In much of the Corn Belt and parts of Europe, the western corn rootworm, Diabrotica virgifera virgifera LeConte, is the most important insect pest of maize. The need for additional basic knowledge of this pest has been highlighted while developing resistance management plans for insecticidal genetically modified crops. This study evaluated the possibility of tracking feeding habits of western corn rootworm larvae using stable carbon isotope signatures. Plants accumulate different ratios of ¹³C:¹²C isotopes, usually expressed as δ¹³C, according to whether they use the C₃ or C₄ photosynthetic pathway. Herbivore biomass is expected to reflect the δ¹³C of the food they eat. For the current experiment, western corn rootworm larvae were grown on different species of plants exhibiting different δ¹³C values. The δ¹³C values were then measured in elytra of emerged beetles. When beetles were unfed, biomass reflected larval feeding. When beetles were fed for 31 d postemergence, δ¹³C values of elytra almost exclusively reflected adult feeding. These results suggest the use of caution in the interpretation of δ¹³C data aiming to document larval diet history when adult feeding history is unknown. The technique was also used to evaluate western corn rootworm larval choice between alternate hosts and maize with and without genetically modified (Bt) traits aimed at their control. Propensity for feeding on alternate hosts versus maize was biased toward feeding on maize regardless whether the maize had Bt or not, suggesting western corn rootworm larvae were not repelled by Bt. These data will be helpful for regulators in interpreting western corn rootworm feeding data on Bt maize.