Anim. Behav., 1996, 52, 875–883 Self–host discrimination by a parasitic wasp: the role of short-term memory TAKATOSHI UENO & TOSHIHARU TANAKA Laboratory of Applied Entomology, Nagoya University (Received 1 September 1994; initial acceptance 17 November 1994; final acceptance 23 January 1996; MS. number: 4725) Abstract. Ovipositing parasitoid wasps deposit marking pheromones on the host that deter conspecifics from superparasitism. In addition to recognizing previously parasitized hosts, female wasps of some species discriminate between hosts parasitized by themselves and by conspecifics (self–host discrimi- nation). The mechanism of self–host discrimination in the parasitic wasp Pimpla nipponica Uchida was examined. A series of experiments, however, failed to show that females of this species deposit chemical markers on a host during oviposition. Females failed to recognize hosts covered either with pieces of cocoon derived from, or plastic tubes used to conceal, hosts they had previously parasitized. They also recognized hosts that had been superparasitized by conspecifics. Two further experiments supported an alternative hypothesis, that females remember hosts they have parasitized before. Rejection of the first parasitized host decreased markedly after females had attacked two or three hosts, and females cooled to induce amnesia failed to recognize self-parasitized hosts. It is suggested that hosts could smell different enough for a female wasp to discriminate between self- and conspecifically parasitized hosts. The role of short-term memory in the foraging strategies of parasitoid wasps is discussed.  1996 The Association for the Study of Animal Behaviour Many parasitic and herbivorous insects, after depositing their eggs, leave behind markers in or on their host (for reviews see van Lenteren 1981; Prokopy 1981; Roitberg & Prokopy 1987; van Alphen & Visser 1990). These are either chemical substances or physical marks (e.g. Vinson 1975; van Lenteren 1981; Prokopy 1981; Strand 1986), which usually deter conspecific females from depositing additional eggs on marked hosts. The markers hence function to reduce competition between immature insects for host resources (e.g. Roitberg & Prokopy 1987). In parasitic insects, females can discriminate between already parasitized and unparasitized hosts and usually avoid ovipositing in the former (e.g. Vinson 1975; van Lenteren 1981; Bakker et al. 1985). They do this by recognizing the markers left on or in the hosts by the ovipositing females (Roitberg & Mangel 1988). The markers are usually species-specific and are hence usually effective only to conspecific females (e.g. Bakker et al. 1985; Turlings et al. 1985; Okuda & Yeargan 1988) but in some cases are recognized by related species (Vet et al. 1984; Bai & Mackauer 1991; Scholz & Höller 1992). In addition, female wasps can discriminate between hosts parasitized by themselves and by other conspecifics (Hubbard et al. 1987; Völkl & Mackauer 1990; Höller et al. 1991; van Dijken et al. 1992; Ueno 1994; Ueno & Tanaka 1994a). This ability is called ‘self–host discrimination’ or ‘self-recognition’ (Höller et al. 1991). Some authors have suggested that markers employed by some parasitoid species are individual-specific, and so allow females to recog- nize self-parasitized hosts (i.e. hosts they have parasitized themselves; Hubbard et al. 1987; Völkl & Mackauer 1990; Höller et al. 1991; van Dijken et al. 1992; Visser 1993; Ueno 1994; Ueno & Tanaka 1994a). Höller et al. (1991) considered that this individuality of markers could be due to specific ratios of chemical components of the markers. However, as yet there is no evidence that a female uses chemical markers as a self–host discrimination cue and that the markers are really individual-specific. Correspondence and present address: T. Ueno, Paddy Crop Insect-Pest Laboratory, National Agriculture Research Center, Tsukuba 305, Japan (email: tueno@narc.affrc.go.jp). T. Tanaka is at the Laboratory of Applied Entomology, Nagoya University, Nagoya 464-01, Japan. 0003–3472/96/110875+09 $25.00/0  1996 The Association for the Study of Animal Behaviour 875