We investigated ovipositional preference of Helicoverpa armigera (Lepidoptera: Noctuidae) for a range of host plants, larval performance on a range of host plants, dispersion rate of a concentrated population of fifth instar larvae, the fitness consequences of cannibalism at two levels of food availability, and the frequencies of cannibalism between larvae of different instars. Eggs laid by adult females were concentrated on some specific plants, despite the wide range of host plants of H. armigera. However, the larvae dispersed when they reached high larval density and eventually exhibited a uniform spatial distribution. Cannibalism frequently occurred under poorer nutritional conditions or greater larval rearing density. The effect of cannibalism on development and pupal weight differed between rich and poor nutritional rearing conditions. The cannibals’ fitness was decreased under suitable food conditions, whereas larvae with a low frequency of cannibalism increased their pupal weight, a potential fitness trait, in unsuitable conditions. These results suggest that cannibalism is a behavior of larval H. armigera that has evolved to remove potential intraspecific competitors and thus to avoid unsuitable conditions.