The biological control of Centaurea spp. In North America: Do insects solve the problem

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Abstract
Spoiled and diffuse knapweed, Centaurea maculosa Lam. and C. diffusa Lam., both of European origin, are two of the most important rangeland weeds in North America. Surveys for potential biological control agents started in 1961, and agents were first introduced in 1970. So far, 11 phytophagous insect species of European origin have been released in North America. Despite legislation (restricted transport), large‐scale application of herbicides and reductions in seed production of up to 95% by the existing seed‐feeding biocontrol agents established, knapweed infestation has further increased during the last decade, covering nearly 2‐8 and 13 million ha for spotted and diffuse knapweed, respectively. In this paper, the biocontrol programme is reviewed critically and the present status analysed. To reach a break‐through in knapweed control, a cumulative stress approach is now envisaged, by extending the stress imposed by the biocontrol agents to increasing competition by the other vegetation through grazing regimes and reseeding programmes. Four steps to optimize the biocontrol effect and which lead to the integration of biocontrol into range management are described and suggestions, both scientific and political, are presented to render biological control more efficient and predictive.

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