Integration of biological control with other methods to restore rangeland infested with spotted and diffuse knapweed

Spotted knapweed (Centaurea stoebe, often erroneously called C. maculosa) and diffuse knapweed (Centaurea diffusa) are invasive alien forbs that have infested extensive areas of rangeland in North America. Biological control agents have been evaluated, permitted and released to control these weeds. However, use of such agents does not necessarily result in restoring the plant community to a desirable state, and may lead to replacing one weed by another. We conducted an experiment to evaluate integrating biological control with several complementary methods to restore vegetation at Yakima, WA and Ft. Carson, CO. Treatments were: 1) releasing biological control agents, 2) seeding desirable grass and forb species, 3) applying sucrose to reduce soil nitrogen availability to promote perennial species, and 4) inoculating plots with soil microbes (including mycorrhizal fungi) from an adjacent native plant community. The insect populations increased dramatically during the study until 2002, when the knapweed populations crashed during a severe drought. Afterward, the knapweed populations failed to recover to pre-release levels, apparently because of the combined effects of reduction of plant vigor and soil seed bank, caused by insect biological control agents, and increased competition with perennial plants. The complementary treatments (seeding, sugar, and soil microbes) were most effective at the site that had the least establishment of native perennial plants at the beginning of the experiment. Efficacy of the complementary treatments depends on soil moisture, which varies from year to year and is not always sufficient in this environment, which reduces reliability.