Use of picloram to enhance establishment of Cyphocleonus achates (Coleoptera : Curculionidae)

Herbicidal control of spotted knapweed, Centaurea maculosa Lamarck, is rare ly cost-effective, and sustainable control may require an integrated approa ch. 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 s potted knapweed using reduced rates of picloram would improve the establish ment of C. achates. At two sites in western Montana, three adult weevil den sities (none, three, and six weevils m(-2)) were released in the fall (1995 at site 1 and 1996 at site 2) in 2-m(2) 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 pe r replication). Spotted knapweed density and spotted knapweed and grass cov er 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 approximate to 500 to approximate to 175 plants m(-2). Cover was reduced from approximate to 60 to 10% at these rates. There was no detectable impact of weevils on spotted knapweed. Weevi l 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 h ighest when spotted knapweed cover was between 30 and 70%. Results show tha n reduced rates of picloram do not limit weevil establishment.