Herbicidal control of spotted knapweed, Centaurea maculosa Lamarck, is rarely cost-effective, and sustainable control may require an integrated approach. Cyphocleonus achates (Fahraeus) is a flightless root-feeding weevil of Eurasian origin that has been introduced into North America for biological control of spotted knapweed. We hypothesized that reducing the density of spotted knapweed using reduced rates of picloram would improve the establishment of C. achates. At two sites in western Montana, three adult weevil densities (none, three, and six weevils m−2) were released in the fall (1995 at site 1 and 1996 at site 2) in 2-m2 plots encircled with enclosures to prevent weevil escape. Six picloram rates (0, 0.03, 0.06, 0.09, 0.12 or 0.15 kg ha−1) were applied the following spring before weevil emergence in a randomized complete-block design with four replications (18 treatments per replication). Spotted knapweed density and spotted knapweed and grass cover were sampled in July each year following treatment. Weevil numbers were counted annually in August. By 1998, picloram rates of >0.09 kg ha−1 had reduced spotted knapweed density from ≈500 to ≈175 plants m−2. Cover was reduced from ≈60 to10% at these rates. There was no detectable impact of weevils on spotted knapweed. Weevil numbers in plots treated with picloram >0.09 kg ha−1 were about half (0.5 weevils per plot) the numbers found in the other picloram treatments (1.25 weevils per plot). Weevil numbers in plots treated with 0.03, 0.06, and 0.09 kg ha−1 were similar to the unsprayed control. Weevil numbers were highest when spotted knapweed cover was between 30 and 70%. Results show than reduced rates of picloram do not limit weevil establishment.
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