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Using two closely related larval parasitoids (Cotesiaspp.) ofPieriscaterpillars we tested the hypothesis that parasitoids are capable of assessing patchprofitability from a distance by showing differential responses to odors from plants infested with different host densities. We furthermore tested whether experience improves this assessment process. The effect of host densities on the olfactory responses of naive and experienced females was studied in two-choice wind tunnel experiments. Naive females ofCotesiaglomerataandC. rubeculadiscriminated between odors from plants with high and low densities. Responsiveness of naive females to odors from host-infested leaves increased with an increase in the total number of feeding hosts. InC. glomeratasensitivity to low host densities increased after experience. The effect of multiple oviposition experiences ofC. glomerataon different plants with different host densities was measured in two-choice situations. Females indeed use experienced host density of a patch as a cue to establish a preference, but the sequence of the experienced host densities influences the behavior to a great extent. The first experience does not entirely fix their behavior. ForC. glomerata,the retention time of learned odors was 3 days. This study illustrates the importance of quantitative differences in infochemicals to host-foraging decisions from a distance. It further demonstrates how experience can modify the parasitoid's response to variation in resource availability.