Variability of Degree-Day Model Parameters for Jacobiasca lybica (Hemiptera: Cicadellidae)1

Jacobiasca lybica (Bergevin) (Hemiptera: Cicadellidae) is a major pest of grapevines in Europe and a secondary pest of citruses in Morocco. Accurate forecasting of its population dynamics is essential for minimizing crop losses, optimizing pest control interventions, and reducing unnecessary insecticide applications. However, existing degree-day (DD) models often fail to capture intra-seasonal variability, overlapping generations, and phenological shifts driven by environmental factors. This study aimed to evaluate the variability of parameters in degree-day models describing the population peaks of J. lybica populations. Field observations were collected from 2017 to 2021 in the Moulouya region of Morocco, and degree-days were calculated using the minimal theoretical 280 DD of the pest. Two candidate functions, Bragg and Beta, were tested for their best-of-fit to the observed population peaks. The Bragg function showed superior overall performance, with the fully flexible model (M5), which is able to optimize peak height, position, and width, providing the best fit in most years (e.g., 2021: R2 = 0.86, RMSE = 22.1). However, a simpler variant (M4), optimizing only peak height and position, outperformed M5 in certain years (e.g., 2017: R2 = 0.994). These findings support the application of Bragg-type models for capturing intra- and inter-annual variations in pest phenology and highlight their potential utility in integrated pest management programs.