Discrimination Between Parasitized and Unparasitized Hosts in the Egg Parasite Trichogramma Embryophagum (Hym.: Trichogrammatidae): a Matter of Learning and Forgetting

Abstract

The ability of the chalcid Trichogramma embryophagum, a parasite of the eggs of moths, to discriminate between unparasitized and parasitized hosts was studied using eggs of the flour moth, Ephestia kuehniella, as hosts. Females of the parasite which have not yet oviposited (inexperienced parasites) accept both unparasitized and parasitized hosts for oviposition (table 1). Parasites which have oviposited once in unparasitized hosts (experienced parasites) will continue to accept unparasitized hosts for oviposition, but reject parasitized ones, which had been parasitized shortly before the second exposure (tables 2 and 3). From the results of these experiments it was concluded that parasites learn to discriminate by ovipositing in unparasitized hosts. The cues relevant for discrimination can be perceived from the surface (chorion) of the parasitized host by the antennae, or from the interior by the ovipositor. Both types of discrimination are based on marks transferred to the host by the ovipositing parasite. In many cases the amount of material left on the chorion does not suffice to prevent drilling by an experienced parasite inspecting the host some minutes after it was infected. In that case the parasitized host is rejected after perforation of the chorion by the ovipositor. The proportion of experienced parasites rejecting a parasitized host is dependent on the length of the interval between the presentations of the unparasitized and the parasitized host. When the interval is ten minutes or less, all parasites discriminate against the latter, but when the interval extends to one hour and a half, all hosts are accepted (fig. 1). The discriminatory ability of the parasite is induced during the actual deposition of the egg in an unparasitized host, i.e. shortly before the withdrawal of the ovipositor. Parasites which were disturbed at earlier phases of the process of parasitization of the unparasitized host, such as drilling or inserting, do not reject parasitized hosts offered within some minutes after the parasite was disturbed (table 4). Incompletely attacked hosts (i.e. attacked up to a certain phase in the process of parasitization) are only rejected by experienced parasites, when the attack progressed up to the phase in which the ovipositor was inserted and a marking substance could be injected (table 5). The effectiveness of discrimination was tested by exposing clusters of two or three hosts to parasites. Parasites provided with excess hosts during the intervals between the tests, so that they could lay eggs at a rate about equal to the rate of production, have a perfect power of discrimination, but parasites ovipositing between the tests at a considerably lower rate superparasitize up to 25 per cent of the hosts. It is argued that the discriminatory ability of this parasite is an adaptation to the parasitization of host eggs which are deposited in clusters.