Technical Abstract: We researched the efficacy of the biological control agent Oberea erythrocephala against the nonnative invasive plant leafy spurge (Euphorbia esula). Our three treatments were release of O. erythrocephala into uncaged plots, release of O. erythrocephala into plots caged to prevent agent escape and control plots caged to prevent agent entry. These treatments were replicated three times at six sites in the western U.S. We measured E. esula biomass for one year following release. We also measured the percentage of E. esula stems showing evidence of O. erythrocephala oviposition for either two or three years following agent release, depending on the site. O. erythrocephala did not demonstrably reduce E. esula biomass in our study. Moreover, compared to the release year, evidence of O. erythrocephala oviposition was much lower after the release year suggesting the agent population was diminishing. This suggests the agent is incapable of building large populations capable of controlling E. esula at the sites we studied. However, after being released, populations of biological control agents sometimes go through long lag phases and then begin rapid population growth, so we cannot completely rule out the possibility that O. erythrocephala might become effective given more time.