1. The importance of learning in parasitoids is likely to depend on how long learning effects persist, the number of learned stimuli that can be retained, and whether learning effects are confined to low ranked hosts.
2. The duration of learning of host-specific stimuli was tested in four strains of Trichogramma nr. brassicae (Hymenoptera: Trichogrammatidae). Both the persistence of learning of the rearing host and oviposition experience were tested in separate experiments.
3. The response to a non-preferred host, Sitotroga cerealella, was measured at different times following experience on that host or an alternative host, Heliothis punctigera. Both types of experience persisted for at least 5 days in all four strains.
4. The effect of oviposition in a highly preferred host, H. punctigera, following oviposition in S. cerealella was also tested. The increased acceptance of S. cerealella following oviposition on this host persisted despite a subsequent oviposition on H. punctigera.
5. The implications for host range at both individual and population levels are discussed.