Sexual size dimorphism (SSD) is widespread and variable among taxa, and the evolution of SSD has been a central topic in evolutionary biology. Whereas many researchers have examined the roles of male competition and female fecundity in the evolution of SSD in insect, however, little is known about the role of viability selection in shaping SSD due to parasitoids although their attacks occur very widely among various insect taxa and some host traits are under selection pressure by them.

The parasitoid, Blaesoxipha japonensis parasitizes the grasshopper, Parapodisma tanbaensis. This grasshopper in adult shows female-biased SSD. We investigated the effects of the mortality caused by B. japonensis on the female-biased SSD of this grasshopper. We examined the relationship between adult body size and mortality caused by B. japonensis by rearing the adult grasshoppers captured in the field.

The results showed higher mortality in smaller female adult hosts, whereas there was no significant relationship between the body size of the male adults and mortality. Those suggested that B. japonensis should favor female-biased SSD in this grasshopper.