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We observed the foraging behavior of Diadegma semiclausum (Hymenoptera: Ichneumonidae), a larval parasitoid of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), in a wind tunnel to determine how interpatch distance affects patch time allocation. Individual female wasps were released onto an experimental patch infested with host larvae and were allowed freely to leave for an identically extra patch placed upwind of the experimental patch with varying interpatch distances. The effects of interpatch distance and within-patch foraging experience on the patch-leaving tendency of the parasitoid were analyzed by means of the proportional hazards model. Increasing interpatch distance and unsuccessful host encounter as a result of host defense decreased the patch-leaving tendency, while successful oviposition and unsuccessful search time since last oviposition increased the patch-leaving tendency. As a result, both patch residence time and number of ovipositions by D. semiclausum increased with increasing interpatch distance, which appears to agree with the general predictions of the marginal value theorem that a parasitoid should stay longer and parasitize more hosts with increasing interpatch distance.