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

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 mini- mizing crop losses, optimizing pest control interventions, and reducing unnec- essary 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 eval- uate the variability of parameters in degree-day models describing the popu- lation peaks of J. lybica populations. Field observations were collected from 2017 to 2021 in the Moulouya region of Morocco, and degree-days were calcu- lated using the minimal theoretical 280 DD of the pest. Two candidate func- tions, 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, posi- tion, and width, providing the best fitin mostyears (e.g., 2021: R2 H 0.86, RMSE H 22.1). However, a simpler variant (M4), optimizing only peak height and position, outperformed M5 in certain years (e.g., 2017: R2 H 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.