Superparasitism is a common condition in which a single host is injected with more than 1 parasitoid egg, but only 1 parasite survives to adulthood, and the remaining animals are eliminated. Here, for the first time, we show that supernumerary Ganaspis xanthopoda, endoparasitoids that invade Drosophila melanogaster, are physiologically suppressed during embryonic development. Whereas the suppressed supernumerary embryos can develop to the first larval instar, their subsequent growth is blocked because they become physically trapped within a novel multicellular envelope that is formed during late embryogenesis. Supernumerary embryos can produce this envelope when cultured in vitro even if they are separated from dominant embryos. Our results suggest that physiological suppression programs supernumerary individuals for developmental arrest, starvation, and necrotic or apoptotic death.

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