Thirty years of field monitoring show that the densities of diffuse knapweed in rangelands in British Columbia declined dramatically following the establishment of the 12th introduced agent, the weevil, Larinus minutus. In addition, field experiments demonstrate the impacts of several widely distributed agents and indicate that L. minutus is a superior agent. Following a fire that temporarily removed biological control agents from one site and allowed the reestablishment of diffuse knapweed, we observed again the impact of L. minutus. Particularly in hot dry springs, feeding damage by adult weevils is sufficient to kill bolting plants. These observations support the prediction that an agent that kills the later developmental stages of knapweed plants can successfully control this invasive weed.