Four species of biocontrol insects (knapweed root weevil, lesser knapweed flower weevil, spotted knapweed seedhead moth, and bronze knapweed root borer) were released at a diffuse knapweed site located about 10 km east of the Colorado Front Range. Two other biocontrol agents (banded gall fly and knapweed seed head fly) were already present at this site. Densities of rosettes and flowering plants, seedhead production per plant, and seeds per seedhead on mowed and unmowed areas were studied for 5 yr, 1997–2001. Of the six biocontrols, five (Urophora spp., bronze knapweed root borer, knapweed root weevil, and lesser knapweed flower weevil) obtained sizable densities relative to weed abundance. Diffuse knapweed declined from 8.3% in absolute cover in June 2000 to 1.9% by September 2001. Vegetation transects closest to the insect release areas showed the largest declines, with diffuse knapweed disappearing entirely on one transect. In contrast, diffuse knapweed abundance at a nearby prairie increased during the same interval from an absolute cover value of 14.5 to 17%. Seed production of diffuse knapweed on the insect release site declined from nearly 5,000 seeds m⁻² in 1997 to less than 100 seeds m⁻² in 2001. Lesser knapweed flower weevil larvae appeared responsible for much of the seed reduction, whereas adults of this species were effective in damaging bolting plants. The extent to which grazing removal and individual insect species contributed to this reduction in diffuse knapweed abundance cannot be identified from this study. These results support the contention that a significant reduction in abundance of diffuse knapweed using insects is possible at least in some regions of the western United States.

Nomenclature: Diffuse knapweed, Centaurea diffusa Lam. CENDI; Alyssum, Alyssum minus (L.) Rothm. AYSMI; Japanese brome, Bromus japonicus Thunb. Ex. Murr. BROJA; field bindweed, Convolvulus arvensis L. CONAR; banded gall fly, Urophora affinis Frauenfeld; knapweed seed head fly, Urophora quadrifasciata Meigen; bronze knapweed root borer, Sphenoptera jugoslavica Obenb.; knapweed root weevil, Cyphocleonus achates Fahraeus; spotted knapweed seedhead moth, Metzneria paucipunctella Zell.; lesser knapweed flower weevil, Larinus minutus Gyllenhal.