We describe a novel behavior in which the seed beetle Mimosestes amicus (Chrysomelidae: Bruchinae) stacks one egg on top of another, which slightly flattens and acts as a shield for the egg concealed underneath (egg-stacking). The eggs of this species are often heavily attacked by the egg parasitoid Uscana semifumipennis (Hymenoptera: Trichogrammatidae), and we investigated whether egg-stacking was associated with combating the risk of egg parasitism. Our results show that egg-stacking significantly reduces egg mortality caused by U. semifumipennis, but the magnitude of this protection varies among sites. The presence or absence of parasitoids corresponded to differences in beetle egg survival while low or high humidity did not, but more importantly, top eggs in a stack were always inviable and never developed into larvae. These 'sacrificial' eggs are also significantly deficient in weight and negatively affect the fitness of any wasp progeny that are developing inside. This study reveals a unique solution to egg parasitism in insects, in which the eggs themselves are manipulated to have a protective function.
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