Sex ratio regulation in Coccygomimus turionella Linnaeus (Hymenoptera: Ichneumonidae) and its ecological implications

Abstract. 1. Sex regulation and its relevance to the ecology of a k-selected pupal parasitoid was investigated in Coccygomimus turionellae.
2. The ratio varied with host size, females predominating in large hosts.
3. Egg transfer experiments and comparative mortality rates among the progeny of virgin and inseminated females demonstrated that the phenomenon was due to parental behaviour.
4. The study of the mechanism confirmed Aubert's hypothesis of host size estimation with the addition that the host had to be exposed from the poles and had to contain the proper host kairomones.
5. Host size was highly correlated to parasitoid fecundity via influences on size and longevity but not the number of ovarioles per female or daily egg production.
6. Host encounters are more limiting than egg production and host size had no effect on host acceptance.
7. Sex regulation allows a maximal host encounter rate at the same time that it maximizes the reproductive potential of female progeny.
8. Low host density increased the production of female progeny resulting in more offspring searching for the limited resource. Concealed hosts and a high host density resulted in a shift towards a Fisher 1:1 sex ratio.