Influence of altitude on larval instar determination of Spodoptera frugiperda (Lepidoptera: Noctuidae) under high-mountain laboratory conditions

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a globally invasive and highly polyphagous pest whose developmental biology has been predominantly characterized under lowland, warm-temperature conditions. However, knowledge of its larval development under tropical high-mountain environments remains limited. This study evaluated the effects of altitude-associated low temperature on larval development, instar number variability, and growth dynamics of S. frugiperda under controlled laboratory conditions representative of high-mountain agroecosystems. Larvae were reared at 17°C and 65% relative humidity, and head capsule widths were measured to determine larval instars and assess developmental plasticity. Larval development was markedly prolonged, instar overlap increased, and deviations from classical growth ratios were observed relative to low-altitude reports. A proportion of individuals exhibited supernumerary instars, indicating strong thermal plasticity. These findings demonstrate that altitude-related thermal constraints significantly modify larval development of S. frugiperda, with direct implications for instar determination, phenological modeling, and integrated pest management strategies in high-altitude tropical cropping systems.