Sex allocation strategies in response to conspecifics’ offspring sex ratio in solitary parasitoids

Parasitoid females adjust their offspring sex ratio in order to maximize their fitness. The optimal sex ratio they produce varies with several factors but especially with competition level. In solitary species, only one adult can emerge from a given host, whatever the number of eggs laid. In some species, the mortality of supernumerary individuals could be due to larval combats. This ability to fight could vary from one sex to another within species. In this way, when females explore an already parasitized host patch, the sex ratio of previous eggs can influence their fitness. These 2 factors could thus strongly influence females’ sex allocation strategies. However, this prediction assumes that parasitoid females can assess the sex of eggs previously laid by conspecifics. We used host acceptance and sex ratio behavior to test this capacity, and our experimental data provide the first evidence for this capacity in a parasitoid species. Females of the solitary ectoparasitoid Anisopteromalus calandrae discriminated the sex of eggs already laid by a conspecific but only when these eggs had reached a certain developmental stage. They adapted their offspring sex ratio as predicted by Hamilton's “sex ratio games” model, allocating the sex of their eggs differentially according to the sex of eggs already on the hosts on which they oviposited. In this way they prevented a lethal larval fight between their sons and the females they could potentially mate after their own emergence, increasing their own fitness and their sons’ reproductive success.