Entomol. exp. appl. 68: 51-58, 1993. 9 1993 Kluwer Academic Publishers. Printed in Belgium. 51 Superparasitism and sex ratio in the solitary parasitoid Epidinocarsis lopezi Marianne J. van Dijken, Pam van Stratum & Jacques J. M. van Alphen Department of Population Biology, University of Leiden, P.O. Box 9516, 2300 RA Leiden, The Netherlands Accepted: December 8, 1992 Key words: super parasitism, sex ratio, solitary parasitoid, Epidinocarsis lopezi, biological control Abstract Highly variable sex ratios are found in the solitary parasitoid Epidinocarsis lopezi, both in the field and in a mass-rearing situation. Superparasitism is one of a number of factors which can influence sex ra- tios in parasitoid Hymenoptera. We show that superparasitism in E. lopezi is common in the field. Sex allocation decisions when parasitizing unparasitized hosts are not different from those with parasitized hosts; neither does differential mortality occur between the sexes in superparasitized hosts. Therefore superparasitism does not contribute to the variable sex ratio of E. lopezi. Both the occurrence of super- parasitism and the sex produced when ovipositing are shown to be functional for E. lopezi. Introduction Parasitoid Hymenoptera possess a haplodiploid reproductive system by which fertilized (diploid) eggs develop into females and unfertilized (hap- loid) eggs develop into males. Mated female par- asitoids can regulate the sex ratio of their progeny by controlling the fertilization of eggs. The abil- ity to control sex ratio allows sex ratios adaptive to a range of conditions to be produced (Charnov, 1982). Consequently, considerable inter- and in- traspecific variation is found in parasitoid sex ra- tios. For the solitary parasitoid Epidinocarsis lopezi (DeSantis) (Hym., Encyrtidae) in Africa, field sex ratios are highly variable, and often male-biased (van Dijken etal., 1991). Small wasps and a male-biased sex ratio were also present in the initial stages of the mass-rearing of this species (P. Neuenschwander, pers. comm.). To gain an understanding of sex ratio allocation strategies in E. lopezi and to improve its mass-rearing, we ex- amine sex ratio variability in E. lopezi. Several evolutionary models have provided po- tential explanations of variable sex allocation (primary sex ratio) in parasitoids. These models involve 1) local male competition (LMC; Hamil- ton, 1967; 1979), 2)host-size distribution (Char- nov, 1979), or 3) superparasitism (for gregarious parasitoids in relation to LMC: Werren, 1980; Suzuki & Iwasa, 1980; for sexual asymmetries: Waage & Godfray, 1985; Godfray, 1986). In ad- dition, differential survival of the sexes in small or superparasitized hosts may play a role by influ- encing the sex ratio at emergence (secondary sex ratio) (see also Charnov, 1982; Waage, 1986 for a summary). The variability in sex ratio for E. lopezi is not caused by a response of female parasitoids to the number of conspecifics exploiting a patch (LMC): primary sex ratios are independent of wasp den- sity (van Dijken et al., 1989). The host size dis-