Helicoverpa zea; HYMENOPTERAN-PARASITES; Microplitis croceipes; Host location, role of parasite olfactory & visual learning Microplitis croceipes; CHEMORECEPTION-; Olfactory learning role in lepidopteran host location; PHOTORECEPTION-; Visual learning role in lepidopteran host location; FORAGING-; Lepidopteran host location; LEARNING-; Olfactory & visual learning roles in lepidopteran host location; LEPIDOPTERAN-HOSTS; Helicoverpa zea; Host location, role of olfactory & visual learning, laboratory study The host foraging behavior of the larval parasitoid Microplitis croceipes (Cresson) (Hymenoptera: Braconidae) was studied in response to various assemblages of hosts and associated cues distributed over a patch of cotton plants located in a flight chamber. Females of the parasitoid developed a preference for stimuli experienced during host encounters. To study odor learning, frass from Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), feeding on either of two different parts of the cotton plant, was offered as volatile stimuli. During training sessions only one of the frass odors was associated with the host. Subsequent choice evaluations revealed that parasitoids preferred whichever frass odor had been associated with the host during training sessions. Thus, it was shown that females can learn to distinguish among frass odors from hosts feeding on different parts of the plant. In the same manner it was shown that parasitoids can be conditioned to visual stimuli. This study demonstrates that parasitoids use olfactory as well as visual learning to concentrate their search on the plant structures that are most profitable in terms of host encounters. Visual and olfactory learning proved to be additive: parasitoids conditioned to a combination of visual and olfactory stimuli displayed a stronger preference than individuals conditioned to either sensory component alone. When conditioned to a combination of stimuli, olfactory learning was demonstrated to be dominant over visual learning. To our knowledge this study is the first account of multisensory conditioning and summational learning effects in insect parasitoids. The significance of multisensory learning in parasitoid foraging is discussed. none