30 years of biological control of invasive spotted knapweed in British Columbia

Spotted knapweed (Centaurea stoebe ssp. micranthos) is a widespread and problematic invader of rangelands in southern British Columbia and much of the Pacific Northwest. Together with diffuse knapweed (C. diffusa), spotted knapweed has been the subject of a North American biocontrol program since the 1970s, resulting in the release of 13 insect agents. Despite effective biocontrol of diffuse knapweed in BC, and widespread reductions in the density and fecundity of spotted knapweed across the province, pockets of abundant spotted knapweed remain with considerable costs to rangeland tenure holders. To test biocontrol efficacy across the invaded range, we sampled more than 20 long-term monitoring sites across southern BC for abundance and density of spotted knapweed and its control agents. We dissected 850 individual knapweed plants, and more than 7500 seed heads to quantify plant morphology, fitness, and control agent presence and abundance. We will present our latest results showing, on average, a long term decline in spotted knapweed under biological control. We augment this finding with demonstrated fitness reductions due to biological control agents across the range, in most cases lowering fecundity below self-sustaining population levels. Despite this, robust populations remain, likely partially reflecting the lag time required for control agents to suppress this long-lived and highly fecund perennial.