none Feeding and movement on cotton of H. zea (Boddie) and H. virescens (F.) when parasitized by M. croceipes were compared to that of unparasitized hosts. First-instar host larvae were placed on cotton plants in the greenhouse (1 larva/plant), and subsets of them were parasitized at instars 2 to 5. Some hosts were left unparasitized as controls. Movement, activity, and feeding were scored daily. Hosts parasitized in instars 2 and 3 damaged 0.41- to 0.72-fold as many squares and 0.22- to 0.59-fold as many bills as those parasitized in instar 5 or unparasitized. Parasitized larvae moved less and fed less often than unparasitized larvae. These results suggest that the within-generation impact of parasitoids of larvae, like M. croceipes, on damage done by Heliothis larvae may have been underestimated in the past.