mL for levonorgestrel and 0.12-50 microg/mL for quinestrol, and the limits of detection were 0.02 and 0.05 microg/mL for levonorgestrel and quinestrol, respectively. Average recovery ranged from 92.5 to 96.3% and inter-day RSDs were less than 7.56%. This method can be applied to the further pharmacokinetic study of levonorgestrel and quinestrol in rat plasma.

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Brandt's voles (Microtus brandti) are small native mammals that inhabit the grasslands of Inner Mongolia, China. The species is considered a pest, particularly during population outbreaks, which have increased in frequency since 1970 from 1 every 7 years to 1 every 3 to 5 years. Using historical records taken between 1948 and 1998, we found that there was significant correlation between years for which the monthly averages of the Southern Oscillation Index (SOI) were consistently high, and years in which outbreaks occurred. Also there was a tendency for outbreaks not to occur in years with precipitation above or below average. For some years at some sites, annual or biannual estimates are available for the density of Brandt's vole populations. We found that the seasonal rate of increase over the non-breeding season from autumn to spring was negatively correlated with the density in autumn. The rate of increase over the breeding season from spring to autumn was independent of the density in spring but instead, reflects the species' preference for habitat with short, sparse grass; populations do not persist in highly degraded grasslands or in tall, dense grassland. The link between outbreaks and climatic indices, and the numerical response of Brandt's voles to particular habitat characteristics, suggest that current grazing intensity tends to maintain grass at low height in years with average precipitation. The substantial increase in livestock numbers over the last 50 years appears to have increased the number of years when the balance of grazing and plant growth favours high rates of increase in Brandt's vole populations, and hence, there has been an increase in the frequency of outbreaks.

Summary 1 Brandt’s vole Microtus brandti 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. 2 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. 3 Survival of voles over winter, a 5–6-month period, depended on age at the onset of winter and the density in autumn. 4 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. 5 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. 6 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.

Ten polymorphic microsatellite loci from Lasiopodomys brandtii have been isolated and characterized. Two to 11 alleles per locus were detected from 52 Brandt's voles samples collected from a single population. Expected heterozygosities ranged from 0.406 to 0.840. For the majority of loci observed heterozygosities were similar to or greater than the expected heterozygosity. One locus pair appeared to be in linkage disequilibrium. The microsatellite markers will enable the studies of genetic diversity, population structure and relatedness in this species, and perhaps in closely related species of vole.