Impacts of the biological control agent Cyphocleonus achates on spotted knapweed, Centaurea maculosa, in experimental plots

The root weevil Cyphocleonus achates was introduced into western Montana in 1988 as a biological control agent against spotted knapweed (Centaurea maculosa), an exotic invasive weed from Eurasia. Spotted knapweed has declining in areas where C. achates was released, and experiments were conducted to evaluate the role this weevil may play in knapweed decline. Field experiments were conducted from 2001 to 2003, and from 2002 to 2004, with groups of 20 knapweed plants exposed to 8 pairs, 2 pairs, or no adult C. achates. Knapweed mortality and vigor of surviving plants were followed for two years. In the 2001–2003 plot experiment, mortality increased with exposure to increasing numbers of C. achates, and root and shoot biomass, and plant height declined in surviving plants. Treatment effects persisted after two years, even though second-year weevil abundance no longer correlated with original treatment levels. In the 2002–2004 experiment, knapweed mortality was relatively high in all treatment groups. A pattern of reduced knapweed vigor with increased exposure to weevils, similar to the 2001–2003 experiment, was observed. This is the first report on impacts by C. achates on spotted knapweed, supporting the hypothesis that knapweed decline at C. achates release sites is due at least in part to their attack.