A larval army caste is found in some parasitic wasps with polyembryonic or clonal proliferation, where many clone larvae emerge from a single egg. In contrast to non-parasitic eusocial Hymenoptera, sterile soldier larvae that protect their clonal reproductives are found in both females and males. Recently, the proportion of soldier larvae has been found to vary radically, depending on the internal conditions of the host, such as multiparasitism by other larval parasites. However, the proportion of male soldier larvae is constant, irrespective of the host internal environment. It is unknown if these traits are heritable. Here we show that a high heritability is found in both sexes, while, in the 6th instar hosts, substantially lower heritability is found in females. These results imply that the structure of the larval caste is determined genetically by both female and male embryonic cells, but more likely modified environmentally in females.