Field and Laboratory Studies for Predicting Annual Generations of Spodoptera littoralis (Boisd.) Using Sex Pheromone Traps and Cumulative Heat Unit Measurements in Egypt.

The Egyptian cotton leafworm Spodoptera littoralis (Boisduval) is a major polyphagous pest in vegetable and cotton crops. Accurate forecasting of its field generations is essential for effective integrated pest management (IPM). Forecasting its generations in the field is traditionally based on the trapping of males, but the use of temperature sums (degree-days, DD) could offer a robust alternative for triggering pest control interventions. The aim of this study was to monitor the flight activity and calculate the summations of degree-days, as well as to compare expected generation peaks and the reliability of S. littoralis males in predicting generations over three successive growing seasons. The study was carried out over three consecutive growing seasons (2017, 2018, and 2019) at Abnob district in Assiut Governorate, Egypt, comparing twice-weekly male captures using pheromone traps and emergence dates predicted by a degree-day model established from physiological data (development threshold: 10.5 °C; thermal constant: 480.6 DD for one generation). Laboratory experiments at four constant temperatures (17, 22, 27, and 32 °C) established the lower developmental thresholds and thermal constants for each life stage: egg (t0 = 11.86 °C; K = 40.10 DD), larva (t0 = 7.56 °C; K = 283.76 DD), pupa (t0 = 12.27 °C; K = 149.50 DD), and pre-oviposition period (t0 = 12.58 °C; K = 23.96 DD). Six successive generations (observed generations) appeared after the overwintering generation each year, with the overwintering generation consistently initiating in early May each season. The capture dates from the male sex-pheromone traps coincided with the adult emergence dates calculated by the accumulated heat model; deviations between observed (the occurrence of the real peak days) and predicted generation dates ranged from -2 to +2 days in the vast majority of cases, except for in the 2nd generation of the 2019 season, when the observed generation occurred 4 days before the expected generation date. Mean accumulated thermal units per generation ranged from 479.62 to 483.78 DD across the three seasons (grand mean: 481.95 DD), with overall mean seasonal deviations of 0.00, -0.14, and -0.43 days, confirming that the model predicts generation peaks to within ±2 days in the vast majority of cases. The integration of the two methods improves the reliability of phenological forecasting for S. littoralis, opens new opportunities for sustainable pest suppression, and improves the timing of control actions. This combined approach is recommended as a practical decision-support tool for IPM programs targeting S. littoralis in Egypt and comparable Mediterranean agroecosystems.