A 20-year evaluation of successes with biological control of spotted knapweed (Centaurea stoebe) in Colorado

Spotted knapweed (Centaurea stoebe) is an invasive plant introduced into North America from Europe that has colonized diverse plant communities across the temperate zone of North America. The impacts of classical biological control insects on this plant have been identified as harmful (benefiting the plant), inconsequential, or exhibiting effective controls. Here we present results that measured C. stoebe densities in Colorado subjected to small numbers of insect releases in 2001 and monitored for density effects on the plant from 2007 to 2020. Seed head weevils (Larinus spp), gall flies (Urophora spp) and root weevils (Cyphocleonus achates) were also measured. In one monitoring transect with ample native perennial and introduced grasses, knapweed stem densities declined over a 14-year interval from about 50 stems/m² to 0/m². At a second site that was disturbed by flood scour midway through the study, stems declined over a 13 year interval from about 40 stems/m² to under 20 stems/m². At a site lacking C. achates, stem densities were measured at 40 stems per m² in 2020, the same densities reported at other sites in 2008. No statistically detectable directional shifts in growing season precipitation or temperatures have occurred across the 2001–2020 interval, and the strongest relationships among variables measured here were directional declines in stem and rosette densities of the knapweed. Seeds/seed head showed positive relationships with precipitation but negative relationships with seed head Larinus spp densities, which in turn showed no directional trends in abundance. The outcome of biological control releases at these Colorado sites for C. stoebe ranged from excellent to negligible, and our work supports the context specific model of biological control, i.e., biological control in conjunction with plant competition are the essential components for reduction of C. stoebe densities.