Technical Abstract: Tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), is the number one pest of cotton, Gossypium hirsitum L., in the mid-South. This pest immigrates into cotton fields from non-cotton hosts during late spring and early summer. Stable carbon isotope (SCI) analysis was used to characterize adult tarnished plant bugs that had developed as immatures on a C3 or C4 plant host. Plant material was collected from hosts of tarnished plant bug during the late spring. Ratios of 13C/12C were produced for each plant host sample indicating host type (C3 versus C4), and both host types were identified using the SCI analysis. Tarnished plant bugs were reared through the nymphal stage on various plant hosts. Wings were removed from the corresponding adults and subjected to SCI analysis, which showed that tarnished plant bug adults reared as immatures on various plant hosts 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. The SCI method can only distinguish between C3 and C4 food sources. Two of the major host plants of tarnished plant bugs during the early growing season are C4 hosts (corn, Zea mays L., and pigweed, Amaranthus spp.). Carbon and nitrogen isotope ratios were analyzed jointly using cluster and discriminant analyses to discriminate between adults that developed as immatures on these two C4 hosts. To evaluate the impact of immature tarnished plant bugs moving from one host type to another on the carbon isotopic signature, nymphs were reared for the first three instars on broccoli, Brassica oleracea L. var. italica, (a C3 plant) and transferred to corn (a C4 plant) for the final two instars. The reverse of this was also performed, and isotope ratios of these insects were compared to those that completed all instars on either broccoli or corn. Results indicated that 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 host plant ecology of tarnished plant bugs in the mid-South. Knowledge of the primary hosts of tarnished plant bugs prior to immigrating to cotton could allow for population management/reduction of these insect populations.