THE BIOLOGICAL-CONTROL OF CENTAUREA SPP IN NORTH-AMERICA - DO INSECTSSOLVE THE PROBLEM

Spotted and diffuse knapweed, Centaurea maculosa Lam. and C. diffusa L am., both of European origin, are two of the most important rangeland weeds in North America. Surveys for potential biological control agent s started in 1961, and agents were first introduced in 1970. So far, 1 1 phytophagous insect species of European origin have been released in North America. Despite legislation (restricted transport), large-scal e application of herbicides and reductions in seed production of up to 95 % by the existing seed-feeding biocontrol agents established, knap weed infestation has further increased during the last decade, coverin g nearly 2.8 and 1.3 million ha for spotted and diffuse knapweed, resp ectively. In this paper, the biocontrol programme is reviewed critical ly and the present status analysed. To reach a break-through in knapwe ed control, a cumulative stress approach is now envisaged, by extendin g the stress imposed by the biocontrol agents to increasing competitio n by the other vegetation through grazing regimes and reseeding progra mmes. Four steps to optimize the biocontrol effect and which lead to t he integration of biocontrol into range management are described and s uggestions, both scientific and political, are presented to render bio logical control more efficient and predictive.