Base temperature and growing degree day for determining the onset and peak of Cydia strobilella (L.) moth flight in Norway spruce seed plantations

The spruce seed moth, Cydia strobilella, is the main pest in Norway spruce Picea abies seed plan− tations, causing significant losses in seed production. Effective pest management strategies require precise phenological data, particularly base temperature (Tb) and accumulated growing degree days (GDD), which are necessary to determine the onset and peak of moth flight. This study aimed to determine the Tb and GDD for the beginning and peak of C. strobilella flights in Poland. Spruce cones were collected from three forest districts (Pniewy, Nowogród, and Łomża) and placed in photoeclectors and rearing containers. Tb was determined using linear regression of development rates at five constant temperatures (6−10°C). No development was observed at 4°C or 5°C; therefore, the base temperature was estimated by extrapolating the regression line below the observed range. The GDD for the onset and peak of flights was cal− culated using the single triangle method with a horizontal cut−off, based on hourly temperature data recorded in the photoeclectors. The results showed that the Tb for C. strobilella was 5.45°C (95% CI: 4.95−5.94°C), which is lower than that of other Cydia species, reflecting its adaptation to boreal and subarctic climates. The beginning of moth flight occurred at approximately 130 GDD, while the flight peak, defined as the first day when the proportion of captured females exceeded that of males, was observed at approximately 180 GDD. These results provide key information for developing precise phenology−based pest management strategies for Norway spruce seed plantations, optimising the timing of protective measures, and minimising seed losses caused by C. strobilella.