Multiple mating is not driven by size and sperm management in black soldier fly (Hermetia illucens)

Abstract

Black soldier fly (BSF), Hermetia illucens (L.) (Diptera: Stratiomyidae), larvae are widely used as bioconverters of agrifood by-products into valuable proteins for animal feed. In adults, females and males have complex reproductive tracts, suggesting a strong pressure on sperm management. Such complexity in reproductive tracts is typically shaped by post-copulatory sexual selection (PCSS). Whereas multiple mating—the foundation of PCSS hypotheses—has been observed in BSF, its underlying phenotypic determinants and implications for sperm storage and use remain unknown. In this study, matings were tested during three consecutive days under laboratory conditions. Spermatozoa were counted in the reproductive tracts of males and females after consecutive matings. The results show that sperm amount in males was not correlated with their size. Mating occurred in 54% of males and 58% of females and was not influenced by size in either males or females; among them, multiple mating was observed in 19% of females and 50% of males. Sperm counts in males and females did not decrease with males' successive matings. Females were observed to store less spermatozoa when they copulated longer. The number and fertility of eggs did not change with female rank. This showed that male sperm production is enough to ensure egg fertilization even after successive matings. In conclusion, based on the obtained results, sperm quantity is not an issue in BSF, and mating strategy must be investigated in other traits, such as sperm quality, paternity biases or other ejaculate components.