BIOLOGICAL TRAITS, MORPHOMETRICS AND SUB-LETHAL EFFECTS OF INSECTICIDES ON FALL ARMYWORM Spodoptera frugiperda (LEPIDOPTERA: NOCTUIDAE) IN MAIZE

Abstract

Maize (Zea mays L.) is an important cereal crop globally and in Bangladesh, but its productivity is threatened by the invasive fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae). This study evaluated the morphometrics, feeding behavior, and reproductive performance of S. frugiperda on maize, and assessed sub‑lethal effects of ten insecticides-Tecorel 18.5 SC, Belt 24 WG, Incipio 20 SC, Delegate 11.7 SC, Vayego 20 SC, Spinomax 44.03 SC, Bio BTK, Biochamak 1% EW, Beauveria bassiana, and Metarhizium anisopliae-under laboratory conditions. Larvae reared on maize cobs exhibited a shorter pre-oviposition period (2.9 days) than larvae reared on maize leaves (3.6 days). They also developed faster, with the sixth larval instar lasting 4.3 days in larvae reared on maize cobs and 5.6 days in those reared on maize leaves. Larvae fed maize leaves treated with sub-lethal concentrations of Delegate 11.7 SC and Spinomax exhibited prolonged larval durations to 21.2 to 22.1 days and pupal durations to 17.3 to 17.8 days, reduced larval body length to 27.8 to 28.2 mm, adult body length (male 12.4 to 13.5 mm, female 13.1 to 13.7 mm), and wing length (male 12.4 to 13.1 mm, female 14.5 to 14.7 mm), suppressed leaf consumption (0.5 to 0.6 g), egg mass production (5.5 to 6.6 egg masses/female), and shortened oviposition period (2.1 to 2.2 days). Bio BTK, Biochamak 1% EW, B. bassiana and M. anisopliae caused moderate reductions in larval length (32.4-33.8 mm), adult body size (male 13.5-14.5 mm, female 13.7-16.1 mm), wing length (male 14.1-14.6 mm, female 15.7-16.1 mm), and egg mass production (10.3-10.7), while maintaining longer adult longevity (male 7.5-8.4 days, female 8.9-9.7 days). These results indicate that chemical insecticides strongly reduced growth and reproduction, while biopesticides moderately affected development and maintained adult survival.