Seasonal dynamics of noctuid larvae and their impact on sugar beet crops

Abstract Background Noctuid larvae are major pests in sugar beet fields, with their populations influenced by temperature and humidity. This study aimed to assess their seasonal dynamics and impact on crop damage under field conditions. Results Field trials were conducted from January to June in 2022, 2023, and 2024, with biweekly sampling of 100 randomly selected sugar beet plants. Larval densities and plant damage were recorded, alongside weather data from a local weather station. The beet armyworm, S. exigua (Hübner) was the most abundant noctuid species, with densities increasing from 630 larvae in 2022 to 710 in 2024. Other species included S. littoralis (Boisduval) (440–470 larvae), Spodoptera litura (Fabricius) (290–350 larvae), and Agrotis segetum (Denis & Schiffermüller) (240–280 larvae). Lesser abundant species were S. eridania (Stoll) (120–150 larvae) and A. ipsilon (Rottemberg) (120–140) larvae. Larval densities and plant damage were significantly influenced by sampling week but not by year. In 2022, the highest larval densities and plant damage occurred during weeks 10–14, coinciding with increased rainfall and rising temperatures. Similar patterns were observed in 2023 and 2024, with peak larval populations and notable damage from S. exigua and S. littoralis during weeks 10 and 14, respectively. Temperature and humidity significantly impacted larval densities, with higher populations recorded at moderate temperatures (14–22 °C) and increased humidity (> 60%). A strong correlation was found between larval populations and plant damage ( r = 0.9966–0.9977), with peak damage occurring during high larval densities. Conclusions Noctuid larval peaks align with moderate weather. Climate-based timing is key for effective pest control.