Population activity of the main insect pests inhabiting sugar beet plants in Assiut Governorate was studied during two successive seasons of 2021/2022 and 2022/2023 at the Farm of the Faculty of Agriculture at Assiut University. The results show that the sugar beet plants were harbored by three main insect species, i.e., Pegomya mixta Vill., Spodoptera littoralis (Boisduval) and Gryllotalpa gryllotalpa (Linnaeus). Results revealed that the highest infestation levels of these insect pests occurred during December and January in both seasons. The results also showed that the activity of the sugar beet fly, Pegomya mixta increased in the Salama and Oscarpoly cultivars during the two seasons. By studying the major weather factors, temperature and relative humidity on the population activity of the major pests on sugar beet plants. Results showed that the population densities of sugar beet fly, Pegomya mixta were relatively higher during the first season than the second one, and it has a higher density than Spodoptera littoralis and Gryllotalpa gryllotalpa in Salama and Oscarpoly cultivars in both seasons. From the current study, we can conclude that Salama and Oscarpoly sugar beet cultivars were sensitive to the infestation with the sugar beet fly, Pegomya mixta, but the sensitivity varies from one cultivar to another.