Melittobia australica (Hymenoptera: Eulophidae) is a gregarious ectoparasitoid of the prepupae and pupae of solitary wasps and bees. The males never disperse from their natal patch, and mating takes place only on the host from which they emerged. We measured the offspring sex ratio of M. australica with differing foundress numbers and examined combat between emerged males. The offspring sex ratios were extremely female biased and almost independent of foundress number in all cases. The population of M. australica used in the experiment was infected with the cytoplasmically inherited symbiotic bacterium Wolbachia. However, although Wolbachia is a potential sex-ratio distorter, noninfected individuals showed the same sex ratio patterns as the Wolbachia-infected individuals. An arena experiment showed that younger males were almost always killed by older males that had eclosed earlier. These results suggested that lethal male-male combat is an additional factor distorting the sex ratio toward a more female-biased sex ratio. This provides a new perspective on current local mate competition models.
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