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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

Three habitual parasites of the pine looper are generally not eliminated by the haemocytic defence reaction of their usual host, because the eggs are laid or the larvae settle behind the epithelium of the gut. This acts as a barrier against the passage of blood cells. This conclusion is supported by the observation that larvae of all three species are encapsulated during abnormally long stays in the host's body cavity. In two species this results from the parasitoid's failure to oviposit in the usual way, being either egg deposition at a less favourable site on the host's integument, or in a less suitable larval instar of the host. In the third species encapsulation results from the failure of the parasitoid's larva to enter the mid-gut due to some unknown factor(s) related to superparasitism.