Proximal mechanisms of the sex ratio and clutch size responses of the wasp Nasonia vitripennis to parasitized hosts

Female Nasonia vitripennis lay fewer eggs and increase the proportion of male offspring when ovipositing in previously parasitized hosts compared to unparasitized hosts. This study examines the location and nature of the cues that females use in these clutch size and sex ratio decisions. Neither the sex ratio nor the clutch size response relies on chemical cues on, or a hole drilled in, the outer shell of the host (the puparium). Rather, the cues for both responses appear to be associated with the host pupa. Females manipulate clutch size but not sex ratio in response to host death: the number of eggs laid on dead hosts is significantly less than on either live hosts or previously parasitized hosts. In addition, the cues that females use for sex ratio manipulation, but not for clutch size manipulation, are local: sex ratio cues are not detected from the end of the host that is opposite the site of parasitization; clutch size cues are. The cues that females use may constrain their sex ratio and clutch size manipulation abilities. One major group of models for sex ratio manipulation in response to environmental conditions is the local mate competition models. These models have been studied best in parasitoid wasps (reviewed in Waage 1986; King in press). Most parasitoid wasps have haplodiploid sex determination, which potentially allows females to control offspring sex ratio by controlling egg fertilization. In addition, many parasitoid wasps seem to have the subdivided population structure assumed by local mate competition models. One prediction of these models is that a mother should often produce a greater sex ratio (proportion of sons) when she oviposits in a previously parasitized host than in an unparasitized host (Suzuki & Iwasa 1980; Werren 1980). The strongest support for this prediction comes from the parasitoid wasp Nasonia vitripennis (e.g. Holmes 1972; Werren 1980). Female N. vitripennis oviposit a greater proportion of sons and