The proportion of females in an animal species directly influences the rate of population growth, and thus is important for understanding population dynamics. Although equal numbers of each sex are produced in many animals, members of some groups, such as the parasitic wasps, often exhibit highly variable sex ratios. Understanding the mechanisms that produce these variable sex ratios is necessary to predict population fluctuations. In addition, variable sex ratios offer a good opportunity to study the process of adaptation, since optimal sex ratios can be predicted using evolutionary models, and the fit to theory can be easily measured experimentally. In a group of minute parasitic wasps in the family Aphelinidae females lay male and female eggs in different hosts. Female eggs are laid inside nymphs of Homoptera, such as whitefly and scale insects, while male eggs are laid inside the body of immature female wasps while they are still developing within the homopteran host. Males are parasitic on female wasps, often of their own species. This study involves investigating the sex allocation behavior of females of an autoparasitoid, Encarsia pergandiella, which parasitizes whiteflies. In particular, the investigator is investigating how females allocate eggs to be sons and daughters given the constraint on sex ratio imposed by the availability of the appropriate hosts. The pattern of egg laying in the two host will be examined over the season when the availability of the two hosts, and the density varies. In the laboratory, factors which may alter the sex allocation behavior such as host density, previous experience and female age will be examined. The results will be interpreted in the context of evolutionary theory. This system offers a unique opportunity to test sex ratio theory. Fisher predicted that natural selection should equalize the investment of resources in each sex. Generally this leads to a 1:1 sex ratio, but in autoparasitoids the predicted sex ratio varies with the limited resource. When eggs are limiting, sons and daughters should be produced in equal numbers, but when hosts are limiting, the sex ratio should reflect the relative availability of the two hosts. In addition, autoparasitoids have been used in many instances to biologically control scale and whitefly pests, often with tremendous economic and environmental benefits due to reduced numbers of pesticide applications. Sex ratio is central to the population biology of these wasps, and, therefore, an understanding of the mechanisms that control sex ratio will improve our ability to evaluate wasp species for their potential to control pest species.
