Sperm production and allocation in response to risk of sperm competition in the black soldier fly Hermetia illucens

In polyandrous species, competition between males for offspring paternity goes on after copulation through the competition of their ejaculates for the fertilisation of females oocytes. Given that males allocating more spermatozoa are favored, different models of sperm competition predict adaptive plasticity in male sperm production and allocation. These predictions were tested experimentally in the black soldier fly (BSF) Hermetia illucens. In this farmed insect, adult biology is little known despite the economic interest of larvae for bioconversion and as an animal feedstuff. Two sets of experiments were carried out to modify the risk of sperm competition perceived by males. The first consisted of placing adult males alone or in groups of 10 - modifying mean risk of sperm competition - and then measuring their sperm production. The second took place at the beginning of copulation; pairs with males from the two mean risk of sperm competition treatments were transferred to different contexts of immediate risk of sperm competition (empty cages, cages containing 10 males, or cages containing 10 females) and the number of spermatozoa stored by the females was counted. Males reared in groups of 10 showed more spermatozoa in their seminal vesicles than males reared alone. Regarding sperm allocation, females that mated in the presence of conspecifics - either 10 males or 10 females - stored more spermatozoa than those that mated alone. This study shows that sperm production and allocation are dependent on sperm competition risk in BSF, revealing a plasticity of reproduction under socio-sexual situations.