Effects of temperature on the ontogeny of sexual size dimorphism in the fall armyworm Spodoptera frugiperda

To date, the ontogeny of insect sexual size dimorphism (SSD) remains poorly understood. In particular, we do not know whether size differences between sexes might change with rearing temperature. Based on a thorough understanding of the life-history traits of the fall armyworm (FAW) Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae), here, we investigated the ontogeny of SSD by measuring larval mass daily during development at 22 and 28°C. We found that temperature significantly influenced SSD ontogeny. Thus, the growth trajectory at 22°C showed that SSD began between the third instar of larval development and the pupal stage but was not manifest at 28°C. The significant difference in larval mass between sexes reared at 22°C was due to the longer duration of the developmental time of males, whereas the disappearance of SSD at 28°C was mainly attributed to both the longer developmental time of female larvae and the higher growth rate of males. Sexual dimorphism in growth rate was observed at both temperatures, being significantly higher in females than in males at 22°C, over the first 6 days before pupation. Conversely, it was significantly higher in males than in females over the first 5 days before pupation, at 28°C. Unlike female-biased insects, FAW individuals showed a decrease in SSD from the pupal to adult stages, as male pupae took significantly longer to develop than female pupae, resulting in males losing more weight than females at the time of eclosion. Consequently, females were larger than males at the adult stage. Furthermore, we found a significant positive correlation between larval developmental time and larval mass, as well as between larval mass and wingspan. These findings pave the way for further research on insect SSD.