The emergence of glyphosate-resistant weed populations threatens the economic viability of genetically modified, glyphosate-resistant crop varieties. This could not only have serious consequences for the welfare of U.S. farmers, but also for environmental quality as farmers turn to more toxic herbicides. The purpose of this paper is to better understand what economic factors have contributed to the rise of resistant weeds. Specifically, I investigate whether externalities associated with weed mobility have led farmers to apply more glyphosate than they would otherwise. To do this I develop a game theoretic model of how farmers choose the amount of glyphosate they apply to their fields and use this model to derive testable hypotheses for how these choices might change when spatial externalities are present. Next, I use state-level data to test these hypotheses.