While once dismissed as an artifact of unnatural laboratory conditions, cannibalism is now widely accepted as a common behavior exhibited by multiple insect taxa, including primarily herbivorous species. In some cases cannibalism provides a means of acquiring nutrients and defensive compounds, reducing competition, and decreasing population density. For plants, inducing insect cannibalism is a defense mechanism to decrease damage by pests. Despite the growing awareness of cannibalism, its causes, and its ecological importance, few model systems exist, largely because observations of cannibalism are not reported. Here, we investigated the cannibalism of pupae by caterpillars of the ranchman's tiger moth, Arctia virginalis, [Lepidoptera: Erebidae] based on an observation of a late instar caterpillar attacking the pupa of a conspecific in our laboratory. We conducted separate laboratory and field experiments to confirm this observation and determine the propensity for cannibalism under more natural conditions. We confirmed our observation of cannibalism in the laboratory experiment but failed to detect evidence of this behavior in the field. Consequently, the frequency of cannibalism, the conditions under which it occurs, and its impact on the population dynamics of A. virginalis are still unknown.