RITY – A phenology model of Ips typographus as a tool for optimization of its monitoring

We developed the RITY-2 phenology model for the spatial and temporal simulation of the seasonal development of Ips typographus based on the PHENIPS model and the data of Integrated Nowcasting through Comprehensive Analysis (INCA). Many parts of PHENIPS were improved with innovative approaches and a new model was developed. European spruce bark beetle phenology was monitored at eight study sites in 2017 and 2018, along with air and bark temperature measurements. RITY-2 predictions are based on air temperature from the INCA system, which is used to calculate the effective bark temperature for beetle development. An innovative procedure was introduced which finds most appropriate spring threshold from which calculation of the phenological model initiates. The onset of Norway spruce infestation in spring was estimated using a lower threshold of 14.5 °C for flight activity and a mean thermal sum of 53.0 degree-days (dd) from 7th March onward. The rate of brood development was calculated from the cumulative effective thermal sum of 30-min temperature data using upper and lower temperature thresholds of 38.9 and 8.3 °C, respectively, and a nonlinear function. For validation, we compared the timing of phenological events in the field with predicted events using both 30-minute recorded data from trap logs in the field and hourly data from INCA. The onset of spring swarming was estimated with a mean absolute error of 2.4 days. The onset of infestation was predicted with a mean absolute error of 4.7 days. The observed onset of emergence of filial beetles was estimated with a mean error of 0.5 dd. The model is calculated according to three scenarios that simulate different stand conditions. RITY-2 explicitly considers the strong effects of regional topography and can be used for precise monitoring of the actual state of bark beetle development at the specific model cell with a spatial resolution of 1 km × 1 km. Additionally, RITY-2 simulates the number of generations, which is necessary to assess the potential impact of bark beetle outbreaks at the regional scale. The model was successfully incorporated into two web applications that serve as tools for the timely deployment of pheromone traps and trap trees for European spruce bark beetle monitoring. The possible application of the RITY-2 model for the whole of Central Europe using data from the INCA system is discussed.