SEX-RATIO CONTROL IN A PARASITIC WASP, NASONIA VITRIPENNIS. II. EXPERIMENTAL ANALYSIS OF AN OPTIMAL SEX-RATIO MODEL

An optimal theory of facultative sex-ratio adjustment (Werren, 1980) was tested using the data from a series of sequential oviposition experiments (Orzack and Parker, 1986). Sex ratios produced by several genotypes in previously parasitized hosts differ significantly from the theoretical prediction. In addition, there is more variance of these "second" sex ratios than would be generated purely by sampling. I outline an alternative model of sex-ratio determination, based upon an imperfect ability of second females to detect previous parasitization, which accounts for the trends observed in the data. These results imply that selection on second sex ratios is weak or that females cannot control sex ratios finely enough to manifest the proper response. This analysis along with other results (Orzack and Parker, 1986; Parker and Orzack, 1985; Grant et al., 1974; Werren et al., 1981; Skinner, 1982) suggests that we need a more comprehensive theory of sex-ratio evolution, one which accounts for the diversity of first and second sex ratio phenotypes in this species.