Instar Determination and Morphometric Analysis of Immature Stages of Sesamia nonagrioides (Lepidoptera: Noctuidae)

Abstract

Instar determination and morphometric analysis of immature stages of Sesamia nonagrioides (Lepidoptera: Noctuidae) were conducted to characterize larval development and establish reliable diagnostic parameters for stage differentiation. Laboratory-reared individuals were monitored from egg hatch to pupation under controlled environmental conditions, and measurements of head capsule width, body length, and other morphological traits were recorded at successive developmental stages. Frequency distribution analysis of head capsule widths confirmed distinct larval instars following Dyar’s rule, enabling accurate separation of developmental stages. Morphometric data revealed a consistent geometric progression in head capsule growth, supporting predictable instar transitions. Significant variation in body length was observed within instars, although head capsule width remained the most stable and reliable parameter for instar identification. Developmental duration varied among instars, with early stages showing faster progression compared to later larval stages. The study also documented key morphological features useful for field identification, including changes in coloration and setal patterns across instars. These results provide a comprehensive baseline for understanding larval development in S. nonagrioides and facilitate accurate population monitoring in maize agroecosystems. The findings are valuable for improving pest surveillance, forecasting infestation levels, and optimizing timing of control measures within integrated pest management programs targeting stem borers.