The estimation of the mortality rate function for a stage-structured population is obtained starting from time-series field data on the abundance of the species. The method is based on the formulation of the mortality as a combination of cubic splines and it is applied to the case of Lobesia botrana, the main pest in the European vineyards, with data collected in a location in the North of Italy. Mortality estimates are based on 3 years of data and are used to obtain the dynamics for two different years. These dynamics give a satisfactory fit of the phenology of the pest. The method presented allows to obtain more flexible shape for the mortality rate functions compared with previously methods applied for the same pest.