Offspring Sex Ratios in Parasitoid Wasps

Laboratory and field studies on about 100 species in sixteen families indicate that several factors can influence offspring sex ratios (males: females) in parasitoid wasps. For many species, the offspring sex ratio increases with one or more of the following: (1) maternal age at ovipositing or number of days since insemination; (2) the age of the male parent or the number of times he has copulated; (3) extreme temperature; (4) decreasing host size, age, or quality; (5) female wasp density; and (6) the number of progeny per host. Other factors which have been shown to affect offspring sex ratios in some species include (1) number of hours since insemination; (2) genetic factors; (3) maternal size; (4) maternal die; (5) polyembryony; (6) photoperiod and relative humidity; (7) hos sex; and (8) host density. These factors may affect offspring sex ratios through females manipulating fertilization of their eggs or through other mechanisms, such as differential mortality or changes in sperm availability. Theoretical development ahs focused primarily on females manipulating their offspring sex ratios in response to host size or female density or both. Host-size models predict a negative relationship between offspring sex ratio and host size. These models assume that host size has a greater effect on the reproductive success of females than of males. Local mate competition (LMC) models predict a positive relationship between offspring sex ratio and female density. A major assumption of these models is that males mate primarily in their natal area. For each model, most ofthe species examined meet the model's general prediction, but models have been rigorously tested for only a few species. Such testing requires supporting data on the assumptions made and examination of alternative explanations, particularly sex-ratio differences that are due to differential mortality.