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 discrimination). 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.
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