Fitness Costs of Broflanilide Resistance: Susceptibility, Resistance Risk and Adaptive Trade-Offs in Spodoptera frugiperda

The fall armyworm (FAW) Spodoptera frugiperda is a polyphagous pest that causes significant damage to various crops and rapidly develops resistance to insecticides. Broflanilide, a novel meta-diamide insecticide, has shown effectiveness against lepidopteran pests, but the risk of resistance and associated fitness costs in FAW remain unclear. This study evaluated the development of resistance to broflanilide over nine generations of selection using the diet incorporation method at the 70% lethal concentration (LC70) concentration. Following nine generations of selection, the LC50 value increased from 0.134 mg/kg to 0.232 mg/kg, showing a 1.73-fold increase in resistance ratio (RR). The calculated heritability of resistance (h2) was 0.084, which suggested that resistance of FAW against broflanilide is evolving at a slow rate. Based on the projected rate of resistance progression, a 10-fold increase in LC50 would take between 30.1 and 66.4 generations, assuming selection mortality rates of 90% and 50%, respectively. Fitness costs were evaluated using age-stage, two-sex life table analysis, revealing reduced fecundity and pupal weight in the broflanilide-selected (Brof-SEL) strain compared to the wild-type. The relative fitness of the Brof-SEL strain was 0.38, indicating trade-offs in biological traits. These findings suggested a low risk of rapid resistance development against broflanilide. However, effective integrated pest management strategies against FAW require the judicious use of this insecticide in combination with biological control measures, including the deployment of parasitoids and predators, to promote a more environmentally sustainable approach.