Despite strong evolutionary pressure for an ovipositing parasitoid to use only hosts with high suitability, there are examples of parasitoid attacks that fail. Here we document one such instance, in which a parasitoid does not discriminate against an inappropriate host life stage, resulting in negative consequences for both the parasitoid and host. The solitary koinobiont parasitoid Encarsia inaron (Walker) was originally imported to North America to control the ash whitefly, Siphoninus phillyreae (Haliday), but readily parasitizes and completes development in other whitefly hosts, including Bemisia tabaci (Gennadius) and Trialeurodes abutiloneus (Haldeman). For both B. tabaci and T. abutiloneus, when younger host instars were parasitized, parasitism was successful and adult wasps emerged. In fourth instar hosts, however, adult parasitoids rarely emerged; rather, adult whiteflies were produced. Dissections over time showed that the wasp larvae continued to develop within these adult whiteflies, but did not successfully complete development. Furthermore, the adult whiteflies suffered reduced fecundity compared with nonparasitized adults and ultimately died as a result of parasitism. Our results suggest that E. inaron may be incapable of arresting the development of hosts parasitized as fourth instars, instead engaging in a developmental race that results either in a win for the parasitoid, or a loss for both the parasitoid and the host.