Plastic responses in sperm expenditure to sperm competition risk in black soldier fly (Hermetia illucens, Diptera) males

In polyandrous species, competition between males for offspring paternity goes on after copulation through the competition of their ejaculates for the fertilisation of the female’s oocytes. Given that males allocating more spermatozoa often have greater fertilisation success, 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 authorized feedstuff. In lab conditions, 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 the 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 treatments were transferred to different contexts of immediate risk of sperm competition: empty cages, cages containing 10 males, or cages containing 10 females. In all cases, spermatozoa transferred to mated females were 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 – received more spermatozoa than those that mated in isolation. In BSF, sperm production and allocation are influenced by the risk of sperm competition, demonstrating reproductive plasticity in response to socio-sexual conditions.