The influence of maize, Zea mays L., and pigweed, Amaranthus spp., on the abundance of tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), adults infesting cotton, Gossypium hirsutum L., in the Mississippi Delta was studied using stable-isotope analyses. Cotton fields adjacent to maize and those farther than 1.6 km from maize were identified in four counties of the Mississippi Delta. Tarnished plant bug adults were collected from the cotton fields weekly throughout the growing season and analyzed with stable-carbon and nitrogen isotopes to determine the natal host. The proportion of tarnished plant bug adults collected from cotton that developed as nymphs on plants that used the C4 photosynthetic pathway, primarily maize and pigweed, peaked from 79–92% during 2008–2009. Distance of cotton fields from maize did not influence the peak proportion of tarnished plant bug adults with a C4 signature from cotton fields, the peak time, or peak duration. Time of the C4 peaks occurred from 28 June to 8 July, and the duration was between 13 and 19 days. Stable-nitrogen isotope analyses indicated maize was the major contributor of C4 tarnished plant bugs infesting cotton compared to pigweed. Nevertheless, pigweed produced a significant proportion of tarnished plant bug adults infesting cotton. Improved control of these weed species could potentially reduce abundance of tarnished plant bugs in cotton. Because maize makes up a significant amount of the crop acreage in the Mississippi Delta, this information can be used to estimate the time of mass movement, thus allowing more efficient use of insecticide to control tarnished plant bugs in cotton. It may also allow future targeting of tarnished plant bugs in field corn before movement to cotton.