Tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), is the major insect pest of cotton, Gossypium hirsutum L., in the mid-South. This pest moves into cotton fields from non-cotton hosts during late spring and early summer. Stable carbon isotope (SCI) analysis was used to characterize plant hosts of tarnished plant bugs, as well as adult tarnished plant bugs that had developed as immatures on the plants. Plants of two host types (C₃ versus C₄) were identified using the SCI analysis, and tarnished plant bug adults reared as immatures on the plants retained a carbon isotopic signature similar to the host plant. Thus, carbon isotope ratios of tarnished plant bugs reared on C₃ plants differed from those reared on C₄ plants. Carbon and nitrogen isotope ratios were analyzed jointly using cluster and discriminant analyses to discriminate between adults that developed as immatures on two primary C₄ hosts (maize, Zea mays L., and pigweed, Amaranthus spp.). These analyses distinguished tarnished plant bugs that developed as nymphs on maize from those that developed on pigweed. To evaluate the impact on the carbon isotopic signature of immature tarnished plant bugs moving from one host type to another, nymphs were reared for the first three instars on broccoli, Brassica oleracea L. var. italica, (a C₃ plant) and transferred to maize (a C₄ plant) for the final two instars. The reverse was also done, and isotope ratios of these insects were compared to those that completed all instars on either broccoli or maize. The isotopic signature of the host plant was primarily obtained by tarnished plant bugs during the last two instars. Results from this study demonstrated the utility of carbon and nitrogen isotope ratio determination for evaluating the ecology of host plants of tarnished plant bugs in the mid-South. Knowledge of the primary hosts of tarnished plant bugs could allow for population management/reduction of these insects before moving into cotton.