Dose-dependent influence of Microplitis rufiventris factors on developmental rate and growth of last-instar Spodoptera littoralis larvae E. M. Hegazi 1 and W. E. Khafagi 2 1 Department of Entomology, Faculty of Agriculture, University of Alexandria, Alexandria, Egypt; 2 Plant Protection Research Institute, Sabahia, Alexandria, Egypt Ms. received: May 27, 2004; accepted: October 14, 2004 Abstract: The solitary endoparasitoid, Microplitis rufiventris, attacks and can develop in earlier instars of Spodoptera littoralis larvae with preference to third-instar larvae. We used the last stadium (sixth instar), a stage which is not naturally parasitized. The newly moulted larvae (0–3 h old) of this stadium were more acceptable for parasitization by the wasp females than the older ones (24 h old). Parasitization by M. rufiventris wasp of last instar S. littoralis larvae leads to dose (no. of eggs + parasitoid factors)-dependent effects which were more pronounced at 20°C than at 27°C. A single oviposition into a sixth instar host larva resulted in normal development of the host. However, superparasitization increased the proportions of developmentally arrested hosts and number of live wasp larvae. Development of supernumerary individuals of the parasitoid in the host larva leads to dose-related adverse effects on host growth and development. The present study may provide interesting opportunities for studying the physiological bases of host–parasitoid interactions and parasitoid intra-specific competition in the biological system considered. Key words: braconid wasp, developmental arrest, last host instar, superparasitism, temperature 1 Introduction Parasitoids often influence growth and development of their hosts, and in many cases these effects are due to parasitoid-associated factors such as polydnaviruses, venom and/or teratocytes (Vinson and Iwantsch, 1980; Coudron, 1991; Lawrence and Lanzrein, 1993). Poly- dnaviruses have been observed only in endoparasitic wasps of the families Icheneumonidae and Braconidae and have accordingly been classified as the genera Ichnovirus and Bracovirus within the family Poly- dnaviridae (Francki et al., 1991). Polydnaviruses rep- licate only in the nuclei of calyx cells of the wasp ovaries, are secreted into the oviduct and injected along with the parasitoid egg into the host (Fleming, 1992; Fleming and Krell, 1993; Stoltz, 1993). Venoms are secretions from the venom gland which are also injected at oviposition; some have paralysing effects but many exert other not yet well-defined functions (Coudron, 1991) or are proposed to synergize polydna- viruses (Stoltz et al., 1988; Stoltz, 1993). Teratocytes are cells derived from the serosal membrane which are released into the host upon hatching of the parasitoid embryo; these cells have been observed only in the families Braconidae, Scelionidae and Platygasteridae (Dahlman, 1990; Dahlman and Vinson, 1993). Calyx fluid or purified polydnaviruses alone induce develop- mental arrest of the host in the gregarious braconid Cotesia congregate (Dushay and Beckage, 1993; Beckage et al., 1994) and in the solitary ichneumonid Campoletis sonorensis (Dover et al., 1987). In other parasitoids, calyx fluid or polydnaviruses affected development only when synergized by coinjected venom, namely the solitary braconids Cardiochiles nigriceps (Guillot and Vinson, 1972), Microplitis demolitor (Strand and Dover, 1991) and Chelonus inanitus (Soller and Lanzrein, 1996) and the gregarious braconid Apanteles karyiai (Tanaka, 1987). Teratocytes are involved in causing developmental arrest in the case of Microplitis croceipes (Zhang and Dahlman, 1989), C. nigriceps (Pennacchio et al., 1992), A. karyiai (Wani et al., 1990) and M. demolitor (Strand and Wong, 1991). In the two latter parasitoids, the combined injection of calyx fluid, venom and teratocytes was most effective and in some cases induced parasitism-like alterations of growth and developmental arrest (Wani et al., 1990; Strand and Wong, 1991). Thus, polydnavirus, venom and terato- cytes are all involved in causing developmental arrest (Lawrence and Lanzrein, 1993) but the relative import- ance of these parasitoid-associated factors varies between the various parasitoids. So far a role of the parasitoid larva in interfering with host development has been demonstrated only in the system C. inanitus– Spodoptera littoralis by parasitoid removal and parasi- toid implantation experiments (Pfister-Wilhelm and JEN 129(1) doi: 10.1111/j.1439-0418.2005.00924.43–51 Ó 2005 Blackwell Verlag, Berlin