An Evaluation of Two Counting Methods to Establish Rodent Densities in Crop fields

The common vole (Microtus arvalis) is the main pest in agricultural areas of Central Europe. It is particularly important to monitor its numbers during spring, and if high numbers are detected, some form of pest management should be considered. In the Czech Republic, the number of active burrows is monitored using the burrow index, BI, which allows estimation of the total number of rodents, saves time and is easy to use. We aimed to assess the relationship between the burrow index and the relative abundance of the rodent species examined by snap trapping in crop fields. Bayesian MCMC algorithms with a zero-inflation model were used for this analysis. The positive relationship between BI and vole abundance occurred in the total sample of all fields and in alfalfa, winter wheat and barley crop fields. A positive relationship between BI and the abundance of the wood mouse (Apodemus sylvaticus), the second most common pest in the area, was only confirmed in barley, and this relationship was negative in winter rape. The positive influence of the degree of weed cover on BI was confirmed in the total sample and in winter rape and alfalfa, but weed cover has a negative effect on BI in barley and winter wheat. In contrast, weed cover did not affect the relative abundance of both rodent species in any of the sampled crops. The presence of shrubs and forests around the fields reduced BI in the whole sample, especially in alfalfa. The relative abundance of the voles was not affected by the presence of shrubs and forests around the crop. Still, a positive influence was confirmed for the abundances of mice in the whole sample and alfalfa. BI can be a reliable indicator of vole abundance in crops with high densities, but it is not very accurate at low densities and in crop fields rarely used by voles, such as sunflower and maize.