The specialist endoparasite Subclytia rotundivertis Fallen (Diptera; Tachiniidae) uses the subsocial parent bug Elasmucha grisea L. (Heteroptera; Acanthosomatidae). The parasite injects a single egg through the upper prothorax of female bugs and after hatching the larval parasites eat their host. Parasitism affected only slightly a female's ability to defend her brood when the nymphs were at the second instar stage. However, with third-instar nymphs, parasitized females responded markedly less to disturbance at all levels of their defensive behaviour. Parasitism reduced female survival and significantly increased the probability of death before the end of maternal care. Thus, the main cost of parasitism to Elasmucha females seems to be a reduced period of maternal care of the nymphs.