ABSTRACT
The oviposition behaviour of the ichneumon flies·Horogenes chrysostictos and Nemeritis canescens has been examined in relation to their multiparasitism of larvae of the moth Ephestia sericarium.
Horogenes is able to distinguish healthy hosts from those that are parasitized by Nemeritis, both when given a simultaneous choice between them and when exposed to each alternately.
Although Horogenes attacks fewer parasitized than healthy hosts, it does not lay proportionately fewer eggs in a parasitized host.
Nemeritis is also able to distinguish healthy hosts from those parasitized by Horogenes. Though it attacks parasitized hosts more readily than does Horogenes, it lays fewer of its eggs in them.
Both species avoid ovipositing in a host containing an advanced larval parasite more frequently than in a host containing a parasite egg.
An Ephestia larva multiparasitized by Horogenes and Nemeritis may therefore contain these parasites at widely different ages, but the host selection behaviour of the adult parasites makes multiparasitism by parasite larvae of similar age more likely in field populations.
