Evolutionarily stable oviposition and sex ratio in parasitoid wasps with single-sex broods

1. The flexibility of hymenopteran sex ratios is well documented, particularly in structured populations featuring sib mating. 2. Using game theoretic models, the present study examines species producing single-sex broods in which sib mating is unlikely, and focuses on the role of population density in determining evolutionarily stable oviposition strategies. 3. Since only mated females can produce offspring of both sexes while unmated females produce only male offspring, mated females are under selection to produce more females overall to balance the primary sex ratio. 4. As the proportion of all females that are mated should increase with density, offspring sex ratio of mated females is strongly linked to density at low to moderate densities. The present study shows that when density becomes low enough for fewer than half of all females to have mated, then female offspring generate higher fitness. 5. In this low density situation, females may gain a fitness benefit from waiting at their emergence site or from using other costly means to find and mate with males before ovipositing. 6. The predicted correspondence between females waiting at the emergence site and fewer than half of females in the population containing sperm, can be tested empirically, as can the somewhat counter-intuitive prediction that greater access to males should yield a more male-biased sex ratio in the offspring of mated females. 7. The present study also indicates how measuring the variance in giving up times by females waiting for males at low density, can provide insight into mechanisms determining waiting times.