none Preferences of Microplitis croceipes for five instars of Heliothis zea and H. virescens was measured experimentally on cotton in field cages. M. corceipes showed strong host-instar preferences. Using Manly's measure, we found wasps preferred third-instar larvae most (b = 0.47), fourth- and second-instar larvae next (b = 0.23 and 0.21, respectively), and first-and fifth-instar larvae least (b = 0.04). Preference did not depend on total host density or on the density of the most preferred instar. The wasps did not prefer one host species over the other. The authors present a model that predicts the proportion of larvae parasitized by the end of each instar, and thus the proportion of a host cohort parasitized in the field. The model requires as input the instar preferences of a parasitoid and an estimate of the density of searching parasitoids; it predicts parasitization fairly well in a collection of larvae from the field.