Development of insecticide resistance in the tarnished plant bug ,Lygus lineolaris (Palisot de Beauvois) has become a major concern for cotton growers in the southern U.S. Resistance seems to be associated with pyrethroid use, and resistance management recommendations encourage limited exposure of tarnished plant bug populations to the pyrethoids. Resistance levels are highest in the delta regions of Arkansas, Louisiana, and Mississippi where populations may be concentrated on cotton for one to two generations in June and July. The highly polyphagous nature of the insect has encouraged scientists to recommend the development of cultural management practices based on vegetation management and manipulation of preferred hosts. The wide host range of the insect also raises questions about the evolution of resistance in such a highly polyphagous pest with large population densities (or refuges) associated with non-crop hosts not treated with insecticide. A simple population model was developed to examine the possible role of population structure (portion of population on crop being sprayed versus that on non-sprayed, non-crop hosts) on resistance evolution in the tarnished plant bug. The model was designed to mimic typical population growth patterns of the tarnished plant bug in the Mississippi Delta and included simulated growth of eight generations per year. Two of the eight generations (the third and fourth) were "bottlenecked" on cotton and exposed to selection from an insecticide. Under these simulated conditions resistance evolved in 101 generations over a 12.6 year period. Pyrethroid insecticides have been used extensively in the Mississippi Delta for nearly 20 years.