Environmental stress drives plasticity in sexual bimaturism in the black soldier fly (Hermetia illucens)

Sexual bimaturism (SBM: differences between males and females in the timing of sexual maturity or adult emergence) is widespread in nature. Theoretical predictions suggest that SBM can evolve either directly by natural selection promoting males to emerge earlier to secure a female partner (direct selection hypothesis) or indirectly, by being physiologically coupled to other life history traits (indirect selection hypothesis), for example, sexual size dimorphisms (SSD). In insects, both processes lead frequently to males emerging earlier than females (protandry). Measuring the plasticity in the magnitude and direction of SBM across a gradient of larval environmental qualities can provide key insights into the underlying drivers of SBM. If SBM has been directly selected, the degree of protandry is expected to be independent of the environmental quality. Here we measured both the plasticity in SBM (timing of emergence of males and females) and SSD in the economically relevant black soldier fly (BSF; Hermetia illucens ) by exposing larvae to multiple types and intensities of environmental stressors. Our results show that SBM can range between protandry and protogyny (females emerging earlier than males) depending on the environmental quality. Additionally, consistent with the indirect selection hypothesis, we found SBM to be coupled with sex differences in the plasticity of growth rates. As environmental quality improves females emerge earlier and larger than males. Furthermore, our results suggest higher plasticity in growth rates and, overall, higher growth rates in BSF females than in males. Our work provides novel empirical evidence that shows that when SBM is coupled to SSD it may lead to high plasticities that can shift between protandry and protogyny. The sensitivity of SBM to environmental quality suggests that it could be employed as a possible bioindicator of ecological stress in the field in species with high SBM plasticity.