Sensitivity analysis, calibration and validation of a phenology model for Pityogenes chalcographus (CHAPY)

The purpose of the study was to develop, calibrate and validate a comprehensive phenological model for the spatiotemporal simulation of the seasonal development of the six-toothed spruce bark beetle, Pityogenes chalcographus (CHAPY). The validation dataset was acquired through monitoring of the bark beetle's phenology at eight sites in Slovenia in 2017 and 2018, along with air and bark temperature measurements. The predictions were made using air temperature from the INCA system (Integrated Nowcasting through Comprehensive Analysis), which is used to calculate the effective bark temperature for beetle development. Since the biology of P. chalcographus is poorly studied, a sensitivity analysis was used to pinpoint the most important model parameters. The first order (main) effect was the highest for the lower developmental threshold (DTL), while the second order (interaction, total) effect was the highest for the optimum temperature (TO). DTL was calibrated with an iterative procedure, and the best result with the lowest mean absolute error (MAE) was achieved at 7.4°C. Effective temperatures in the range between TO and the upper developmental threshold were calculated with a nonlinear function whose parameters were appropriately calibrated. The spring date threshold when the model calculation starts was calibrated with an iterative procedure and set at 9th March, which had the minimum MAE. The onset of Norway spruce infestation in spring was estimated using a lower threshold of 15.6°C for flight activity and a mean thermal sum of 216.5 degree-days (dd) from 9th March onward. The observed mean thermal sum required for total development of filial beetles was 652.8 ± 22.7°C, while the predicted mean thermal sum was 635.4 ± 31.4°C. Re-emergence of parental beetles occurred when 52.7% of the minimum thermal sum for total development was reached. The relative duration of the egg, larval and combination of the pupal and teneral adult developmental stages was 9.4%, 58.2% and 32.4%, respectively. Mass swarming concluded in the end of August when daylength was lower than 13.6 h, which was determined with the independent dataset of 1,017 pheromone traps. The diapause initiation at a daylength < 13.6 h is included in the model as an assumption. Successful hibernation of established broods is predicted by assessing the developmental stage of initiated generations at the 31st December. For validation, we compared the timing of phenological events in the field with predicted events using both 30-minute recorded data at study sites in the field and hourly data from the INCA. The time of spring swarming was estimated with a MAE of 5.6 days. The onset of infestation was predicted with a MAE of 6.0 days. The predicted onset of emergence of filial beetles was estimated with a MAE of 2.1 days. Additionally, CHAPY simulates the number of generations. CHAPY was successfully incorporated into two publicly available web applications. Development of the model revealed several knowledge gaps in P. chalcographus phenology, thus providing opportunities for further research of the second most damaging bark beetle of Norway spruce in Central Europe and for further improvement of the CHAPY model. Potential applications of the model for monitoring and management of P. chalcographus are discussed.