SIBLING RIVALRY AND BROOD SEX-RATIOS IN POLYEMBRYONIC WASPS

FEMALE-BIASED sex ratios are predicted under local mate competition where offspring of one or a few females mate in isolated subpopulations1. Haplodiploid sex determination allows parasitic wasps to bias their sex ratios by controlling fertilization2, but parental control is not solely responsible for the broods of polyembryonic wasps. The eggs of these parasites divide to form many offspring and often two larval morphs3,4. Precocious larvae die without pupating while reproductive larvae become adults. Copidosoma floridanum (Hymenoptera: Encyrtidae) produces 1,000-1,400 offspring per host and most broods contain both sexes (mixed)5. Mixed broods are female-biased and wasps mate before dispersing, but these broods develop from the mother laying one male and one female egg and adult males probably obtain additional matings away from the host5,6. Thus, intersexual conflict may exist over the number of offspring to produce per host. Here we show that precocious larvae in mixed broods are predominantly female and that they bias the sex ratio by killing males.