Selection of insecticides, determitionof applying timing and times, and establishment of dynamic economic threshold on sweet corn for the invasive Spodoptera frugiperda in tropical Asia

Abstract The fall armyworm Spodoptera frugiperda (Smith) (FAW; Lepidoptera: Noctuidae) is a migratory corn pest that causes major yield losses globally. In the (sub-)tropics, extensive FAW feeding damage regularly triggers the over-use of high-risk chemical insecticides and thereby deepens smallholder poverty vulnerability. In this study, we compared field efficacy of chlorfenapyr, spinetoram, chlorantraniliprole, cyantraniliprole and gauged their effects on FAW infestation dynamics in Hainan (China). Based upon an empirical assessment of how insecticide application frequency at different corn phenological stages affects yield and net (farmgate) income, we equally defined a dynamic, stage-specific economic threshold (ET). Overall, chlorantraniliprole had the highest efficacy at 80.69% over a 7-day period. Across treatments, FAW infestation levels ranged between 16.00–120.75% and peaked during early vegetative (V6) and silking stages (R1). While corn yields were highest for four or five spray applications of chlorantraniliprole, net income declined markedly after 5–7 applications. By accounting for variable input and corn prices, a dynamic ET was calculated. ET values for FAW larvae ranged between 7.77–13.10, 17.47–33.25, 24.89–46.52, and 12.44–21.78 per 100 plants at V4–V6, V8–V10, V12–V14, and VT–R1 stages in 2021 and 2022, respectively. The ETs of this study lower than the action threshold proposed in Colombia. Further communication and enforcement of those ETs is key to implementing economically-sound and environmentally-responsible pest management. As such, our study provides baseline information to advance integrated pest management (IPM) and resolve insecticide-induced externalities in FAW-invaded corn cropping systems across (sub-)tropical Asia.