Environmental Stress Drives Plasticity in Sexual Bimaturism in the Black Soldier Fly ( Hermetia illucens Linnaeus, 1758)

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 favoring 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 the plasticity in SBM in the economically relevant black soldier fly (BSF; Hermetia illucens Linnaeus, 1758; Stratiomyidae: Hermetiinae) by exposing larvae to nutrient availability and humidity stress. 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 the degree of protandry to decrease as larval environmental quality improves. This ultimately leads to protogyny in optimal conditions. This result is consistent with the BSF life history traits and similar patterns observed in other dipterans. We suggest that SBM may be coupled to SSD in the BSF, thus leading to high plasticity 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.