none Behavioral interations among Cotesia urabae Austin and Allen, Dolichogenidea eucalypti Austin and Allen (Hymenoptera: Braconidae), and their host Uraba lugens Walker, the gum leaf skeletonizer (Lepidoptera: Noctuidae), were observed at three host sizes over a 20-min period. These sizes were first instar (small, gregarious) fourth-fifth instar (mid, gregarious) sixth-seventh instar (large, solitary) larvae. Unlike C. urabae, D. eucalypti used its legs to hold small larvae before ovipositor insertion. D. eucalypti also visited patches of small larvae more frequently, proceeded less often through patches of mid larvae, and made significantly fewer ovipositions in mid and large larvae. Small larvae responded to both parasitoids by dispersing outward, while mid larvae responsed to parasitoids by moving inward to form a denser group. Larvae reared or thrashed after each parasitoid visit, especially mid larvae, and some continued to do so for up to 2 h after parasitoid departure. Mid and large larvae occasionally injured parasitoids by biting their appendages. By rearing or thrashing immediately prior to an encounter with a parasitoid, mid and large larvae decreased the likelihood of being parasitized by up to 50%.