Print PageSpotted knapweed (Centaurea stoebe) is a perennial forb introduced into North America in the late nineteenth century. It has since spread throughout almost all of the United States and much of Canada, displacing native vegetation in both disturbed and undisturbed areas. This study focused on a population of C. stoebe in the Left Hand Canyon watershed in the foothills of the Front Range of the Rocky Mountains in Boulder County, CO. A paired-plot, multifactorial design was used to quantify the abundance of C. stoebe across the landscape, its representation in the seed bank, and its response to the action of several predatory insects that had been introduced as biological control agents. These measurements were stratified in such a way to test the effects of landscape, localized distrurbance (gopher (Thomomys talpoides) mounds), and insect predation. Specifically, vegetative cover, seed abundance (both in soil and in seedheads), seedling density, plant age and size, insect abundance, and insect-caused tissue damage were measured on north facing slopes, south facing slopes and in a riparian meadow, in areas that had been disturbed and not disturbed.
Results/Conclusions
No disturbance effects and few significant responses in the C. stoebe population across aspects were detected, although seed production was significantly higher in the riparian corridor vs. either slope type. There was distinct variation in biocontrol insect distribution across aspects. The root-boring weevil Cyphocleonus achates was more abundant on south-facing slopes and, to a lesser extent, in the meadow. The seedhead weevil Larinus minitus was more abundant in the meadow, with limited distribution on north-facing slopes. We offer several complementary explanations for these phenomena, and suggest how management strategies might be informed by these observations.
