Classical biological control of invasive plants: Limits and possibilities

Classical biological control has been used as a strategy in North America since 1940.  There have been many successes since then, and methods to evaluate specificity and to regulate the authorization of releases have progressed.  However, the most notable successes have been of alien plants that have invaded rangelands and aquatic habitats, not of weeds in cultivated crops.  Nevertheless, in the latter systems, weeds that are resistant to herbicides, or that can propagate by root fragments, may be reasonable targets for classical biological control.  Furthermore, augmentation and conservation techniques have largely been ignored in weed biological control, but could be important and more feasible in intensively managed ecosystems.