Before embarking on the 5–10 yr effort it can take to transfer plant resistance (R) genes to adapted crop cultivars, a question must be asked: is the pest a sufficient threat to warrant this effort? We used the recently discovered female-produced sex pheromone of the Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae), to explore this question for populations in the Upper Great Plains. Methods for pheromone trapping were established and trapping data were used to explore geographic distribution, phenology, and density. The pheromone lure remained attractive for up to 10 d and only attracted male Hessian flies. Traps placed within the crop canopy caught flies but traps placed above the crop canopy did not. Hessian flies were trapped throughout North Dakota starting in the spring and continuing through the summer and autumn. Densities were low in the spring but increased greatly during the early part of the summer, with peak adult emergence taking place at a time (July/August) when spring wheat was being harvested and winter wheat had not yet been planted. In the autumn, adults were found at a time when winter wheat seedlings are growing. The discovery of flies on Conservation Reserve Program land supports the idea that pasture grasses serve as alternate hosts. We conclude that the Hessian fly is a risk to wheat in the Upper Great Plains and predict that global warming and the increasing cultivation of winter wheat will add to this risk.