Food plants play a significant role in influencing the ontogeny of sexual size dimorphism in the fall armyworm, Spodoptera frugiperda

The ontogenetic timing of sexual size dimorphism (SSD) emergence in insects, as well as the influence of larval dietary quality on dimorphic growth patterns, remains poorly understood. In this study, we systematically investigated the effects of host plant species – corn versus sugarcane – on life history traits and developmental growth trajectories in Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), using a large sample size. Larvae reared on corn exhibited significantly shorter larval development durations, greater pupal masses, higher growth rates, reduced SSD, and less weight loss compared to those reared on sugarcane. The onset of SSD in individuals fed on corn was first detected 1 day prior to pupation, during the fifth instar, whereas in those fed on sugarcane, SSD became evident approximately 10 days earlier, during the third instar. This difference in the timing of SSD emergence is attributable to the prolonged developmental period observed in males. Female larvae achieved higher growth rates than males, primarily due to their extended larval development duration. Additionally, male larvae reared on corn showed a significant positive correlation between larval mass and development time, while those reared on sugarcane exhibited a slight negative correlation. These findings collectively enhance our understanding of the ontogeny of SSD and highlight the role of nutritional environment in shaping sex-specific developmental trajectories.