Sex ratio, the proportion of a progeny that is female, is an extremely important fitness trait. It is particularly interesting in relation to haplodiploidy, a variety of parthenogenesis in which organisms reproduce without the benefit of fertilization. This study arrives at conclusions drawn from new empirical studies: that biased sex ratios are characteristic of haplodiploid species; that these species are characteristically colonizing species with genetics suited both to spatially and temporally unpredictable environments; that manipulation of haplodiploid biological control agents, or pests that are to be controlled, depends on understanding the determinants of sex ratio; and that because evolutionary theory predicts that haplodiploids have the capacity to evolve faster than diploid organisms, haplodiploid species are the organisms of choice in biological control strategies.