First come, first served: sperm plug-mediated sperm precedence in the black soldier fly ( Hermetia illucens )

Abstract Males of many species transfer plugs made of sperm and/or coagulated seminal fluid to the female reproductive tract. One potential function of these plugs in the context of sperm competition is to block access to rival sperm, thereby promoting first-male sperm precedence. Recently, sperm plugs have been observed in the black soldier fly, Hermetia illucens. Despite growing interest in this species as livestock feed, its reproductive biology remains understudied, although the structure of the female reproductive tract suggests strong sexual selection pressures. We investigated whether first-male sperm precedence occurs in the black soldier fly by experimentally testing how mating order affects paternity in polyandrous females. Females mated sequentially with two males, and offspring paternity was assessed using single nucleotide polymorphism markers. The results showed that the first male sired most offspring (79 ± 5%), with higher precedence when remating was delayed. Male size also influenced paternity: larger second males relative to the first male gained a higher share of offspring. These findings support a role for sperm plugs in blocking rival sperm and suggest the existence of sperm precedence mechanisms, offering new insights into the interactions between the male ejaculate and female reproductive tract in the black soldier fly.